Initial import

1 files changed, 4228 insertions, 0 deletions

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
new file mode 100644
index 000000000..ab5811545
--- /dev/null
+++ b/fs/btrfs/scrub.c
@@ -0,0 +1,4228 @@
+/*
+ * Copyright (C) 2011, 2012 STRATO.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+
+#include <linux/blkdev.h>
+#include <linux/ratelimit.h>
+#include "ctree.h"
+#include "volumes.h"
+#include "disk-io.h"
+#include "ordered-data.h"
+#include "transaction.h"
+#include "backref.h"
+#include "extent_io.h"
+#include "dev-replace.h"
+#include "check-integrity.h"
+#include "rcu-string.h"
+#include "raid56.h"
+
+/*
+ * This is only the first step towards a full-features scrub. It reads all
+ * extent and super block and verifies the checksums. In case a bad checksum
+ * is found or the extent cannot be read, good data will be written back if
+ * any can be found.
+ *
+ * Future enhancements:
+ *  - In case an unrepairable extent is encountered, track which files are
+ *    affected and report them
+ *  - track and record media errors, throw out bad devices
+ *  - add a mode to also read unallocated space
+ */
+
+struct scrub_block;
+struct scrub_ctx;
+
+/*
+ * the following three values only influence the performance.
+ * The last one configures the number of parallel and outstanding I/O
+ * operations. The first two values configure an upper limit for the number
+ * of (dynamically allocated) pages that are added to a bio.
+ */
+#define SCRUB_PAGES_PER_RD_BIO	32	/* 128k per bio */
+#define SCRUB_PAGES_PER_WR_BIO	32	/* 128k per bio */
+#define SCRUB_BIOS_PER_SCTX	64	/* 8MB per device in flight */
+
+/*
+ * the following value times PAGE_SIZE needs to be large enough to match the
+ * largest node/leaf/sector size that shall be supported.
+ * Values larger than BTRFS_STRIPE_LEN are not supported.
+ */
+#define SCRUB_MAX_PAGES_PER_BLOCK	16	/* 64k per node/leaf/sector */
+
+struct scrub_recover {
+	atomic_t		refs;
+	struct btrfs_bio	*bbio;
+	u64			map_length;
+};
+
+struct scrub_page {
+	struct scrub_block	*sblock;
+	struct page		*page;
+	struct btrfs_device	*dev;
+	struct list_head	list;
+	u64			flags;  /* extent flags */
+	u64			generation;
+	u64			logical;
+	u64			physical;
+	u64			physical_for_dev_replace;
+	atomic_t		refs;
+	struct {
+		unsigned int	mirror_num:8;
+		unsigned int	have_csum:1;
+		unsigned int	io_error:1;
+	};
+	u8			csum[BTRFS_CSUM_SIZE];
+
+	struct scrub_recover	*recover;
+};
+
+struct scrub_bio {
+	int			index;
+	struct scrub_ctx	*sctx;
+	struct btrfs_device	*dev;
+	struct bio		*bio;
+	int			err;
+	u64			logical;
+	u64			physical;
+#if SCRUB_PAGES_PER_WR_BIO >= SCRUB_PAGES_PER_RD_BIO
+	struct scrub_page	*pagev[SCRUB_PAGES_PER_WR_BIO];
+#else
+	struct scrub_page	*pagev[SCRUB_PAGES_PER_RD_BIO];
+#endif
+	int			page_count;
+	int			next_free;
+	struct btrfs_work	work;
+};
+
+struct scrub_block {
+	struct scrub_page	*pagev[SCRUB_MAX_PAGES_PER_BLOCK];
+	int			page_count;
+	atomic_t		outstanding_pages;
+	atomic_t		refs; /* free mem on transition to zero */
+	struct scrub_ctx	*sctx;
+	struct scrub_parity	*sparity;
+	struct {
+		unsigned int	header_error:1;
+		unsigned int	checksum_error:1;
+		unsigned int	no_io_error_seen:1;
+		unsigned int	generation_error:1; /* also sets header_error */
+
+		/* The following is for the data used to check parity */
+		/* It is for the data with checksum */
+		unsigned int	data_corrected:1;
+	};
+};
+
+/* Used for the chunks with parity stripe such RAID5/6 */
+struct scrub_parity {
+	struct scrub_ctx	*sctx;
+
+	struct btrfs_device	*scrub_dev;
+
+	u64			logic_start;
+
+	u64			logic_end;
+
+	int			nsectors;
+
+	int			stripe_len;
+
+	atomic_t		refs;
+
+	struct list_head	spages;
+
+	/* Work of parity check and repair */
+	struct btrfs_work	work;
+
+	/* Mark the parity blocks which have data */
+	unsigned long		*dbitmap;
+
+	/*
+	 * Mark the parity blocks which have data, but errors happen when
+	 * read data or check data
+	 */
+	unsigned long		*ebitmap;
+
+	unsigned long		bitmap[0];
+};
+
+struct scrub_wr_ctx {
+	struct scrub_bio *wr_curr_bio;
+	struct btrfs_device *tgtdev;
+	int pages_per_wr_bio; /* <= SCRUB_PAGES_PER_WR_BIO */
+	atomic_t flush_all_writes;
+	struct mutex wr_lock;
+};
+
+struct scrub_ctx {
+	struct scrub_bio	*bios[SCRUB_BIOS_PER_SCTX];
+	struct btrfs_root	*dev_root;
+	int			first_free;
+	int			curr;
+	atomic_t		bios_in_flight;
+	atomic_t		workers_pending;
+	spinlock_t		list_lock;
+	wait_queue_head_t	list_wait;
+	u16			csum_size;
+	struct list_head	csum_list;
+	atomic_t		cancel_req;
+	int			readonly;
+	int			pages_per_rd_bio;
+	u32			sectorsize;
+	u32			nodesize;
+
+	int			is_dev_replace;
+	struct scrub_wr_ctx	wr_ctx;
+
+	/*
+	 * statistics
+	 */
+	struct btrfs_scrub_progress stat;
+	spinlock_t		stat_lock;
+
+	/*
+	 * Use a ref counter to avoid use-after-free issues. Scrub workers
+	 * decrement bios_in_flight and workers_pending and then do a wakeup
+	 * on the list_wait wait queue. We must ensure the main scrub task
+	 * doesn't free the scrub context before or while the workers are
+	 * doing the wakeup() call.
+	 */
+	atomic_t                refs;
+};
+
+struct scrub_fixup_nodatasum {
+	struct scrub_ctx	*sctx;
+	struct btrfs_device	*dev;
+	u64			logical;
+	struct btrfs_root	*root;
+	struct btrfs_work	work;
+	int			mirror_num;
+};
+
+struct scrub_nocow_inode {
+	u64			inum;
+	u64			offset;
+	u64			root;
+	struct list_head	list;
+};
+
+struct scrub_copy_nocow_ctx {
+	struct scrub_ctx	*sctx;
+	u64			logical;
+	u64			len;
+	int			mirror_num;
+	u64			physical_for_dev_replace;
+	struct list_head	inodes;
+	struct btrfs_work	work;
+};
+
+struct scrub_warning {
+	struct btrfs_path	*path;
+	u64			extent_item_size;
+	const char		*errstr;
+	sector_t		sector;
+	u64			logical;
+	struct btrfs_device	*dev;
+};
+
+static void scrub_pending_bio_inc(struct scrub_ctx *sctx);
+static void scrub_pending_bio_dec(struct scrub_ctx *sctx);
+static void scrub_pending_trans_workers_inc(struct scrub_ctx *sctx);
+static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx);
+static int scrub_handle_errored_block(struct scrub_block *sblock_to_check);
+static int scrub_setup_recheck_block(struct scrub_block *original_sblock,
+				     struct scrub_block *sblocks_for_recheck);
+static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
+				struct scrub_block *sblock, int is_metadata,
+				int have_csum, u8 *csum, u64 generation,
+				u16 csum_size, int retry_failed_mirror);
+static void scrub_recheck_block_checksum(struct btrfs_fs_info *fs_info,
+					 struct scrub_block *sblock,
+					 int is_metadata, int have_csum,
+					 const u8 *csum, u64 generation,
+					 u16 csum_size);
+static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
+					     struct scrub_block *sblock_good);
+static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
+					    struct scrub_block *sblock_good,
+					    int page_num, int force_write);
+static void scrub_write_block_to_dev_replace(struct scrub_block *sblock);
+static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
+					   int page_num);
+static int scrub_checksum_data(struct scrub_block *sblock);
+static int scrub_checksum_tree_block(struct scrub_block *sblock);
+static int scrub_checksum_super(struct scrub_block *sblock);
+static void scrub_block_get(struct scrub_block *sblock);
+static void scrub_block_put(struct scrub_block *sblock);
+static void scrub_page_get(struct scrub_page *spage);
+static void scrub_page_put(struct scrub_page *spage);
+static void scrub_parity_get(struct scrub_parity *sparity);
+static void scrub_parity_put(struct scrub_parity *sparity);
+static int scrub_add_page_to_rd_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage);
+static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
+		       u64 physical, struct btrfs_device *dev, u64 flags,
+		       u64 gen, int mirror_num, u8 *csum, int force,
+		       u64 physical_for_dev_replace);
+static void scrub_bio_end_io(struct bio *bio, int err);
+static void scrub_bio_end_io_worker(struct btrfs_work *work);
+static void scrub_block_complete(struct scrub_block *sblock);
+static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
+			       u64 extent_logical, u64 extent_len,
+			       u64 *extent_physical,
+			       struct btrfs_device **extent_dev,
+			       int *extent_mirror_num);
+static int scrub_setup_wr_ctx(struct scrub_ctx *sctx,
+			      struct scrub_wr_ctx *wr_ctx,
+			      struct btrfs_fs_info *fs_info,
+			      struct btrfs_device *dev,
+			      int is_dev_replace);
+static void scrub_free_wr_ctx(struct scrub_wr_ctx *wr_ctx);
+static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage);
+static void scrub_wr_submit(struct scrub_ctx *sctx);
+static void scrub_wr_bio_end_io(struct bio *bio, int err);
+static void scrub_wr_bio_end_io_worker(struct btrfs_work *work);
+static int write_page_nocow(struct scrub_ctx *sctx,
+			    u64 physical_for_dev_replace, struct page *page);
+static int copy_nocow_pages_for_inode(u64 inum, u64 offset, u64 root,
+				      struct scrub_copy_nocow_ctx *ctx);
+static int copy_nocow_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
+			    int mirror_num, u64 physical_for_dev_replace);
+static void copy_nocow_pages_worker(struct btrfs_work *work);
+static void __scrub_blocked_if_needed(struct btrfs_fs_info *fs_info);
+static void scrub_blocked_if_needed(struct btrfs_fs_info *fs_info);
+static void scrub_put_ctx(struct scrub_ctx *sctx);
+
+
+static void scrub_pending_bio_inc(struct scrub_ctx *sctx)
+{
+	atomic_inc(&sctx->refs);
+	atomic_inc(&sctx->bios_in_flight);
+}
+
+static void scrub_pending_bio_dec(struct scrub_ctx *sctx)
+{
+	atomic_dec(&sctx->bios_in_flight);
+	wake_up(&sctx->list_wait);
+	scrub_put_ctx(sctx);
+}
+
+static void __scrub_blocked_if_needed(struct btrfs_fs_info *fs_info)
+{
+	while (atomic_read(&fs_info->scrub_pause_req)) {
+		mutex_unlock(&fs_info->scrub_lock);
+		wait_event(fs_info->scrub_pause_wait,
+		   atomic_read(&fs_info->scrub_pause_req) == 0);
+		mutex_lock(&fs_info->scrub_lock);
+	}
+}
+
+static void scrub_blocked_if_needed(struct btrfs_fs_info *fs_info)
+{
+	atomic_inc(&fs_info->scrubs_paused);
+	wake_up(&fs_info->scrub_pause_wait);
+
+	mutex_lock(&fs_info->scrub_lock);
+	__scrub_blocked_if_needed(fs_info);
+	atomic_dec(&fs_info->scrubs_paused);
+	mutex_unlock(&fs_info->scrub_lock);
+
+	wake_up(&fs_info->scrub_pause_wait);
+}
+
+/*
+ * used for workers that require transaction commits (i.e., for the
+ * NOCOW case)
+ */
+static void scrub_pending_trans_workers_inc(struct scrub_ctx *sctx)
+{
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+
+	atomic_inc(&sctx->refs);
+	/*
+	 * increment scrubs_running to prevent cancel requests from
+	 * completing as long as a worker is running. we must also
+	 * increment scrubs_paused to prevent deadlocking on pause
+	 * requests used for transactions commits (as the worker uses a
+	 * transaction context). it is safe to regard the worker
+	 * as paused for all matters practical. effectively, we only
+	 * avoid cancellation requests from completing.
+	 */
+	mutex_lock(&fs_info->scrub_lock);
+	atomic_inc(&fs_info->scrubs_running);
+	atomic_inc(&fs_info->scrubs_paused);
+	mutex_unlock(&fs_info->scrub_lock);
+
+	/*
+	 * check if @scrubs_running=@scrubs_paused condition
+	 * inside wait_event() is not an atomic operation.
+	 * which means we may inc/dec @scrub_running/paused
+	 * at any time. Let's wake up @scrub_pause_wait as
+	 * much as we can to let commit transaction blocked less.
+	 */
+	wake_up(&fs_info->scrub_pause_wait);
+
+	atomic_inc(&sctx->workers_pending);
+}
+
+/* used for workers that require transaction commits */
+static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx)
+{
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+
+	/*
+	 * see scrub_pending_trans_workers_inc() why we're pretending
+	 * to be paused in the scrub counters
+	 */
+	mutex_lock(&fs_info->scrub_lock);
+	atomic_dec(&fs_info->scrubs_running);
+	atomic_dec(&fs_info->scrubs_paused);
+	mutex_unlock(&fs_info->scrub_lock);
+	atomic_dec(&sctx->workers_pending);
+	wake_up(&fs_info->scrub_pause_wait);
+	wake_up(&sctx->list_wait);
+	scrub_put_ctx(sctx);
+}
+
+static void scrub_free_csums(struct scrub_ctx *sctx)
+{
+	while (!list_empty(&sctx->csum_list)) {
+		struct btrfs_ordered_sum *sum;
+		sum = list_first_entry(&sctx->csum_list,
+				       struct btrfs_ordered_sum, list);
+		list_del(&sum->list);
+		kfree(sum);
+	}
+}
+
+static noinline_for_stack void scrub_free_ctx(struct scrub_ctx *sctx)
+{
+	int i;
+
+	if (!sctx)
+		return;
+
+	scrub_free_wr_ctx(&sctx->wr_ctx);
+
+	/* this can happen when scrub is cancelled */
+	if (sctx->curr != -1) {
+		struct scrub_bio *sbio = sctx->bios[sctx->curr];
+
+		for (i = 0; i < sbio->page_count; i++) {
+			WARN_ON(!sbio->pagev[i]->page);
+			scrub_block_put(sbio->pagev[i]->sblock);
+		}
+		bio_put(sbio->bio);
+	}
+
+	for (i = 0; i < SCRUB_BIOS_PER_SCTX; ++i) {
+		struct scrub_bio *sbio = sctx->bios[i];
+
+		if (!sbio)
+			break;
+		kfree(sbio);
+	}
+
+	scrub_free_csums(sctx);
+	kfree(sctx);
+}
+
+static void scrub_put_ctx(struct scrub_ctx *sctx)
+{
+	if (atomic_dec_and_test(&sctx->refs))
+		scrub_free_ctx(sctx);
+}
+
+static noinline_for_stack
+struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int is_dev_replace)
+{
+	struct scrub_ctx *sctx;
+	int		i;
+	struct btrfs_fs_info *fs_info = dev->dev_root->fs_info;
+	int pages_per_rd_bio;
+	int ret;
+
+	/*
+	 * the setting of pages_per_rd_bio is correct for scrub but might
+	 * be wrong for the dev_replace code where we might read from
+	 * different devices in the initial huge bios. However, that
+	 * code is able to correctly handle the case when adding a page
+	 * to a bio fails.
+	 */
+	if (dev->bdev)
+		pages_per_rd_bio = min_t(int, SCRUB_PAGES_PER_RD_BIO,
+					 bio_get_nr_vecs(dev->bdev));
+	else
+		pages_per_rd_bio = SCRUB_PAGES_PER_RD_BIO;
+	sctx = kzalloc(sizeof(*sctx), GFP_NOFS);
+	if (!sctx)
+		goto nomem;
+	atomic_set(&sctx->refs, 1);
+	sctx->is_dev_replace = is_dev_replace;
+	sctx->pages_per_rd_bio = pages_per_rd_bio;
+	sctx->curr = -1;
+	sctx->dev_root = dev->dev_root;
+	for (i = 0; i < SCRUB_BIOS_PER_SCTX; ++i) {
+		struct scrub_bio *sbio;
+
+		sbio = kzalloc(sizeof(*sbio), GFP_NOFS);
+		if (!sbio)
+			goto nomem;
+		sctx->bios[i] = sbio;
+
+		sbio->index = i;
+		sbio->sctx = sctx;
+		sbio->page_count = 0;
+		btrfs_init_work(&sbio->work, btrfs_scrub_helper,
+				scrub_bio_end_io_worker, NULL, NULL);
+
+		if (i != SCRUB_BIOS_PER_SCTX - 1)
+			sctx->bios[i]->next_free = i + 1;
+		else
+			sctx->bios[i]->next_free = -1;
+	}
+	sctx->first_free = 0;
+	sctx->nodesize = dev->dev_root->nodesize;
+	sctx->sectorsize = dev->dev_root->sectorsize;
+	atomic_set(&sctx->bios_in_flight, 0);
+	atomic_set(&sctx->workers_pending, 0);
+	atomic_set(&sctx->cancel_req, 0);
+	sctx->csum_size = btrfs_super_csum_size(fs_info->super_copy);
+	INIT_LIST_HEAD(&sctx->csum_list);
+
+	spin_lock_init(&sctx->list_lock);
+	spin_lock_init(&sctx->stat_lock);
+	init_waitqueue_head(&sctx->list_wait);
+
+	ret = scrub_setup_wr_ctx(sctx, &sctx->wr_ctx, fs_info,
+				 fs_info->dev_replace.tgtdev, is_dev_replace);
+	if (ret) {
+		scrub_free_ctx(sctx);
+		return ERR_PTR(ret);
+	}
+	return sctx;
+
+nomem:
+	scrub_free_ctx(sctx);
+	return ERR_PTR(-ENOMEM);
+}
+
+static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root,
+				     void *warn_ctx)
+{
+	u64 isize;
+	u32 nlink;
+	int ret;
+	int i;
+	struct extent_buffer *eb;
+	struct btrfs_inode_item *inode_item;
+	struct scrub_warning *swarn = warn_ctx;
+	struct btrfs_fs_info *fs_info = swarn->dev->dev_root->fs_info;
+	struct inode_fs_paths *ipath = NULL;
+	struct btrfs_root *local_root;
+	struct btrfs_key root_key;
+	struct btrfs_key key;
+
+	root_key.objectid = root;
+	root_key.type = BTRFS_ROOT_ITEM_KEY;
+	root_key.offset = (u64)-1;
+	local_root = btrfs_read_fs_root_no_name(fs_info, &root_key);
+	if (IS_ERR(local_root)) {
+		ret = PTR_ERR(local_root);
+		goto err;
+	}
+
+	/*
+	 * this makes the path point to (inum INODE_ITEM ioff)
+	 */
+	key.objectid = inum;
+	key.type = BTRFS_INODE_ITEM_KEY;
+	key.offset = 0;
+
+	ret = btrfs_search_slot(NULL, local_root, &key, swarn->path, 0, 0);
+	if (ret) {
+		btrfs_release_path(swarn->path);
+		goto err;
+	}
+
+	eb = swarn->path->nodes[0];
+	inode_item = btrfs_item_ptr(eb, swarn->path->slots[0],
+					struct btrfs_inode_item);
+	isize = btrfs_inode_size(eb, inode_item);
+	nlink = btrfs_inode_nlink(eb, inode_item);
+	btrfs_release_path(swarn->path);
+
+	ipath = init_ipath(4096, local_root, swarn->path);
+	if (IS_ERR(ipath)) {
+		ret = PTR_ERR(ipath);
+		ipath = NULL;
+		goto err;
+	}
+	ret = paths_from_inode(inum, ipath);
+
+	if (ret < 0)
+		goto err;
+
+	/*
+	 * we deliberately ignore the bit ipath might have been too small to
+	 * hold all of the paths here
+	 */
+	for (i = 0; i < ipath->fspath->elem_cnt; ++i)
+		printk_in_rcu(KERN_WARNING "BTRFS: %s at logical %llu on dev "
+			"%s, sector %llu, root %llu, inode %llu, offset %llu, "
+			"length %llu, links %u (path: %s)\n", swarn->errstr,
+			swarn->logical, rcu_str_deref(swarn->dev->name),
+			(unsigned long long)swarn->sector, root, inum, offset,
+			min(isize - offset, (u64)PAGE_SIZE), nlink,
+			(char *)(unsigned long)ipath->fspath->val[i]);
+
+	free_ipath(ipath);
+	return 0;
+
+err:
+	printk_in_rcu(KERN_WARNING "BTRFS: %s at logical %llu on dev "
+		"%s, sector %llu, root %llu, inode %llu, offset %llu: path "
+		"resolving failed with ret=%d\n", swarn->errstr,
+		swarn->logical, rcu_str_deref(swarn->dev->name),
+		(unsigned long long)swarn->sector, root, inum, offset, ret);
+
+	free_ipath(ipath);
+	return 0;
+}
+
+static void scrub_print_warning(const char *errstr, struct scrub_block *sblock)
+{
+	struct btrfs_device *dev;
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_path *path;
+	struct btrfs_key found_key;
+	struct extent_buffer *eb;
+	struct btrfs_extent_item *ei;
+	struct scrub_warning swarn;
+	unsigned long ptr = 0;
+	u64 extent_item_pos;
+	u64 flags = 0;
+	u64 ref_root;
+	u32 item_size;
+	u8 ref_level;
+	int ret;
+
+	WARN_ON(sblock->page_count < 1);
+	dev = sblock->pagev[0]->dev;
+	fs_info = sblock->sctx->dev_root->fs_info;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return;
+
+	swarn.sector = (sblock->pagev[0]->physical) >> 9;
+	swarn.logical = sblock->pagev[0]->logical;
+	swarn.errstr = errstr;
+	swarn.dev = NULL;
+
+	ret = extent_from_logical(fs_info, swarn.logical, path, &found_key,
+				  &flags);
+	if (ret < 0)
+		goto out;
+
+	extent_item_pos = swarn.logical - found_key.objectid;
+	swarn.extent_item_size = found_key.offset;
+
+	eb = path->nodes[0];
+	ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
+	item_size = btrfs_item_size_nr(eb, path->slots[0]);
+
+	if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
+		do {
+			ret = tree_backref_for_extent(&ptr, eb, &found_key, ei,
+						      item_size, &ref_root,
+						      &ref_level);
+			printk_in_rcu(KERN_WARNING
+				"BTRFS: %s at logical %llu on dev %s, "
+				"sector %llu: metadata %s (level %d) in tree "
+				"%llu\n", errstr, swarn.logical,
+				rcu_str_deref(dev->name),
+				(unsigned long long)swarn.sector,
+				ref_level ? "node" : "leaf",
+				ret < 0 ? -1 : ref_level,
+				ret < 0 ? -1 : ref_root);
+		} while (ret != 1);
+		btrfs_release_path(path);
+	} else {
+		btrfs_release_path(path);
+		swarn.path = path;
+		swarn.dev = dev;
+		iterate_extent_inodes(fs_info, found_key.objectid,
+					extent_item_pos, 1,
+					scrub_print_warning_inode, &swarn);
+	}
+
+out:
+	btrfs_free_path(path);
+}
+
+static int scrub_fixup_readpage(u64 inum, u64 offset, u64 root, void *fixup_ctx)
+{
+	struct page *page = NULL;
+	unsigned long index;
+	struct scrub_fixup_nodatasum *fixup = fixup_ctx;
+	int ret;
+	int corrected = 0;
+	struct btrfs_key key;
+	struct inode *inode = NULL;
+	struct btrfs_fs_info *fs_info;
+	u64 end = offset + PAGE_SIZE - 1;
+	struct btrfs_root *local_root;
+	int srcu_index;
+
+	key.objectid = root;
+	key.type = BTRFS_ROOT_ITEM_KEY;
+	key.offset = (u64)-1;
+
+	fs_info = fixup->root->fs_info;
+	srcu_index = srcu_read_lock(&fs_info->subvol_srcu);
+
+	local_root = btrfs_read_fs_root_no_name(fs_info, &key);
+	if (IS_ERR(local_root)) {
+		srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
+		return PTR_ERR(local_root);
+	}
+
+	key.type = BTRFS_INODE_ITEM_KEY;
+	key.objectid = inum;
+	key.offset = 0;
+	inode = btrfs_iget(fs_info->sb, &key, local_root, NULL);
+	srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
+	if (IS_ERR(inode))
+		return PTR_ERR(inode);
+
+	index = offset >> PAGE_CACHE_SHIFT;
+
+	page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
+	if (!page) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	if (PageUptodate(page)) {
+		if (PageDirty(page)) {
+			/*
+			 * we need to write the data to the defect sector. the
+			 * data that was in that sector is not in memory,
+			 * because the page was modified. we must not write the
+			 * modified page to that sector.
+			 *
+			 * TODO: what could be done here: wait for the delalloc
+			 *       runner to write out that page (might involve
+			 *       COW) and see whether the sector is still
+			 *       referenced afterwards.
+			 *
+			 * For the meantime, we'll treat this error
+			 * incorrectable, although there is a chance that a
+			 * later scrub will find the bad sector again and that
+			 * there's no dirty page in memory, then.
+			 */
+			ret = -EIO;
+			goto out;
+		}
+		ret = repair_io_failure(inode, offset, PAGE_SIZE,
+					fixup->logical, page,
+					offset - page_offset(page),
+					fixup->mirror_num);
+		unlock_page(page);
+		corrected = !ret;
+	} else {
+		/*
+		 * we need to get good data first. the general readpage path
+		 * will call repair_io_failure for us, we just have to make
+		 * sure we read the bad mirror.
+		 */
+		ret = set_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
+					EXTENT_DAMAGED, GFP_NOFS);
+		if (ret) {
+			/* set_extent_bits should give proper error */
+			WARN_ON(ret > 0);
+			if (ret > 0)
+				ret = -EFAULT;
+			goto out;
+		}
+
+		ret = extent_read_full_page(&BTRFS_I(inode)->io_tree, page,
+						btrfs_get_extent,
+						fixup->mirror_num);
+		wait_on_page_locked(page);
+
+		corrected = !test_range_bit(&BTRFS_I(inode)->io_tree, offset,
+						end, EXTENT_DAMAGED, 0, NULL);
+		if (!corrected)
+			clear_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
+						EXTENT_DAMAGED, GFP_NOFS);
+	}
+
+out:
+	if (page)
+		put_page(page);
+
+	iput(inode);
+
+	if (ret < 0)
+		return ret;
+
+	if (ret == 0 && corrected) {
+		/*
+		 * we only need to call readpage for one of the inodes belonging
+		 * to this extent. so make iterate_extent_inodes stop
+		 */
+		return 1;
+	}
+
+	return -EIO;
+}
+
+static void scrub_fixup_nodatasum(struct btrfs_work *work)
+{
+	int ret;
+	struct scrub_fixup_nodatasum *fixup;
+	struct scrub_ctx *sctx;
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_path *path;
+	int uncorrectable = 0;
+
+	fixup = container_of(work, struct scrub_fixup_nodatasum, work);
+	sctx = fixup->sctx;
+
+	path = btrfs_alloc_path();
+	if (!path) {
+		spin_lock(&sctx->stat_lock);
+		++sctx->stat.malloc_errors;
+		spin_unlock(&sctx->stat_lock);
+		uncorrectable = 1;
+		goto out;
+	}
+
+	trans = btrfs_join_transaction(fixup->root);
+	if (IS_ERR(trans)) {
+		uncorrectable = 1;
+		goto out;
+	}
+
+	/*
+	 * the idea is to trigger a regular read through the standard path. we
+	 * read a page from the (failed) logical address by specifying the
+	 * corresponding copynum of the failed sector. thus, that readpage is
+	 * expected to fail.
+	 * that is the point where on-the-fly error correction will kick in
+	 * (once it's finished) and rewrite the failed sector if a good copy
+	 * can be found.
+	 */
+	ret = iterate_inodes_from_logical(fixup->logical, fixup->root->fs_info,
+						path, scrub_fixup_readpage,
+						fixup);
+	if (ret < 0) {
+		uncorrectable = 1;
+		goto out;
+	}
+	WARN_ON(ret != 1);
+
+	spin_lock(&sctx->stat_lock);
+	++sctx->stat.corrected_errors;
+	spin_unlock(&sctx->stat_lock);
+
+out:
+	if (trans && !IS_ERR(trans))
+		btrfs_end_transaction(trans, fixup->root);
+	if (uncorrectable) {
+		spin_lock(&sctx->stat_lock);
+		++sctx->stat.uncorrectable_errors;
+		spin_unlock(&sctx->stat_lock);
+		btrfs_dev_replace_stats_inc(
+			&sctx->dev_root->fs_info->dev_replace.
+			num_uncorrectable_read_errors);
+		printk_ratelimited_in_rcu(KERN_ERR "BTRFS: "
+		    "unable to fixup (nodatasum) error at logical %llu on dev %s\n",
+			fixup->logical, rcu_str_deref(fixup->dev->name));
+	}
+
+	btrfs_free_path(path);
+	kfree(fixup);
+
+	scrub_pending_trans_workers_dec(sctx);
+}
+
+static inline void scrub_get_recover(struct scrub_recover *recover)
+{
+	atomic_inc(&recover->refs);
+}
+
+static inline void scrub_put_recover(struct scrub_recover *recover)
+{
+	if (atomic_dec_and_test(&recover->refs)) {
+		btrfs_put_bbio(recover->bbio);
+		kfree(recover);
+	}
+}
+
+/*
+ * scrub_handle_errored_block gets called when either verification of the
+ * pages failed or the bio failed to read, e.g. with EIO. In the latter
+ * case, this function handles all pages in the bio, even though only one
+ * may be bad.
+ * The goal of this function is to repair the errored block by using the
+ * contents of one of the mirrors.
+ */
+static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
+{
+	struct scrub_ctx *sctx = sblock_to_check->sctx;
+	struct btrfs_device *dev;
+	struct btrfs_fs_info *fs_info;
+	u64 length;
+	u64 logical;
+	u64 generation;
+	unsigned int failed_mirror_index;
+	unsigned int is_metadata;
+	unsigned int have_csum;
+	u8 *csum;
+	struct scrub_block *sblocks_for_recheck; /* holds one for each mirror */
+	struct scrub_block *sblock_bad;
+	int ret;
+	int mirror_index;
+	int page_num;
+	int success;
+	static DEFINE_RATELIMIT_STATE(_rs, DEFAULT_RATELIMIT_INTERVAL,
+				      DEFAULT_RATELIMIT_BURST);
+
+	BUG_ON(sblock_to_check->page_count < 1);
+	fs_info = sctx->dev_root->fs_info;
+	if (sblock_to_check->pagev[0]->flags & BTRFS_EXTENT_FLAG_SUPER) {
+		/*
+		 * if we find an error in a super block, we just report it.
+		 * They will get written with the next transaction commit
+		 * anyway
+		 */
+		spin_lock(&sctx->stat_lock);
+		++sctx->stat.super_errors;
+		spin_unlock(&sctx->stat_lock);
+		return 0;
+	}
+	length = sblock_to_check->page_count * PAGE_SIZE;
+	logical = sblock_to_check->pagev[0]->logical;
+	generation = sblock_to_check->pagev[0]->generation;
+	BUG_ON(sblock_to_check->pagev[0]->mirror_num < 1);
+	failed_mirror_index = sblock_to_check->pagev[0]->mirror_num - 1;
+	is_metadata = !(sblock_to_check->pagev[0]->flags &
+			BTRFS_EXTENT_FLAG_DATA);
+	have_csum = sblock_to_check->pagev[0]->have_csum;
+	csum = sblock_to_check->pagev[0]->csum;
+	dev = sblock_to_check->pagev[0]->dev;
+
+	if (sctx->is_dev_replace && !is_metadata && !have_csum) {
+		sblocks_for_recheck = NULL;
+		goto nodatasum_case;
+	}
+
+	/*
+	 * read all mirrors one after the other. This includes to
+	 * re-read the extent or metadata block that failed (that was
+	 * the cause that this fixup code is called) another time,
+	 * page by page this time in order to know which pages
+	 * caused I/O errors and which ones are good (for all mirrors).
+	 * It is the goal to handle the situation when more than one
+	 * mirror contains I/O errors, but the errors do not
+	 * overlap, i.e. the data can be repaired by selecting the
+	 * pages from those mirrors without I/O error on the
+	 * particular pages. One example (with blocks >= 2 * PAGE_SIZE)
+	 * would be that mirror #1 has an I/O error on the first page,
+	 * the second page is good, and mirror #2 has an I/O error on
+	 * the second page, but the first page is good.
+	 * Then the first page of the first mirror can be repaired by
+	 * taking the first page of the second mirror, and the
+	 * second page of the second mirror can be repaired by
+	 * copying the contents of the 2nd page of the 1st mirror.
+	 * One more note: if the pages of one mirror contain I/O
+	 * errors, the checksum cannot be verified. In order to get
+	 * the best data for repairing, the first attempt is to find
+	 * a mirror without I/O errors and with a validated checksum.
+	 * Only if this is not possible, the pages are picked from
+	 * mirrors with I/O errors without considering the checksum.
+	 * If the latter is the case, at the end, the checksum of the
+	 * repaired area is verified in order to correctly maintain
+	 * the statistics.
+	 */
+
+	sblocks_for_recheck = kcalloc(BTRFS_MAX_MIRRORS,
+				      sizeof(*sblocks_for_recheck), GFP_NOFS);
+	if (!sblocks_for_recheck) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		sctx->stat.read_errors++;
+		sctx->stat.uncorrectable_errors++;
+		spin_unlock(&sctx->stat_lock);
+		btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
+		goto out;
+	}
+
+	/* setup the context, map the logical blocks and alloc the pages */
+	ret = scrub_setup_recheck_block(sblock_to_check, sblocks_for_recheck);
+	if (ret) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.read_errors++;
+		sctx->stat.uncorrectable_errors++;
+		spin_unlock(&sctx->stat_lock);
+		btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
+		goto out;
+	}
+	BUG_ON(failed_mirror_index >= BTRFS_MAX_MIRRORS);
+	sblock_bad = sblocks_for_recheck + failed_mirror_index;
+
+	/* build and submit the bios for the failed mirror, check checksums */
+	scrub_recheck_block(fs_info, sblock_bad, is_metadata, have_csum,
+			    csum, generation, sctx->csum_size, 1);
+
+	if (!sblock_bad->header_error && !sblock_bad->checksum_error &&
+	    sblock_bad->no_io_error_seen) {
+		/*
+		 * the error disappeared after reading page by page, or
+		 * the area was part of a huge bio and other parts of the
+		 * bio caused I/O errors, or the block layer merged several
+		 * read requests into one and the error is caused by a
+		 * different bio (usually one of the two latter cases is
+		 * the cause)
+		 */
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.unverified_errors++;
+		sblock_to_check->data_corrected = 1;
+		spin_unlock(&sctx->stat_lock);
+
+		if (sctx->is_dev_replace)
+			scrub_write_block_to_dev_replace(sblock_bad);
+		goto out;
+	}
+
+	if (!sblock_bad->no_io_error_seen) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.read_errors++;
+		spin_unlock(&sctx->stat_lock);
+		if (__ratelimit(&_rs))
+			scrub_print_warning("i/o error", sblock_to_check);
+		btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
+	} else if (sblock_bad->checksum_error) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.csum_errors++;
+		spin_unlock(&sctx->stat_lock);
+		if (__ratelimit(&_rs))
+			scrub_print_warning("checksum error", sblock_to_check);
+		btrfs_dev_stat_inc_and_print(dev,
+					     BTRFS_DEV_STAT_CORRUPTION_ERRS);
+	} else if (sblock_bad->header_error) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.verify_errors++;
+		spin_unlock(&sctx->stat_lock);
+		if (__ratelimit(&_rs))
+			scrub_print_warning("checksum/header error",
+					    sblock_to_check);
+		if (sblock_bad->generation_error)
+			btrfs_dev_stat_inc_and_print(dev,
+				BTRFS_DEV_STAT_GENERATION_ERRS);
+		else
+			btrfs_dev_stat_inc_and_print(dev,
+				BTRFS_DEV_STAT_CORRUPTION_ERRS);
+	}
+
+	if (sctx->readonly) {
+		ASSERT(!sctx->is_dev_replace);
+		goto out;
+	}
+
+	if (!is_metadata && !have_csum) {
+		struct scrub_fixup_nodatasum *fixup_nodatasum;
+
+		WARN_ON(sctx->is_dev_replace);
+
+nodatasum_case:
+
+		/*
+		 * !is_metadata and !have_csum, this means that the data
+		 * might not be COW'ed, that it might be modified
+		 * concurrently. The general strategy to work on the
+		 * commit root does not help in the case when COW is not
+		 * used.
+		 */
+		fixup_nodatasum = kzalloc(sizeof(*fixup_nodatasum), GFP_NOFS);
+		if (!fixup_nodatasum)
+			goto did_not_correct_error;
+		fixup_nodatasum->sctx = sctx;
+		fixup_nodatasum->dev = dev;
+		fixup_nodatasum->logical = logical;
+		fixup_nodatasum->root = fs_info->extent_root;
+		fixup_nodatasum->mirror_num = failed_mirror_index + 1;
+		scrub_pending_trans_workers_inc(sctx);
+		btrfs_init_work(&fixup_nodatasum->work, btrfs_scrub_helper,
+				scrub_fixup_nodatasum, NULL, NULL);
+		btrfs_queue_work(fs_info->scrub_workers,
+				 &fixup_nodatasum->work);
+		goto out;
+	}
+
+	/*
+	 * now build and submit the bios for the other mirrors, check
+	 * checksums.
+	 * First try to pick the mirror which is completely without I/O
+	 * errors and also does not have a checksum error.
+	 * If one is found, and if a checksum is present, the full block
+	 * that is known to contain an error is rewritten. Afterwards
+	 * the block is known to be corrected.
+	 * If a mirror is found which is completely correct, and no
+	 * checksum is present, only those pages are rewritten that had
+	 * an I/O error in the block to be repaired, since it cannot be
+	 * determined, which copy of the other pages is better (and it
+	 * could happen otherwise that a correct page would be
+	 * overwritten by a bad one).
+	 */
+	for (mirror_index = 0;
+	     mirror_index < BTRFS_MAX_MIRRORS &&
+	     sblocks_for_recheck[mirror_index].page_count > 0;
+	     mirror_index++) {
+		struct scrub_block *sblock_other;
+
+		if (mirror_index == failed_mirror_index)
+			continue;
+		sblock_other = sblocks_for_recheck + mirror_index;
+
+		/* build and submit the bios, check checksums */
+		scrub_recheck_block(fs_info, sblock_other, is_metadata,
+				    have_csum, csum, generation,
+				    sctx->csum_size, 0);
+
+		if (!sblock_other->header_error &&
+		    !sblock_other->checksum_error &&
+		    sblock_other->no_io_error_seen) {
+			if (sctx->is_dev_replace) {
+				scrub_write_block_to_dev_replace(sblock_other);
+				goto corrected_error;
+			} else {
+				ret = scrub_repair_block_from_good_copy(
+						sblock_bad, sblock_other);
+				if (!ret)
+					goto corrected_error;
+			}
+		}
+	}
+
+	if (sblock_bad->no_io_error_seen && !sctx->is_dev_replace)
+		goto did_not_correct_error;
+
+	/*
+	 * In case of I/O errors in the area that is supposed to be
+	 * repaired, continue by picking good copies of those pages.
+	 * Select the good pages from mirrors to rewrite bad pages from
+	 * the area to fix. Afterwards verify the checksum of the block
+	 * that is supposed to be repaired. This verification step is
+	 * only done for the purpose of statistic counting and for the
+	 * final scrub report, whether errors remain.
+	 * A perfect algorithm could make use of the checksum and try
+	 * all possible combinations of pages from the different mirrors
+	 * until the checksum verification succeeds. For example, when
+	 * the 2nd page of mirror #1 faces I/O errors, and the 2nd page
+	 * of mirror #2 is readable but the final checksum test fails,
+	 * then the 2nd page of mirror #3 could be tried, whether now
+	 * the final checksum succeedes. But this would be a rare
+	 * exception and is therefore not implemented. At least it is
+	 * avoided that the good copy is overwritten.
+	 * A more useful improvement would be to pick the sectors
+	 * without I/O error based on sector sizes (512 bytes on legacy
+	 * disks) instead of on PAGE_SIZE. Then maybe 512 byte of one
+	 * mirror could be repaired by taking 512 byte of a different
+	 * mirror, even if other 512 byte sectors in the same PAGE_SIZE
+	 * area are unreadable.
+	 */
+	success = 1;
+	for (page_num = 0; page_num < sblock_bad->page_count;
+	     page_num++) {
+		struct scrub_page *page_bad = sblock_bad->pagev[page_num];
+		struct scrub_block *sblock_other = NULL;
+
+		/* skip no-io-error page in scrub */
+		if (!page_bad->io_error && !sctx->is_dev_replace)
+			continue;
+
+		/* try to find no-io-error page in mirrors */
+		if (page_bad->io_error) {
+			for (mirror_index = 0;
+			     mirror_index < BTRFS_MAX_MIRRORS &&
+			     sblocks_for_recheck[mirror_index].page_count > 0;
+			     mirror_index++) {
+				if (!sblocks_for_recheck[mirror_index].
+				    pagev[page_num]->io_error) {
+					sblock_other = sblocks_for_recheck +
+						       mirror_index;
+					break;
+				}
+			}
+			if (!sblock_other)
+				success = 0;
+		}
+
+		if (sctx->is_dev_replace) {
+			/*
+			 * did not find a mirror to fetch the page
+			 * from. scrub_write_page_to_dev_replace()
+			 * handles this case (page->io_error), by
+			 * filling the block with zeros before
+			 * submitting the write request
+			 */
+			if (!sblock_other)
+				sblock_other = sblock_bad;
+
+			if (scrub_write_page_to_dev_replace(sblock_other,
+							    page_num) != 0) {
+				btrfs_dev_replace_stats_inc(
+					&sctx->dev_root->
+					fs_info->dev_replace.
+					num_write_errors);
+				success = 0;
+			}
+		} else if (sblock_other) {
+			ret = scrub_repair_page_from_good_copy(sblock_bad,
+							       sblock_other,
+							       page_num, 0);
+			if (0 == ret)
+				page_bad->io_error = 0;
+			else
+				success = 0;
+		}
+	}
+
+	if (success && !sctx->is_dev_replace) {
+		if (is_metadata || have_csum) {
+			/*
+			 * need to verify the checksum now that all
+			 * sectors on disk are repaired (the write
+			 * request for data to be repaired is on its way).
+			 * Just be lazy and use scrub_recheck_block()
+			 * which re-reads the data before the checksum
+			 * is verified, but most likely the data comes out
+			 * of the page cache.
+			 */
+			scrub_recheck_block(fs_info, sblock_bad,
+					    is_metadata, have_csum, csum,
+					    generation, sctx->csum_size, 1);
+			if (!sblock_bad->header_error &&
+			    !sblock_bad->checksum_error &&
+			    sblock_bad->no_io_error_seen)
+				goto corrected_error;
+			else
+				goto did_not_correct_error;
+		} else {
+corrected_error:
+			spin_lock(&sctx->stat_lock);
+			sctx->stat.corrected_errors++;
+			sblock_to_check->data_corrected = 1;
+			spin_unlock(&sctx->stat_lock);
+			printk_ratelimited_in_rcu(KERN_ERR
+				"BTRFS: fixed up error at logical %llu on dev %s\n",
+				logical, rcu_str_deref(dev->name));
+		}
+	} else {
+did_not_correct_error:
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.uncorrectable_errors++;
+		spin_unlock(&sctx->stat_lock);
+		printk_ratelimited_in_rcu(KERN_ERR
+			"BTRFS: unable to fixup (regular) error at logical %llu on dev %s\n",
+			logical, rcu_str_deref(dev->name));
+	}
+
+out:
+	if (sblocks_for_recheck) {
+		for (mirror_index = 0; mirror_index < BTRFS_MAX_MIRRORS;
+		     mirror_index++) {
+			struct scrub_block *sblock = sblocks_for_recheck +
+						     mirror_index;
+			struct scrub_recover *recover;
+			int page_index;
+
+			for (page_index = 0; page_index < sblock->page_count;
+			     page_index++) {
+				sblock->pagev[page_index]->sblock = NULL;
+				recover = sblock->pagev[page_index]->recover;
+				if (recover) {
+					scrub_put_recover(recover);
+					sblock->pagev[page_index]->recover =
+									NULL;
+				}
+				scrub_page_put(sblock->pagev[page_index]);
+			}
+		}
+		kfree(sblocks_for_recheck);
+	}
+
+	return 0;
+}
+
+static inline int scrub_nr_raid_mirrors(struct btrfs_bio *bbio)
+{
+	if (bbio->map_type & BTRFS_BLOCK_GROUP_RAID5)
+		return 2;
+	else if (bbio->map_type & BTRFS_BLOCK_GROUP_RAID6)
+		return 3;
+	else
+		return (int)bbio->num_stripes;
+}
+
+static inline void scrub_stripe_index_and_offset(u64 logical, u64 map_type,
+						 u64 *raid_map,
+						 u64 mapped_length,
+						 int nstripes, int mirror,
+						 int *stripe_index,
+						 u64 *stripe_offset)
+{
+	int i;
+
+	if (map_type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+		/* RAID5/6 */
+		for (i = 0; i < nstripes; i++) {
+			if (raid_map[i] == RAID6_Q_STRIPE ||
+			    raid_map[i] == RAID5_P_STRIPE)
+				continue;
+
+			if (logical >= raid_map[i] &&
+			    logical < raid_map[i] + mapped_length)
+				break;
+		}
+
+		*stripe_index = i;
+		*stripe_offset = logical - raid_map[i];
+	} else {
+		/* The other RAID type */
+		*stripe_index = mirror;
+		*stripe_offset = 0;
+	}
+}
+
+static int scrub_setup_recheck_block(struct scrub_block *original_sblock,
+				     struct scrub_block *sblocks_for_recheck)
+{
+	struct scrub_ctx *sctx = original_sblock->sctx;
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+	u64 length = original_sblock->page_count * PAGE_SIZE;
+	u64 logical = original_sblock->pagev[0]->logical;
+	struct scrub_recover *recover;
+	struct btrfs_bio *bbio;
+	u64 sublen;
+	u64 mapped_length;
+	u64 stripe_offset;
+	int stripe_index;
+	int page_index = 0;
+	int mirror_index;
+	int nmirrors;
+	int ret;
+
+	/*
+	 * note: the two members refs and outstanding_pages
+	 * are not used (and not set) in the blocks that are used for
+	 * the recheck procedure
+	 */
+
+	while (length > 0) {
+		sublen = min_t(u64, length, PAGE_SIZE);
+		mapped_length = sublen;
+		bbio = NULL;
+
+		/*
+		 * with a length of PAGE_SIZE, each returned stripe
+		 * represents one mirror
+		 */
+		ret = btrfs_map_sblock(fs_info, REQ_GET_READ_MIRRORS, logical,
+				       &mapped_length, &bbio, 0, 1);
+		if (ret || !bbio || mapped_length < sublen) {
+			btrfs_put_bbio(bbio);
+			return -EIO;
+		}
+
+		recover = kzalloc(sizeof(struct scrub_recover), GFP_NOFS);
+		if (!recover) {
+			btrfs_put_bbio(bbio);
+			return -ENOMEM;
+		}
+
+		atomic_set(&recover->refs, 1);
+		recover->bbio = bbio;
+		recover->map_length = mapped_length;
+
+		BUG_ON(page_index >= SCRUB_PAGES_PER_RD_BIO);
+
+		nmirrors = min(scrub_nr_raid_mirrors(bbio), BTRFS_MAX_MIRRORS);
+
+		for (mirror_index = 0; mirror_index < nmirrors;
+		     mirror_index++) {
+			struct scrub_block *sblock;
+			struct scrub_page *page;
+
+			sblock = sblocks_for_recheck + mirror_index;
+			sblock->sctx = sctx;
+			page = kzalloc(sizeof(*page), GFP_NOFS);
+			if (!page) {
+leave_nomem:
+				spin_lock(&sctx->stat_lock);
+				sctx->stat.malloc_errors++;
+				spin_unlock(&sctx->stat_lock);
+				scrub_put_recover(recover);
+				return -ENOMEM;
+			}
+			scrub_page_get(page);
+			sblock->pagev[page_index] = page;
+			page->logical = logical;
+
+			scrub_stripe_index_and_offset(logical,
+						      bbio->map_type,
+						      bbio->raid_map,
+						      mapped_length,
+						      bbio->num_stripes -
+						      bbio->num_tgtdevs,
+						      mirror_index,
+						      &stripe_index,
+						      &stripe_offset);
+			page->physical = bbio->stripes[stripe_index].physical +
+					 stripe_offset;
+			page->dev = bbio->stripes[stripe_index].dev;
+
+			BUG_ON(page_index >= original_sblock->page_count);
+			page->physical_for_dev_replace =
+				original_sblock->pagev[page_index]->
+				physical_for_dev_replace;
+			/* for missing devices, dev->bdev is NULL */
+			page->mirror_num = mirror_index + 1;
+			sblock->page_count++;
+			page->page = alloc_page(GFP_NOFS);
+			if (!page->page)
+				goto leave_nomem;
+
+			scrub_get_recover(recover);
+			page->recover = recover;
+		}
+		scrub_put_recover(recover);
+		length -= sublen;
+		logical += sublen;
+		page_index++;
+	}
+
+	return 0;
+}
+
+struct scrub_bio_ret {
+	struct completion event;
+	int error;
+};
+
+static void scrub_bio_wait_endio(struct bio *bio, int error)
+{
+	struct scrub_bio_ret *ret = bio->bi_private;
+
+	ret->error = error;
+	complete(&ret->event);
+}
+
+static inline int scrub_is_page_on_raid56(struct scrub_page *page)
+{
+	return page->recover &&
+	       (page->recover->bbio->map_type & BTRFS_BLOCK_GROUP_RAID56_MASK);
+}
+
+static int scrub_submit_raid56_bio_wait(struct btrfs_fs_info *fs_info,
+					struct bio *bio,
+					struct scrub_page *page)
+{
+	struct scrub_bio_ret done;
+	int ret;
+
+	init_completion(&done.event);
+	done.error = 0;
+	bio->bi_iter.bi_sector = page->logical >> 9;
+	bio->bi_private = &done;
+	bio->bi_end_io = scrub_bio_wait_endio;
+
+	ret = raid56_parity_recover(fs_info->fs_root, bio, page->recover->bbio,
+				    page->recover->map_length,
+				    page->mirror_num, 0);
+	if (ret)
+		return ret;
+
+	wait_for_completion(&done.event);
+	if (done.error)
+		return -EIO;
+
+	return 0;
+}
+
+/*
+ * this function will check the on disk data for checksum errors, header
+ * errors and read I/O errors. If any I/O errors happen, the exact pages
+ * which are errored are marked as being bad. The goal is to enable scrub
+ * to take those pages that are not errored from all the mirrors so that
+ * the pages that are errored in the just handled mirror can be repaired.
+ */
+static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
+				struct scrub_block *sblock, int is_metadata,
+				int have_csum, u8 *csum, u64 generation,
+				u16 csum_size, int retry_failed_mirror)
+{
+	int page_num;
+
+	sblock->no_io_error_seen = 1;
+	sblock->header_error = 0;
+	sblock->checksum_error = 0;
+
+	for (page_num = 0; page_num < sblock->page_count; page_num++) {
+		struct bio *bio;
+		struct scrub_page *page = sblock->pagev[page_num];
+
+		if (page->dev->bdev == NULL) {
+			page->io_error = 1;
+			sblock->no_io_error_seen = 0;
+			continue;
+		}
+
+		WARN_ON(!page->page);
+		bio = btrfs_io_bio_alloc(GFP_NOFS, 1);
+		if (!bio) {
+			page->io_error = 1;
+			sblock->no_io_error_seen = 0;
+			continue;
+		}
+		bio->bi_bdev = page->dev->bdev;
+
+		bio_add_page(bio, page->page, PAGE_SIZE, 0);
+		if (!retry_failed_mirror && scrub_is_page_on_raid56(page)) {
+			if (scrub_submit_raid56_bio_wait(fs_info, bio, page))
+				sblock->no_io_error_seen = 0;
+		} else {
+			bio->bi_iter.bi_sector = page->physical >> 9;
+
+			if (btrfsic_submit_bio_wait(READ, bio))
+				sblock->no_io_error_seen = 0;
+		}
+
+		bio_put(bio);
+	}
+
+	if (sblock->no_io_error_seen)
+		scrub_recheck_block_checksum(fs_info, sblock, is_metadata,
+					     have_csum, csum, generation,
+					     csum_size);
+
+	return;
+}
+
+static inline int scrub_check_fsid(u8 fsid[],
+				   struct scrub_page *spage)
+{
+	struct btrfs_fs_devices *fs_devices = spage->dev->fs_devices;
+	int ret;
+
+	ret = memcmp(fsid, fs_devices->fsid, BTRFS_UUID_SIZE);
+	return !ret;
+}
+
+static void scrub_recheck_block_checksum(struct btrfs_fs_info *fs_info,
+					 struct scrub_block *sblock,
+					 int is_metadata, int have_csum,
+					 const u8 *csum, u64 generation,
+					 u16 csum_size)
+{
+	int page_num;
+	u8 calculated_csum[BTRFS_CSUM_SIZE];
+	u32 crc = ~(u32)0;
+	void *mapped_buffer;
+
+	WARN_ON(!sblock->pagev[0]->page);
+	if (is_metadata) {
+		struct btrfs_header *h;
+
+		mapped_buffer = kmap_atomic(sblock->pagev[0]->page);
+		h = (struct btrfs_header *)mapped_buffer;
+
+		if (sblock->pagev[0]->logical != btrfs_stack_header_bytenr(h) ||
+		    !scrub_check_fsid(h->fsid, sblock->pagev[0]) ||
+		    memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
+			   BTRFS_UUID_SIZE)) {
+			sblock->header_error = 1;
+		} else if (generation != btrfs_stack_header_generation(h)) {
+			sblock->header_error = 1;
+			sblock->generation_error = 1;
+		}
+		csum = h->csum;
+	} else {
+		if (!have_csum)
+			return;
+
+		mapped_buffer = kmap_atomic(sblock->pagev[0]->page);
+	}
+
+	for (page_num = 0;;) {
+		if (page_num == 0 && is_metadata)
+			crc = btrfs_csum_data(
+				((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE,
+				crc, PAGE_SIZE - BTRFS_CSUM_SIZE);
+		else
+			crc = btrfs_csum_data(mapped_buffer, crc, PAGE_SIZE);
+
+		kunmap_atomic(mapped_buffer);
+		page_num++;
+		if (page_num >= sblock->page_count)
+			break;
+		WARN_ON(!sblock->pagev[page_num]->page);
+
+		mapped_buffer = kmap_atomic(sblock->pagev[page_num]->page);
+	}
+
+	btrfs_csum_final(crc, calculated_csum);
+	if (memcmp(calculated_csum, csum, csum_size))
+		sblock->checksum_error = 1;
+}
+
+static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
+					     struct scrub_block *sblock_good)
+{
+	int page_num;
+	int ret = 0;
+
+	for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
+		int ret_sub;
+
+		ret_sub = scrub_repair_page_from_good_copy(sblock_bad,
+							   sblock_good,
+							   page_num, 1);
+		if (ret_sub)
+			ret = ret_sub;
+	}
+
+	return ret;
+}
+
+static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
+					    struct scrub_block *sblock_good,
+					    int page_num, int force_write)
+{
+	struct scrub_page *page_bad = sblock_bad->pagev[page_num];
+	struct scrub_page *page_good = sblock_good->pagev[page_num];
+
+	BUG_ON(page_bad->page == NULL);
+	BUG_ON(page_good->page == NULL);
+	if (force_write || sblock_bad->header_error ||
+	    sblock_bad->checksum_error || page_bad->io_error) {
+		struct bio *bio;
+		int ret;
+
+		if (!page_bad->dev->bdev) {
+			printk_ratelimited(KERN_WARNING "BTRFS: "
+				"scrub_repair_page_from_good_copy(bdev == NULL) "
+				"is unexpected!\n");
+			return -EIO;
+		}
+
+		bio = btrfs_io_bio_alloc(GFP_NOFS, 1);
+		if (!bio)
+			return -EIO;
+		bio->bi_bdev = page_bad->dev->bdev;
+		bio->bi_iter.bi_sector = page_bad->physical >> 9;
+
+		ret = bio_add_page(bio, page_good->page, PAGE_SIZE, 0);
+		if (PAGE_SIZE != ret) {
+			bio_put(bio);
+			return -EIO;
+		}
+
+		if (btrfsic_submit_bio_wait(WRITE, bio)) {
+			btrfs_dev_stat_inc_and_print(page_bad->dev,
+				BTRFS_DEV_STAT_WRITE_ERRS);
+			btrfs_dev_replace_stats_inc(
+				&sblock_bad->sctx->dev_root->fs_info->
+				dev_replace.num_write_errors);
+			bio_put(bio);
+			return -EIO;
+		}
+		bio_put(bio);
+	}
+
+	return 0;
+}
+
+static void scrub_write_block_to_dev_replace(struct scrub_block *sblock)
+{
+	int page_num;
+
+	/*
+	 * This block is used for the check of the parity on the source device,
+	 * so the data needn't be written into the destination device.
+	 */
+	if (sblock->sparity)
+		return;
+
+	for (page_num = 0; page_num < sblock->page_count; page_num++) {
+		int ret;
+
+		ret = scrub_write_page_to_dev_replace(sblock, page_num);
+		if (ret)
+			btrfs_dev_replace_stats_inc(
+				&sblock->sctx->dev_root->fs_info->dev_replace.
+				num_write_errors);
+	}
+}
+
+static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
+					   int page_num)
+{
+	struct scrub_page *spage = sblock->pagev[page_num];
+
+	BUG_ON(spage->page == NULL);
+	if (spage->io_error) {
+		void *mapped_buffer = kmap_atomic(spage->page);
+
+		memset(mapped_buffer, 0, PAGE_CACHE_SIZE);
+		flush_dcache_page(spage->page);
+		kunmap_atomic(mapped_buffer);
+	}
+	return scrub_add_page_to_wr_bio(sblock->sctx, spage);
+}
+
+static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage)
+{
+	struct scrub_wr_ctx *wr_ctx = &sctx->wr_ctx;
+	struct scrub_bio *sbio;
+	int ret;
+
+	mutex_lock(&wr_ctx->wr_lock);
+again:
+	if (!wr_ctx->wr_curr_bio) {
+		wr_ctx->wr_curr_bio = kzalloc(sizeof(*wr_ctx->wr_curr_bio),
+					      GFP_NOFS);
+		if (!wr_ctx->wr_curr_bio) {
+			mutex_unlock(&wr_ctx->wr_lock);
+			return -ENOMEM;
+		}
+		wr_ctx->wr_curr_bio->sctx = sctx;
+		wr_ctx->wr_curr_bio->page_count = 0;
+	}
+	sbio = wr_ctx->wr_curr_bio;
+	if (sbio->page_count == 0) {
+		struct bio *bio;
+
+		sbio->physical = spage->physical_for_dev_replace;
+		sbio->logical = spage->logical;
+		sbio->dev = wr_ctx->tgtdev;
+		bio = sbio->bio;
+		if (!bio) {
+			bio = btrfs_io_bio_alloc(GFP_NOFS, wr_ctx->pages_per_wr_bio);
+			if (!bio) {
+				mutex_unlock(&wr_ctx->wr_lock);
+				return -ENOMEM;
+			}
+			sbio->bio = bio;
+		}
+
+		bio->bi_private = sbio;
+		bio->bi_end_io = scrub_wr_bio_end_io;
+		bio->bi_bdev = sbio->dev->bdev;
+		bio->bi_iter.bi_sector = sbio->physical >> 9;
+		sbio->err = 0;
+	} else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
+		   spage->physical_for_dev_replace ||
+		   sbio->logical + sbio->page_count * PAGE_SIZE !=
+		   spage->logical) {
+		scrub_wr_submit(sctx);
+		goto again;
+	}
+
+	ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
+	if (ret != PAGE_SIZE) {
+		if (sbio->page_count < 1) {
+			bio_put(sbio->bio);
+			sbio->bio = NULL;
+			mutex_unlock(&wr_ctx->wr_lock);
+			return -EIO;
+		}
+		scrub_wr_submit(sctx);
+		goto again;
+	}
+
+	sbio->pagev[sbio->page_count] = spage;
+	scrub_page_get(spage);
+	sbio->page_count++;
+	if (sbio->page_count == wr_ctx->pages_per_wr_bio)
+		scrub_wr_submit(sctx);
+	mutex_unlock(&wr_ctx->wr_lock);
+
+	return 0;
+}
+
+static void scrub_wr_submit(struct scrub_ctx *sctx)
+{
+	struct scrub_wr_ctx *wr_ctx = &sctx->wr_ctx;
+	struct scrub_bio *sbio;
+
+	if (!wr_ctx->wr_curr_bio)
+		return;
+
+	sbio = wr_ctx->wr_curr_bio;
+	wr_ctx->wr_curr_bio = NULL;
+	WARN_ON(!sbio->bio->bi_bdev);
+	scrub_pending_bio_inc(sctx);
+	/* process all writes in a single worker thread. Then the block layer
+	 * orders the requests before sending them to the driver which
+	 * doubled the write performance on spinning disks when measured
+	 * with Linux 3.5 */
+	btrfsic_submit_bio(WRITE, sbio->bio);
+}
+
+static void scrub_wr_bio_end_io(struct bio *bio, int err)
+{
+	struct scrub_bio *sbio = bio->bi_private;
+	struct btrfs_fs_info *fs_info = sbio->dev->dev_root->fs_info;
+
+	sbio->err = err;
+	sbio->bio = bio;
+
+	btrfs_init_work(&sbio->work, btrfs_scrubwrc_helper,
+			 scrub_wr_bio_end_io_worker, NULL, NULL);
+	btrfs_queue_work(fs_info->scrub_wr_completion_workers, &sbio->work);
+}
+
+static void scrub_wr_bio_end_io_worker(struct btrfs_work *work)
+{
+	struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
+	struct scrub_ctx *sctx = sbio->sctx;
+	int i;
+
+	WARN_ON(sbio->page_count > SCRUB_PAGES_PER_WR_BIO);
+	if (sbio->err) {
+		struct btrfs_dev_replace *dev_replace =
+			&sbio->sctx->dev_root->fs_info->dev_replace;
+
+		for (i = 0; i < sbio->page_count; i++) {
+			struct scrub_page *spage = sbio->pagev[i];
+
+			spage->io_error = 1;
+			btrfs_dev_replace_stats_inc(&dev_replace->
+						    num_write_errors);
+		}
+	}
+
+	for (i = 0; i < sbio->page_count; i++)
+		scrub_page_put(sbio->pagev[i]);
+
+	bio_put(sbio->bio);
+	kfree(sbio);
+	scrub_pending_bio_dec(sctx);
+}
+
+static int scrub_checksum(struct scrub_block *sblock)
+{
+	u64 flags;
+	int ret;
+
+	WARN_ON(sblock->page_count < 1);
+	flags = sblock->pagev[0]->flags;
+	ret = 0;
+	if (flags & BTRFS_EXTENT_FLAG_DATA)
+		ret = scrub_checksum_data(sblock);
+	else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
+		ret = scrub_checksum_tree_block(sblock);
+	else if (flags & BTRFS_EXTENT_FLAG_SUPER)
+		(void)scrub_checksum_super(sblock);
+	else
+		WARN_ON(1);
+	if (ret)
+		scrub_handle_errored_block(sblock);
+
+	return ret;
+}
+
+static int scrub_checksum_data(struct scrub_block *sblock)
+{
+	struct scrub_ctx *sctx = sblock->sctx;
+	u8 csum[BTRFS_CSUM_SIZE];
+	u8 *on_disk_csum;
+	struct page *page;
+	void *buffer;
+	u32 crc = ~(u32)0;
+	int fail = 0;
+	u64 len;
+	int index;
+
+	BUG_ON(sblock->page_count < 1);
+	if (!sblock->pagev[0]->have_csum)
+		return 0;
+
+	on_disk_csum = sblock->pagev[0]->csum;
+	page = sblock->pagev[0]->page;
+	buffer = kmap_atomic(page);
+
+	len = sctx->sectorsize;
+	index = 0;
+	for (;;) {
+		u64 l = min_t(u64, len, PAGE_SIZE);
+
+		crc = btrfs_csum_data(buffer, crc, l);
+		kunmap_atomic(buffer);
+		len -= l;
+		if (len == 0)
+			break;
+		index++;
+		BUG_ON(index >= sblock->page_count);
+		BUG_ON(!sblock->pagev[index]->page);
+		page = sblock->pagev[index]->page;
+		buffer = kmap_atomic(page);
+	}
+
+	btrfs_csum_final(crc, csum);
+	if (memcmp(csum, on_disk_csum, sctx->csum_size))
+		fail = 1;
+
+	return fail;
+}
+
+static int scrub_checksum_tree_block(struct scrub_block *sblock)
+{
+	struct scrub_ctx *sctx = sblock->sctx;
+	struct btrfs_header *h;
+	struct btrfs_root *root = sctx->dev_root;
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	u8 calculated_csum[BTRFS_CSUM_SIZE];
+	u8 on_disk_csum[BTRFS_CSUM_SIZE];
+	struct page *page;
+	void *mapped_buffer;
+	u64 mapped_size;
+	void *p;
+	u32 crc = ~(u32)0;
+	int fail = 0;
+	int crc_fail = 0;
+	u64 len;
+	int index;
+
+	BUG_ON(sblock->page_count < 1);
+	page = sblock->pagev[0]->page;
+	mapped_buffer = kmap_atomic(page);
+	h = (struct btrfs_header *)mapped_buffer;
+	memcpy(on_disk_csum, h->csum, sctx->csum_size);
+
+	/*
+	 * we don't use the getter functions here, as we
+	 * a) don't have an extent buffer and
+	 * b) the page is already kmapped
+	 */
+
+	if (sblock->pagev[0]->logical != btrfs_stack_header_bytenr(h))
+		++fail;
+
+	if (sblock->pagev[0]->generation != btrfs_stack_header_generation(h))
+		++fail;
+
+	if (!scrub_check_fsid(h->fsid, sblock->pagev[0]))
+		++fail;
+
+	if (memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
+		   BTRFS_UUID_SIZE))
+		++fail;
+
+	len = sctx->nodesize - BTRFS_CSUM_SIZE;
+	mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
+	p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
+	index = 0;
+	for (;;) {
+		u64 l = min_t(u64, len, mapped_size);
+
+		crc = btrfs_csum_data(p, crc, l);
+		kunmap_atomic(mapped_buffer);
+		len -= l;
+		if (len == 0)
+			break;
+		index++;
+		BUG_ON(index >= sblock->page_count);
+		BUG_ON(!sblock->pagev[index]->page);
+		page = sblock->pagev[index]->page;
+		mapped_buffer = kmap_atomic(page);
+		mapped_size = PAGE_SIZE;
+		p = mapped_buffer;
+	}
+
+	btrfs_csum_final(crc, calculated_csum);
+	if (memcmp(calculated_csum, on_disk_csum, sctx->csum_size))
+		++crc_fail;
+
+	return fail || crc_fail;
+}
+
+static int scrub_checksum_super(struct scrub_block *sblock)
+{
+	struct btrfs_super_block *s;
+	struct scrub_ctx *sctx = sblock->sctx;
+	u8 calculated_csum[BTRFS_CSUM_SIZE];
+	u8 on_disk_csum[BTRFS_CSUM_SIZE];
+	struct page *page;
+	void *mapped_buffer;
+	u64 mapped_size;
+	void *p;
+	u32 crc = ~(u32)0;
+	int fail_gen = 0;
+	int fail_cor = 0;
+	u64 len;
+	int index;
+
+	BUG_ON(sblock->page_count < 1);
+	page = sblock->pagev[0]->page;
+	mapped_buffer = kmap_atomic(page);
+	s = (struct btrfs_super_block *)mapped_buffer;
+	memcpy(on_disk_csum, s->csum, sctx->csum_size);
+
+	if (sblock->pagev[0]->logical != btrfs_super_bytenr(s))
+		++fail_cor;
+
+	if (sblock->pagev[0]->generation != btrfs_super_generation(s))
+		++fail_gen;
+
+	if (!scrub_check_fsid(s->fsid, sblock->pagev[0]))
+		++fail_cor;
+
+	len = BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE;
+	mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
+	p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
+	index = 0;
+	for (;;) {
+		u64 l = min_t(u64, len, mapped_size);
+
+		crc = btrfs_csum_data(p, crc, l);
+		kunmap_atomic(mapped_buffer);
+		len -= l;
+		if (len == 0)
+			break;
+		index++;
+		BUG_ON(index >= sblock->page_count);
+		BUG_ON(!sblock->pagev[index]->page);
+		page = sblock->pagev[index]->page;
+		mapped_buffer = kmap_atomic(page);
+		mapped_size = PAGE_SIZE;
+		p = mapped_buffer;
+	}
+
+	btrfs_csum_final(crc, calculated_csum);
+	if (memcmp(calculated_csum, on_disk_csum, sctx->csum_size))
+		++fail_cor;
+
+	if (fail_cor + fail_gen) {
+		/*
+		 * if we find an error in a super block, we just report it.
+		 * They will get written with the next transaction commit
+		 * anyway
+		 */
+		spin_lock(&sctx->stat_lock);
+		++sctx->stat.super_errors;
+		spin_unlock(&sctx->stat_lock);
+		if (fail_cor)
+			btrfs_dev_stat_inc_and_print(sblock->pagev[0]->dev,
+				BTRFS_DEV_STAT_CORRUPTION_ERRS);
+		else
+			btrfs_dev_stat_inc_and_print(sblock->pagev[0]->dev,
+				BTRFS_DEV_STAT_GENERATION_ERRS);
+	}
+
+	return fail_cor + fail_gen;
+}
+
+static void scrub_block_get(struct scrub_block *sblock)
+{
+	atomic_inc(&sblock->refs);
+}
+
+static void scrub_block_put(struct scrub_block *sblock)
+{
+	if (atomic_dec_and_test(&sblock->refs)) {
+		int i;
+
+		if (sblock->sparity)
+			scrub_parity_put(sblock->sparity);
+
+		for (i = 0; i < sblock->page_count; i++)
+			scrub_page_put(sblock->pagev[i]);
+		kfree(sblock);
+	}
+}
+
+static void scrub_page_get(struct scrub_page *spage)
+{
+	atomic_inc(&spage->refs);
+}
+
+static void scrub_page_put(struct scrub_page *spage)
+{
+	if (atomic_dec_and_test(&spage->refs)) {
+		if (spage->page)
+			__free_page(spage->page);
+		kfree(spage);
+	}
+}
+
+static void scrub_submit(struct scrub_ctx *sctx)
+{
+	struct scrub_bio *sbio;
+
+	if (sctx->curr == -1)
+		return;
+
+	sbio = sctx->bios[sctx->curr];
+	sctx->curr = -1;
+	scrub_pending_bio_inc(sctx);
+
+	if (!sbio->bio->bi_bdev) {
+		/*
+		 * this case should not happen. If btrfs_map_block() is
+		 * wrong, it could happen for dev-replace operations on
+		 * missing devices when no mirrors are available, but in
+		 * this case it should already fail the mount.
+		 * This case is handled correctly (but _very_ slowly).
+		 */
+		printk_ratelimited(KERN_WARNING
+			"BTRFS: scrub_submit(bio bdev == NULL) is unexpected!\n");
+		bio_endio(sbio->bio, -EIO);
+	} else {
+		btrfsic_submit_bio(READ, sbio->bio);
+	}
+}
+
+static int scrub_add_page_to_rd_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage)
+{
+	struct scrub_block *sblock = spage->sblock;
+	struct scrub_bio *sbio;
+	int ret;
+
+again:
+	/*
+	 * grab a fresh bio or wait for one to become available
+	 */
+	while (sctx->curr == -1) {
+		spin_lock(&sctx->list_lock);
+		sctx->curr = sctx->first_free;
+		if (sctx->curr != -1) {
+			sctx->first_free = sctx->bios[sctx->curr]->next_free;
+			sctx->bios[sctx->curr]->next_free = -1;
+			sctx->bios[sctx->curr]->page_count = 0;
+			spin_unlock(&sctx->list_lock);
+		} else {
+			spin_unlock(&sctx->list_lock);
+			wait_event(sctx->list_wait, sctx->first_free != -1);
+		}
+	}
+	sbio = sctx->bios[sctx->curr];
+	if (sbio->page_count == 0) {
+		struct bio *bio;
+
+		sbio->physical = spage->physical;
+		sbio->logical = spage->logical;
+		sbio->dev = spage->dev;
+		bio = sbio->bio;
+		if (!bio) {
+			bio = btrfs_io_bio_alloc(GFP_NOFS, sctx->pages_per_rd_bio);
+			if (!bio)
+				return -ENOMEM;
+			sbio->bio = bio;
+		}
+
+		bio->bi_private = sbio;
+		bio->bi_end_io = scrub_bio_end_io;
+		bio->bi_bdev = sbio->dev->bdev;
+		bio->bi_iter.bi_sector = sbio->physical >> 9;
+		sbio->err = 0;
+	} else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
+		   spage->physical ||
+		   sbio->logical + sbio->page_count * PAGE_SIZE !=
+		   spage->logical ||
+		   sbio->dev != spage->dev) {
+		scrub_submit(sctx);
+		goto again;
+	}
+
+	sbio->pagev[sbio->page_count] = spage;
+	ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
+	if (ret != PAGE_SIZE) {
+		if (sbio->page_count < 1) {
+			bio_put(sbio->bio);
+			sbio->bio = NULL;
+			return -EIO;
+		}
+		scrub_submit(sctx);
+		goto again;
+	}
+
+	scrub_block_get(sblock); /* one for the page added to the bio */
+	atomic_inc(&sblock->outstanding_pages);
+	sbio->page_count++;
+	if (sbio->page_count == sctx->pages_per_rd_bio)
+		scrub_submit(sctx);
+
+	return 0;
+}
+
+static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
+		       u64 physical, struct btrfs_device *dev, u64 flags,
+		       u64 gen, int mirror_num, u8 *csum, int force,
+		       u64 physical_for_dev_replace)
+{
+	struct scrub_block *sblock;
+	int index;
+
+	sblock = kzalloc(sizeof(*sblock), GFP_NOFS);
+	if (!sblock) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+
+	/* one ref inside this function, plus one for each page added to
+	 * a bio later on */
+	atomic_set(&sblock->refs, 1);
+	sblock->sctx = sctx;
+	sblock->no_io_error_seen = 1;
+
+	for (index = 0; len > 0; index++) {
+		struct scrub_page *spage;
+		u64 l = min_t(u64, len, PAGE_SIZE);
+
+		spage = kzalloc(sizeof(*spage), GFP_NOFS);
+		if (!spage) {
+leave_nomem:
+			spin_lock(&sctx->stat_lock);
+			sctx->stat.malloc_errors++;
+			spin_unlock(&sctx->stat_lock);
+			scrub_block_put(sblock);
+			return -ENOMEM;
+		}
+		BUG_ON(index >= SCRUB_MAX_PAGES_PER_BLOCK);
+		scrub_page_get(spage);
+		sblock->pagev[index] = spage;
+		spage->sblock = sblock;
+		spage->dev = dev;
+		spage->flags = flags;
+		spage->generation = gen;
+		spage->logical = logical;
+		spage->physical = physical;
+		spage->physical_for_dev_replace = physical_for_dev_replace;
+		spage->mirror_num = mirror_num;
+		if (csum) {
+			spage->have_csum = 1;
+			memcpy(spage->csum, csum, sctx->csum_size);
+		} else {
+			spage->have_csum = 0;
+		}
+		sblock->page_count++;
+		spage->page = alloc_page(GFP_NOFS);
+		if (!spage->page)
+			goto leave_nomem;
+		len -= l;
+		logical += l;
+		physical += l;
+		physical_for_dev_replace += l;
+	}
+
+	WARN_ON(sblock->page_count == 0);
+	for (index = 0; index < sblock->page_count; index++) {
+		struct scrub_page *spage = sblock->pagev[index];
+		int ret;
+
+		ret = scrub_add_page_to_rd_bio(sctx, spage);
+		if (ret) {
+			scrub_block_put(sblock);
+			return ret;
+		}
+	}
+
+	if (force)
+		scrub_submit(sctx);
+
+	/* last one frees, either here or in bio completion for last page */
+	scrub_block_put(sblock);
+	return 0;
+}
+
+static void scrub_bio_end_io(struct bio *bio, int err)
+{
+	struct scrub_bio *sbio = bio->bi_private;
+	struct btrfs_fs_info *fs_info = sbio->dev->dev_root->fs_info;
+
+	sbio->err = err;
+	sbio->bio = bio;
+
+	btrfs_queue_work(fs_info->scrub_workers, &sbio->work);
+}
+
+static void scrub_bio_end_io_worker(struct btrfs_work *work)
+{
+	struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
+	struct scrub_ctx *sctx = sbio->sctx;
+	int i;
+
+	BUG_ON(sbio->page_count > SCRUB_PAGES_PER_RD_BIO);
+	if (sbio->err) {
+		for (i = 0; i < sbio->page_count; i++) {
+			struct scrub_page *spage = sbio->pagev[i];
+
+			spage->io_error = 1;
+			spage->sblock->no_io_error_seen = 0;
+		}
+	}
+
+	/* now complete the scrub_block items that have all pages completed */
+	for (i = 0; i < sbio->page_count; i++) {
+		struct scrub_page *spage = sbio->pagev[i];
+		struct scrub_block *sblock = spage->sblock;
+
+		if (atomic_dec_and_test(&sblock->outstanding_pages))
+			scrub_block_complete(sblock);
+		scrub_block_put(sblock);
+	}
+
+	bio_put(sbio->bio);
+	sbio->bio = NULL;
+	spin_lock(&sctx->list_lock);
+	sbio->next_free = sctx->first_free;
+	sctx->first_free = sbio->index;
+	spin_unlock(&sctx->list_lock);
+
+	if (sctx->is_dev_replace &&
+	    atomic_read(&sctx->wr_ctx.flush_all_writes)) {
+		mutex_lock(&sctx->wr_ctx.wr_lock);
+		scrub_wr_submit(sctx);
+		mutex_unlock(&sctx->wr_ctx.wr_lock);
+	}
+
+	scrub_pending_bio_dec(sctx);
+}
+
+static inline void __scrub_mark_bitmap(struct scrub_parity *sparity,
+				       unsigned long *bitmap,
+				       u64 start, u64 len)
+{
+	u32 offset;
+	int nsectors;
+	int sectorsize = sparity->sctx->dev_root->sectorsize;
+
+	if (len >= sparity->stripe_len) {
+		bitmap_set(bitmap, 0, sparity->nsectors);
+		return;
+	}
+
+	start -= sparity->logic_start;
+	start = div_u64_rem(start, sparity->stripe_len, &offset);
+	offset /= sectorsize;
+	nsectors = (int)len / sectorsize;
+
+	if (offset + nsectors <= sparity->nsectors) {
+		bitmap_set(bitmap, offset, nsectors);
+		return;
+	}
+
+	bitmap_set(bitmap, offset, sparity->nsectors - offset);
+	bitmap_set(bitmap, 0, nsectors - (sparity->nsectors - offset));
+}
+
+static inline void scrub_parity_mark_sectors_error(struct scrub_parity *sparity,
+						   u64 start, u64 len)
+{
+	__scrub_mark_bitmap(sparity, sparity->ebitmap, start, len);
+}
+
+static inline void scrub_parity_mark_sectors_data(struct scrub_parity *sparity,
+						  u64 start, u64 len)
+{
+	__scrub_mark_bitmap(sparity, sparity->dbitmap, start, len);
+}
+
+static void scrub_block_complete(struct scrub_block *sblock)
+{
+	int corrupted = 0;
+
+	if (!sblock->no_io_error_seen) {
+		corrupted = 1;
+		scrub_handle_errored_block(sblock);
+	} else {
+		/*
+		 * if has checksum error, write via repair mechanism in
+		 * dev replace case, otherwise write here in dev replace
+		 * case.
+		 */
+		corrupted = scrub_checksum(sblock);
+		if (!corrupted && sblock->sctx->is_dev_replace)
+			scrub_write_block_to_dev_replace(sblock);
+	}
+
+	if (sblock->sparity && corrupted && !sblock->data_corrected) {
+		u64 start = sblock->pagev[0]->logical;
+		u64 end = sblock->pagev[sblock->page_count - 1]->logical +
+			  PAGE_SIZE;
+
+		scrub_parity_mark_sectors_error(sblock->sparity,
+						start, end - start);
+	}
+}
+
+static int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u64 len,
+			   u8 *csum)
+{
+	struct btrfs_ordered_sum *sum = NULL;
+	unsigned long index;
+	unsigned long num_sectors;
+
+	while (!list_empty(&sctx->csum_list)) {
+		sum = list_first_entry(&sctx->csum_list,
+				       struct btrfs_ordered_sum, list);
+		if (sum->bytenr > logical)
+			return 0;
+		if (sum->bytenr + sum->len > logical)
+			break;
+
+		++sctx->stat.csum_discards;
+		list_del(&sum->list);
+		kfree(sum);
+		sum = NULL;
+	}
+	if (!sum)
+		return 0;
+
+	index = ((u32)(logical - sum->bytenr)) / sctx->sectorsize;
+	num_sectors = sum->len / sctx->sectorsize;
+	memcpy(csum, sum->sums + index, sctx->csum_size);
+	if (index == num_sectors - 1) {
+		list_del(&sum->list);
+		kfree(sum);
+	}
+	return 1;
+}
+
+/* scrub extent tries to collect up to 64 kB for each bio */
+static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
+			u64 physical, struct btrfs_device *dev, u64 flags,
+			u64 gen, int mirror_num, u64 physical_for_dev_replace)
+{
+	int ret;
+	u8 csum[BTRFS_CSUM_SIZE];
+	u32 blocksize;
+
+	if (flags & BTRFS_EXTENT_FLAG_DATA) {
+		blocksize = sctx->sectorsize;
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.data_extents_scrubbed++;
+		sctx->stat.data_bytes_scrubbed += len;
+		spin_unlock(&sctx->stat_lock);
+	} else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
+		blocksize = sctx->nodesize;
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.tree_extents_scrubbed++;
+		sctx->stat.tree_bytes_scrubbed += len;
+		spin_unlock(&sctx->stat_lock);
+	} else {
+		blocksize = sctx->sectorsize;
+		WARN_ON(1);
+	}
+
+	while (len) {
+		u64 l = min_t(u64, len, blocksize);
+		int have_csum = 0;
+
+		if (flags & BTRFS_EXTENT_FLAG_DATA) {
+			/* push csums to sbio */
+			have_csum = scrub_find_csum(sctx, logical, l, csum);
+			if (have_csum == 0)
+				++sctx->stat.no_csum;
+			if (sctx->is_dev_replace && !have_csum) {
+				ret = copy_nocow_pages(sctx, logical, l,
+						       mirror_num,
+						      physical_for_dev_replace);
+				goto behind_scrub_pages;
+			}
+		}
+		ret = scrub_pages(sctx, logical, l, physical, dev, flags, gen,
+				  mirror_num, have_csum ? csum : NULL, 0,
+				  physical_for_dev_replace);
+behind_scrub_pages:
+		if (ret)
+			return ret;
+		len -= l;
+		logical += l;
+		physical += l;
+		physical_for_dev_replace += l;
+	}
+	return 0;
+}
+
+static int scrub_pages_for_parity(struct scrub_parity *sparity,
+				  u64 logical, u64 len,
+				  u64 physical, struct btrfs_device *dev,
+				  u64 flags, u64 gen, int mirror_num, u8 *csum)
+{
+	struct scrub_ctx *sctx = sparity->sctx;
+	struct scrub_block *sblock;
+	int index;
+
+	sblock = kzalloc(sizeof(*sblock), GFP_NOFS);
+	if (!sblock) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+
+	/* one ref inside this function, plus one for each page added to
+	 * a bio later on */
+	atomic_set(&sblock->refs, 1);
+	sblock->sctx = sctx;
+	sblock->no_io_error_seen = 1;
+	sblock->sparity = sparity;
+	scrub_parity_get(sparity);
+
+	for (index = 0; len > 0; index++) {
+		struct scrub_page *spage;
+		u64 l = min_t(u64, len, PAGE_SIZE);
+
+		spage = kzalloc(sizeof(*spage), GFP_NOFS);
+		if (!spage) {
+leave_nomem:
+			spin_lock(&sctx->stat_lock);
+			sctx->stat.malloc_errors++;
+			spin_unlock(&sctx->stat_lock);
+			scrub_block_put(sblock);
+			return -ENOMEM;
+		}
+		BUG_ON(index >= SCRUB_MAX_PAGES_PER_BLOCK);
+		/* For scrub block */
+		scrub_page_get(spage);
+		sblock->pagev[index] = spage;
+		/* For scrub parity */
+		scrub_page_get(spage);
+		list_add_tail(&spage->list, &sparity->spages);
+		spage->sblock = sblock;
+		spage->dev = dev;
+		spage->flags = flags;
+		spage->generation = gen;
+		spage->logical = logical;
+		spage->physical = physical;
+		spage->mirror_num = mirror_num;
+		if (csum) {
+			spage->have_csum = 1;
+			memcpy(spage->csum, csum, sctx->csum_size);
+		} else {
+			spage->have_csum = 0;
+		}
+		sblock->page_count++;
+		spage->page = alloc_page(GFP_NOFS);
+		if (!spage->page)
+			goto leave_nomem;
+		len -= l;
+		logical += l;
+		physical += l;
+	}
+
+	WARN_ON(sblock->page_count == 0);
+	for (index = 0; index < sblock->page_count; index++) {
+		struct scrub_page *spage = sblock->pagev[index];
+		int ret;
+
+		ret = scrub_add_page_to_rd_bio(sctx, spage);
+		if (ret) {
+			scrub_block_put(sblock);
+			return ret;
+		}
+	}
+
+	/* last one frees, either here or in bio completion for last page */
+	scrub_block_put(sblock);
+	return 0;
+}
+
+static int scrub_extent_for_parity(struct scrub_parity *sparity,
+				   u64 logical, u64 len,
+				   u64 physical, struct btrfs_device *dev,
+				   u64 flags, u64 gen, int mirror_num)
+{
+	struct scrub_ctx *sctx = sparity->sctx;
+	int ret;
+	u8 csum[BTRFS_CSUM_SIZE];
+	u32 blocksize;
+
+	if (flags & BTRFS_EXTENT_FLAG_DATA) {
+		blocksize = sctx->sectorsize;
+	} else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
+		blocksize = sctx->nodesize;
+	} else {
+		blocksize = sctx->sectorsize;
+		WARN_ON(1);
+	}
+
+	while (len) {
+		u64 l = min_t(u64, len, blocksize);
+		int have_csum = 0;
+
+		if (flags & BTRFS_EXTENT_FLAG_DATA) {
+			/* push csums to sbio */
+			have_csum = scrub_find_csum(sctx, logical, l, csum);
+			if (have_csum == 0)
+				goto skip;
+		}
+		ret = scrub_pages_for_parity(sparity, logical, l, physical, dev,
+					     flags, gen, mirror_num,
+					     have_csum ? csum : NULL);
+		if (ret)
+			return ret;
+skip:
+		len -= l;
+		logical += l;
+		physical += l;
+	}
+	return 0;
+}
+
+/*
+ * Given a physical address, this will calculate it's
+ * logical offset. if this is a parity stripe, it will return
+ * the most left data stripe's logical offset.
+ *
+ * return 0 if it is a data stripe, 1 means parity stripe.
+ */
+static int get_raid56_logic_offset(u64 physical, int num,
+				   struct map_lookup *map, u64 *offset,
+				   u64 *stripe_start)
+{
+	int i;
+	int j = 0;
+	u64 stripe_nr;
+	u64 last_offset;
+	u32 stripe_index;
+	u32 rot;
+
+	last_offset = (physical - map->stripes[num].physical) *
+		      nr_data_stripes(map);
+	if (stripe_start)
+		*stripe_start = last_offset;
+
+	*offset = last_offset;
+	for (i = 0; i < nr_data_stripes(map); i++) {
+		*offset = last_offset + i * map->stripe_len;
+
+		stripe_nr = div_u64(*offset, map->stripe_len);
+		stripe_nr = div_u64(stripe_nr, nr_data_stripes(map));
+
+		/* Work out the disk rotation on this stripe-set */
+		stripe_nr = div_u64_rem(stripe_nr, map->num_stripes, &rot);
+		/* calculate which stripe this data locates */
+		rot += i;
+		stripe_index = rot % map->num_stripes;
+		if (stripe_index == num)
+			return 0;
+		if (stripe_index < num)
+			j++;
+	}
+	*offset = last_offset + j * map->stripe_len;
+	return 1;
+}
+
+static void scrub_free_parity(struct scrub_parity *sparity)
+{
+	struct scrub_ctx *sctx = sparity->sctx;
+	struct scrub_page *curr, *next;
+	int nbits;
+
+	nbits = bitmap_weight(sparity->ebitmap, sparity->nsectors);
+	if (nbits) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.read_errors += nbits;
+		sctx->stat.uncorrectable_errors += nbits;
+		spin_unlock(&sctx->stat_lock);
+	}
+
+	list_for_each_entry_safe(curr, next, &sparity->spages, list) {
+		list_del_init(&curr->list);
+		scrub_page_put(curr);
+	}
+
+	kfree(sparity);
+}
+
+static void scrub_parity_bio_endio(struct bio *bio, int error)
+{
+	struct scrub_parity *sparity = (struct scrub_parity *)bio->bi_private;
+	struct scrub_ctx *sctx = sparity->sctx;
+
+	if (error)
+		bitmap_or(sparity->ebitmap, sparity->ebitmap, sparity->dbitmap,
+			  sparity->nsectors);
+
+	scrub_free_parity(sparity);
+	scrub_pending_bio_dec(sctx);
+	bio_put(bio);
+}
+
+static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
+{
+	struct scrub_ctx *sctx = sparity->sctx;
+	struct bio *bio;
+	struct btrfs_raid_bio *rbio;
+	struct scrub_page *spage;
+	struct btrfs_bio *bbio = NULL;
+	u64 length;
+	int ret;
+
+	if (!bitmap_andnot(sparity->dbitmap, sparity->dbitmap, sparity->ebitmap,
+			   sparity->nsectors))
+		goto out;
+
+	length = sparity->logic_end - sparity->logic_start + 1;
+	ret = btrfs_map_sblock(sctx->dev_root->fs_info, WRITE,
+			       sparity->logic_start,
+			       &length, &bbio, 0, 1);
+	if (ret || !bbio || !bbio->raid_map)
+		goto bbio_out;
+
+	bio = btrfs_io_bio_alloc(GFP_NOFS, 0);
+	if (!bio)
+		goto bbio_out;
+
+	bio->bi_iter.bi_sector = sparity->logic_start >> 9;
+	bio->bi_private = sparity;
+	bio->bi_end_io = scrub_parity_bio_endio;
+
+	rbio = raid56_parity_alloc_scrub_rbio(sctx->dev_root, bio, bbio,
+					      length, sparity->scrub_dev,
+					      sparity->dbitmap,
+					      sparity->nsectors);
+	if (!rbio)
+		goto rbio_out;
+
+	list_for_each_entry(spage, &sparity->spages, list)
+		raid56_parity_add_scrub_pages(rbio, spage->page,
+					      spage->logical);
+
+	scrub_pending_bio_inc(sctx);
+	raid56_parity_submit_scrub_rbio(rbio);
+	return;
+
+rbio_out:
+	bio_put(bio);
+bbio_out:
+	btrfs_put_bbio(bbio);
+	bitmap_or(sparity->ebitmap, sparity->ebitmap, sparity->dbitmap,
+		  sparity->nsectors);
+	spin_lock(&sctx->stat_lock);
+	sctx->stat.malloc_errors++;
+	spin_unlock(&sctx->stat_lock);
+out:
+	scrub_free_parity(sparity);
+}
+
+static inline int scrub_calc_parity_bitmap_len(int nsectors)
+{
+	return DIV_ROUND_UP(nsectors, BITS_PER_LONG) * (BITS_PER_LONG / 8);
+}
+
+static void scrub_parity_get(struct scrub_parity *sparity)
+{
+	atomic_inc(&sparity->refs);
+}
+
+static void scrub_parity_put(struct scrub_parity *sparity)
+{
+	if (!atomic_dec_and_test(&sparity->refs))
+		return;
+
+	scrub_parity_check_and_repair(sparity);
+}
+
+static noinline_for_stack int scrub_raid56_parity(struct scrub_ctx *sctx,
+						  struct map_lookup *map,
+						  struct btrfs_device *sdev,
+						  struct btrfs_path *path,
+						  u64 logic_start,
+						  u64 logic_end)
+{
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+	struct btrfs_root *root = fs_info->extent_root;
+	struct btrfs_root *csum_root = fs_info->csum_root;
+	struct btrfs_extent_item *extent;
+	u64 flags;
+	int ret;
+	int slot;
+	struct extent_buffer *l;
+	struct btrfs_key key;
+	u64 generation;
+	u64 extent_logical;
+	u64 extent_physical;
+	u64 extent_len;
+	struct btrfs_device *extent_dev;
+	struct scrub_parity *sparity;
+	int nsectors;
+	int bitmap_len;
+	int extent_mirror_num;
+	int stop_loop = 0;
+
+	nsectors = map->stripe_len / root->sectorsize;
+	bitmap_len = scrub_calc_parity_bitmap_len(nsectors);
+	sparity = kzalloc(sizeof(struct scrub_parity) + 2 * bitmap_len,
+			  GFP_NOFS);
+	if (!sparity) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+
+	sparity->stripe_len = map->stripe_len;
+	sparity->nsectors = nsectors;
+	sparity->sctx = sctx;
+	sparity->scrub_dev = sdev;
+	sparity->logic_start = logic_start;
+	sparity->logic_end = logic_end;
+	atomic_set(&sparity->refs, 1);
+	INIT_LIST_HEAD(&sparity->spages);
+	sparity->dbitmap = sparity->bitmap;
+	sparity->ebitmap = (void *)sparity->bitmap + bitmap_len;
+
+	ret = 0;
+	while (logic_start < logic_end) {
+		if (btrfs_fs_incompat(fs_info, SKINNY_METADATA))
+			key.type = BTRFS_METADATA_ITEM_KEY;
+		else
+			key.type = BTRFS_EXTENT_ITEM_KEY;
+		key.objectid = logic_start;
+		key.offset = (u64)-1;
+
+		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+		if (ret < 0)
+			goto out;
+
+		if (ret > 0) {
+			ret = btrfs_previous_extent_item(root, path, 0);
+			if (ret < 0)
+				goto out;
+			if (ret > 0) {
+				btrfs_release_path(path);
+				ret = btrfs_search_slot(NULL, root, &key,
+							path, 0, 0);
+				if (ret < 0)
+					goto out;
+			}
+		}
+
+		stop_loop = 0;
+		while (1) {
+			u64 bytes;
+
+			l = path->nodes[0];
+			slot = path->slots[0];
+			if (slot >= btrfs_header_nritems(l)) {
+				ret = btrfs_next_leaf(root, path);
+				if (ret == 0)
+					continue;
+				if (ret < 0)
+					goto out;
+
+				stop_loop = 1;
+				break;
+			}
+			btrfs_item_key_to_cpu(l, &key, slot);
+
+			if (key.type == BTRFS_METADATA_ITEM_KEY)
+				bytes = root->nodesize;
+			else
+				bytes = key.offset;
+
+			if (key.objectid + bytes <= logic_start)
+				goto next;
+
+			if (key.type != BTRFS_EXTENT_ITEM_KEY &&
+			    key.type != BTRFS_METADATA_ITEM_KEY)
+				goto next;
+
+			if (key.objectid > logic_end) {
+				stop_loop = 1;
+				break;
+			}
+
+			while (key.objectid >= logic_start + map->stripe_len)
+				logic_start += map->stripe_len;
+
+			extent = btrfs_item_ptr(l, slot,
+						struct btrfs_extent_item);
+			flags = btrfs_extent_flags(l, extent);
+			generation = btrfs_extent_generation(l, extent);
+
+			if (key.objectid < logic_start &&
+			    (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)) {
+				btrfs_err(fs_info,
+					  "scrub: tree block %llu spanning stripes, ignored. logical=%llu",
+					   key.objectid, logic_start);
+				goto next;
+			}
+again:
+			extent_logical = key.objectid;
+			extent_len = bytes;
+
+			if (extent_logical < logic_start) {
+				extent_len -= logic_start - extent_logical;
+				extent_logical = logic_start;
+			}
+
+			if (extent_logical + extent_len >
+			    logic_start + map->stripe_len)
+				extent_len = logic_start + map->stripe_len -
+					     extent_logical;
+
+			scrub_parity_mark_sectors_data(sparity, extent_logical,
+						       extent_len);
+
+			scrub_remap_extent(fs_info, extent_logical,
+					   extent_len, &extent_physical,
+					   &extent_dev,
+					   &extent_mirror_num);
+
+			ret = btrfs_lookup_csums_range(csum_root,
+						extent_logical,
+						extent_logical + extent_len - 1,
+						&sctx->csum_list, 1);
+			if (ret)
+				goto out;
+
+			ret = scrub_extent_for_parity(sparity, extent_logical,
+						      extent_len,
+						      extent_physical,
+						      extent_dev, flags,
+						      generation,
+						      extent_mirror_num);
+			if (ret)
+				goto out;
+
+			scrub_free_csums(sctx);
+			if (extent_logical + extent_len <
+			    key.objectid + bytes) {
+				logic_start += map->stripe_len;
+
+				if (logic_start >= logic_end) {
+					stop_loop = 1;
+					break;
+				}
+
+				if (logic_start < key.objectid + bytes) {
+					cond_resched();
+					goto again;
+				}
+			}
+next:
+			path->slots[0]++;
+		}
+
+		btrfs_release_path(path);
+
+		if (stop_loop)
+			break;
+
+		logic_start += map->stripe_len;
+	}
+out:
+	if (ret < 0)
+		scrub_parity_mark_sectors_error(sparity, logic_start,
+						logic_end - logic_start + 1);
+	scrub_parity_put(sparity);
+	scrub_submit(sctx);
+	mutex_lock(&sctx->wr_ctx.wr_lock);
+	scrub_wr_submit(sctx);
+	mutex_unlock(&sctx->wr_ctx.wr_lock);
+
+	btrfs_release_path(path);
+	return ret < 0 ? ret : 0;
+}
+
+static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
+					   struct map_lookup *map,
+					   struct btrfs_device *scrub_dev,
+					   int num, u64 base, u64 length,
+					   int is_dev_replace)
+{
+	struct btrfs_path *path, *ppath;
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+	struct btrfs_root *root = fs_info->extent_root;
+	struct btrfs_root *csum_root = fs_info->csum_root;
+	struct btrfs_extent_item *extent;
+	struct blk_plug plug;
+	u64 flags;
+	int ret;
+	int slot;
+	u64 nstripes;
+	struct extent_buffer *l;
+	struct btrfs_key key;
+	u64 physical;
+	u64 logical;
+	u64 logic_end;
+	u64 physical_end;
+	u64 generation;
+	int mirror_num;
+	struct reada_control *reada1;
+	struct reada_control *reada2;
+	struct btrfs_key key_start;
+	struct btrfs_key key_end;
+	u64 increment = map->stripe_len;
+	u64 offset;
+	u64 extent_logical;
+	u64 extent_physical;
+	u64 extent_len;
+	u64 stripe_logical;
+	u64 stripe_end;
+	struct btrfs_device *extent_dev;
+	int extent_mirror_num;
+	int stop_loop = 0;
+
+	physical = map->stripes[num].physical;
+	offset = 0;
+	nstripes = div_u64(length, map->stripe_len);
+	if (map->type & BTRFS_BLOCK_GROUP_RAID0) {
+		offset = map->stripe_len * num;
+		increment = map->stripe_len * map->num_stripes;
+		mirror_num = 1;
+	} else if (map->type & BTRFS_BLOCK_GROUP_RAID10) {
+		int factor = map->num_stripes / map->sub_stripes;
+		offset = map->stripe_len * (num / map->sub_stripes);
+		increment = map->stripe_len * factor;
+		mirror_num = num % map->sub_stripes + 1;
+	} else if (map->type & BTRFS_BLOCK_GROUP_RAID1) {
+		increment = map->stripe_len;
+		mirror_num = num % map->num_stripes + 1;
+	} else if (map->type & BTRFS_BLOCK_GROUP_DUP) {
+		increment = map->stripe_len;
+		mirror_num = num % map->num_stripes + 1;
+	} else if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+		get_raid56_logic_offset(physical, num, map, &offset, NULL);
+		increment = map->stripe_len * nr_data_stripes(map);
+		mirror_num = 1;
+	} else {
+		increment = map->stripe_len;
+		mirror_num = 1;
+	}
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	ppath = btrfs_alloc_path();
+	if (!ppath) {
+		btrfs_free_path(path);
+		return -ENOMEM;
+	}
+
+	/*
+	 * work on commit root. The related disk blocks are static as
+	 * long as COW is applied. This means, it is save to rewrite
+	 * them to repair disk errors without any race conditions
+	 */
+	path->search_commit_root = 1;
+	path->skip_locking = 1;
+
+	ppath->search_commit_root = 1;
+	ppath->skip_locking = 1;
+	/*
+	 * trigger the readahead for extent tree csum tree and wait for
+	 * completion. During readahead, the scrub is officially paused
+	 * to not hold off transaction commits
+	 */
+	logical = base + offset;
+	physical_end = physical + nstripes * map->stripe_len;
+	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+		get_raid56_logic_offset(physical_end, num,
+					map, &logic_end, NULL);
+		logic_end += base;
+	} else {
+		logic_end = logical + increment * nstripes;
+	}
+	wait_event(sctx->list_wait,
+		   atomic_read(&sctx->bios_in_flight) == 0);
+	scrub_blocked_if_needed(fs_info);
+
+	/* FIXME it might be better to start readahead at commit root */
+	key_start.objectid = logical;
+	key_start.type = BTRFS_EXTENT_ITEM_KEY;
+	key_start.offset = (u64)0;
+	key_end.objectid = logic_end;
+	key_end.type = BTRFS_METADATA_ITEM_KEY;
+	key_end.offset = (u64)-1;
+	reada1 = btrfs_reada_add(root, &key_start, &key_end);
+
+	key_start.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
+	key_start.type = BTRFS_EXTENT_CSUM_KEY;
+	key_start.offset = logical;
+	key_end.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
+	key_end.type = BTRFS_EXTENT_CSUM_KEY;
+	key_end.offset = logic_end;
+	reada2 = btrfs_reada_add(csum_root, &key_start, &key_end);
+
+	if (!IS_ERR(reada1))
+		btrfs_reada_wait(reada1);
+	if (!IS_ERR(reada2))
+		btrfs_reada_wait(reada2);
+
+
+	/*
+	 * collect all data csums for the stripe to avoid seeking during
+	 * the scrub. This might currently (crc32) end up to be about 1MB
+	 */
+	blk_start_plug(&plug);
+
+	/*
+	 * now find all extents for each stripe and scrub them
+	 */
+	ret = 0;
+	while (physical < physical_end) {
+		/* for raid56, we skip parity stripe */
+		if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+			ret = get_raid56_logic_offset(physical, num,
+					map, &logical, &stripe_logical);
+			logical += base;
+			if (ret) {
+				stripe_logical += base;
+				stripe_end = stripe_logical + increment - 1;
+				ret = scrub_raid56_parity(sctx, map, scrub_dev,
+						ppath, stripe_logical,
+						stripe_end);
+				if (ret)
+					goto out;
+				goto skip;
+			}
+		}
+		/*
+		 * canceled?
+		 */
+		if (atomic_read(&fs_info->scrub_cancel_req) ||
+		    atomic_read(&sctx->cancel_req)) {
+			ret = -ECANCELED;
+			goto out;
+		}
+		/*
+		 * check to see if we have to pause
+		 */
+		if (atomic_read(&fs_info->scrub_pause_req)) {
+			/* push queued extents */
+			atomic_set(&sctx->wr_ctx.flush_all_writes, 1);
+			scrub_submit(sctx);
+			mutex_lock(&sctx->wr_ctx.wr_lock);
+			scrub_wr_submit(sctx);
+			mutex_unlock(&sctx->wr_ctx.wr_lock);
+			wait_event(sctx->list_wait,
+				   atomic_read(&sctx->bios_in_flight) == 0);
+			atomic_set(&sctx->wr_ctx.flush_all_writes, 0);
+			scrub_blocked_if_needed(fs_info);
+		}
+
+		if (btrfs_fs_incompat(fs_info, SKINNY_METADATA))
+			key.type = BTRFS_METADATA_ITEM_KEY;
+		else
+			key.type = BTRFS_EXTENT_ITEM_KEY;
+		key.objectid = logical;
+		key.offset = (u64)-1;
+
+		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+		if (ret < 0)
+			goto out;
+
+		if (ret > 0) {
+			ret = btrfs_previous_extent_item(root, path, 0);
+			if (ret < 0)
+				goto out;
+			if (ret > 0) {
+				/* there's no smaller item, so stick with the
+				 * larger one */
+				btrfs_release_path(path);
+				ret = btrfs_search_slot(NULL, root, &key,
+							path, 0, 0);
+				if (ret < 0)
+					goto out;
+			}
+		}
+
+		stop_loop = 0;
+		while (1) {
+			u64 bytes;
+
+			l = path->nodes[0];
+			slot = path->slots[0];
+			if (slot >= btrfs_header_nritems(l)) {
+				ret = btrfs_next_leaf(root, path);
+				if (ret == 0)
+					continue;
+				if (ret < 0)
+					goto out;
+
+				stop_loop = 1;
+				break;
+			}
+			btrfs_item_key_to_cpu(l, &key, slot);
+
+			if (key.type == BTRFS_METADATA_ITEM_KEY)
+				bytes = root->nodesize;
+			else
+				bytes = key.offset;
+
+			if (key.objectid + bytes <= logical)
+				goto next;
+
+			if (key.type != BTRFS_EXTENT_ITEM_KEY &&
+			    key.type != BTRFS_METADATA_ITEM_KEY)
+				goto next;
+
+			if (key.objectid >= logical + map->stripe_len) {
+				/* out of this device extent */
+				if (key.objectid >= logic_end)
+					stop_loop = 1;
+				break;
+			}
+
+			extent = btrfs_item_ptr(l, slot,
+						struct btrfs_extent_item);
+			flags = btrfs_extent_flags(l, extent);
+			generation = btrfs_extent_generation(l, extent);
+
+			if (key.objectid < logical &&
+			    (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)) {
+				btrfs_err(fs_info,
+					   "scrub: tree block %llu spanning "
+					   "stripes, ignored. logical=%llu",
+				       key.objectid, logical);
+				goto next;
+			}
+
+again:
+			extent_logical = key.objectid;
+			extent_len = bytes;
+
+			/*
+			 * trim extent to this stripe
+			 */
+			if (extent_logical < logical) {
+				extent_len -= logical - extent_logical;
+				extent_logical = logical;
+			}
+			if (extent_logical + extent_len >
+			    logical + map->stripe_len) {
+				extent_len = logical + map->stripe_len -
+					     extent_logical;
+			}
+
+			extent_physical = extent_logical - logical + physical;
+			extent_dev = scrub_dev;
+			extent_mirror_num = mirror_num;
+			if (is_dev_replace)
+				scrub_remap_extent(fs_info, extent_logical,
+						   extent_len, &extent_physical,
+						   &extent_dev,
+						   &extent_mirror_num);
+
+			ret = btrfs_lookup_csums_range(csum_root, logical,
+						logical + map->stripe_len - 1,
+						&sctx->csum_list, 1);
+			if (ret)
+				goto out;
+
+			ret = scrub_extent(sctx, extent_logical, extent_len,
+					   extent_physical, extent_dev, flags,
+					   generation, extent_mirror_num,
+					   extent_logical - logical + physical);
+			if (ret)
+				goto out;
+
+			scrub_free_csums(sctx);
+			if (extent_logical + extent_len <
+			    key.objectid + bytes) {
+				if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+					/*
+					 * loop until we find next data stripe
+					 * or we have finished all stripes.
+					 */
+loop:
+					physical += map->stripe_len;
+					ret = get_raid56_logic_offset(physical,
+							num, map, &logical,
+							&stripe_logical);
+					logical += base;
+
+					if (ret && physical < physical_end) {
+						stripe_logical += base;
+						stripe_end = stripe_logical +
+								increment - 1;
+						ret = scrub_raid56_parity(sctx,
+							map, scrub_dev, ppath,
+							stripe_logical,
+							stripe_end);
+						if (ret)
+							goto out;
+						goto loop;
+					}
+				} else {
+					physical += map->stripe_len;
+					logical += increment;
+				}
+				if (logical < key.objectid + bytes) {
+					cond_resched();
+					goto again;
+				}
+
+				if (physical >= physical_end) {
+					stop_loop = 1;
+					break;
+				}
+			}
+next:
+			path->slots[0]++;
+		}
+		btrfs_release_path(path);
+skip:
+		logical += increment;
+		physical += map->stripe_len;
+		spin_lock(&sctx->stat_lock);
+		if (stop_loop)
+			sctx->stat.last_physical = map->stripes[num].physical +
+						   length;
+		else
+			sctx->stat.last_physical = physical;
+		spin_unlock(&sctx->stat_lock);
+		if (stop_loop)
+			break;
+	}
+out:
+	/* push queued extents */
+	scrub_submit(sctx);
+	mutex_lock(&sctx->wr_ctx.wr_lock);
+	scrub_wr_submit(sctx);
+	mutex_unlock(&sctx->wr_ctx.wr_lock);
+
+	blk_finish_plug(&plug);
+	btrfs_free_path(path);
+	btrfs_free_path(ppath);
+	return ret < 0 ? ret : 0;
+}
+
+static noinline_for_stack int scrub_chunk(struct scrub_ctx *sctx,
+					  struct btrfs_device *scrub_dev,
+					  u64 chunk_tree, u64 chunk_objectid,
+					  u64 chunk_offset, u64 length,
+					  u64 dev_offset, int is_dev_replace)
+{
+	struct btrfs_mapping_tree *map_tree =
+		&sctx->dev_root->fs_info->mapping_tree;
+	struct map_lookup *map;
+	struct extent_map *em;
+	int i;
+	int ret = 0;
+
+	read_lock(&map_tree->map_tree.lock);
+	em = lookup_extent_mapping(&map_tree->map_tree, chunk_offset, 1);
+	read_unlock(&map_tree->map_tree.lock);
+
+	if (!em)
+		return -EINVAL;
+
+	map = (struct map_lookup *)em->bdev;
+	if (em->start != chunk_offset)
+		goto out;
+
+	if (em->len < length)
+		goto out;
+
+	for (i = 0; i < map->num_stripes; ++i) {
+		if (map->stripes[i].dev->bdev == scrub_dev->bdev &&
+		    map->stripes[i].physical == dev_offset) {
+			ret = scrub_stripe(sctx, map, scrub_dev, i,
+					   chunk_offset, length,
+					   is_dev_replace);
+			if (ret)
+				goto out;
+		}
+	}
+out:
+	free_extent_map(em);
+
+	return ret;
+}
+
+static noinline_for_stack
+int scrub_enumerate_chunks(struct scrub_ctx *sctx,
+			   struct btrfs_device *scrub_dev, u64 start, u64 end,
+			   int is_dev_replace)
+{
+	struct btrfs_dev_extent *dev_extent = NULL;
+	struct btrfs_path *path;
+	struct btrfs_root *root = sctx->dev_root;
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	u64 length;
+	u64 chunk_tree;
+	u64 chunk_objectid;
+	u64 chunk_offset;
+	int ret;
+	int slot;
+	struct extent_buffer *l;
+	struct btrfs_key key;
+	struct btrfs_key found_key;
+	struct btrfs_block_group_cache *cache;
+	struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	path->reada = 2;
+	path->search_commit_root = 1;
+	path->skip_locking = 1;
+
+	key.objectid = scrub_dev->devid;
+	key.offset = 0ull;
+	key.type = BTRFS_DEV_EXTENT_KEY;
+
+	while (1) {
+		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+		if (ret < 0)
+			break;
+		if (ret > 0) {
+			if (path->slots[0] >=
+			    btrfs_header_nritems(path->nodes[0])) {
+				ret = btrfs_next_leaf(root, path);
+				if (ret)
+					break;
+			}
+		}
+
+		l = path->nodes[0];
+		slot = path->slots[0];
+
+		btrfs_item_key_to_cpu(l, &found_key, slot);
+
+		if (found_key.objectid != scrub_dev->devid)
+			break;
+
+		if (found_key.type != BTRFS_DEV_EXTENT_KEY)
+			break;
+
+		if (found_key.offset >= end)
+			break;
+
+		if (found_key.offset < key.offset)
+			break;
+
+		dev_extent = btrfs_item_ptr(l, slot, struct btrfs_dev_extent);
+		length = btrfs_dev_extent_length(l, dev_extent);
+
+		if (found_key.offset + length <= start)
+			goto skip;
+
+		chunk_tree = btrfs_dev_extent_chunk_tree(l, dev_extent);
+		chunk_objectid = btrfs_dev_extent_chunk_objectid(l, dev_extent);
+		chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);
+
+		/*
+		 * get a reference on the corresponding block group to prevent
+		 * the chunk from going away while we scrub it
+		 */
+		cache = btrfs_lookup_block_group(fs_info, chunk_offset);
+
+		/* some chunks are removed but not committed to disk yet,
+		 * continue scrubbing */
+		if (!cache)
+			goto skip;
+
+		dev_replace->cursor_right = found_key.offset + length;
+		dev_replace->cursor_left = found_key.offset;
+		dev_replace->item_needs_writeback = 1;
+		ret = scrub_chunk(sctx, scrub_dev, chunk_tree, chunk_objectid,
+				  chunk_offset, length, found_key.offset,
+				  is_dev_replace);
+
+		/*
+		 * flush, submit all pending read and write bios, afterwards
+		 * wait for them.
+		 * Note that in the dev replace case, a read request causes
+		 * write requests that are submitted in the read completion
+		 * worker. Therefore in the current situation, it is required
+		 * that all write requests are flushed, so that all read and
+		 * write requests are really completed when bios_in_flight
+		 * changes to 0.
+		 */
+		atomic_set(&sctx->wr_ctx.flush_all_writes, 1);
+		scrub_submit(sctx);
+		mutex_lock(&sctx->wr_ctx.wr_lock);
+		scrub_wr_submit(sctx);
+		mutex_unlock(&sctx->wr_ctx.wr_lock);
+
+		wait_event(sctx->list_wait,
+			   atomic_read(&sctx->bios_in_flight) == 0);
+		atomic_inc(&fs_info->scrubs_paused);
+		wake_up(&fs_info->scrub_pause_wait);
+
+		/*
+		 * must be called before we decrease @scrub_paused.
+		 * make sure we don't block transaction commit while
+		 * we are waiting pending workers finished.
+		 */
+		wait_event(sctx->list_wait,
+			   atomic_read(&sctx->workers_pending) == 0);
+		atomic_set(&sctx->wr_ctx.flush_all_writes, 0);
+
+		mutex_lock(&fs_info->scrub_lock);
+		__scrub_blocked_if_needed(fs_info);
+		atomic_dec(&fs_info->scrubs_paused);
+		mutex_unlock(&fs_info->scrub_lock);
+		wake_up(&fs_info->scrub_pause_wait);
+
+		btrfs_put_block_group(cache);
+		if (ret)
+			break;
+		if (is_dev_replace &&
+		    atomic64_read(&dev_replace->num_write_errors) > 0) {
+			ret = -EIO;
+			break;
+		}
+		if (sctx->stat.malloc_errors > 0) {
+			ret = -ENOMEM;
+			break;
+		}
+
+		dev_replace->cursor_left = dev_replace->cursor_right;
+		dev_replace->item_needs_writeback = 1;
+skip:
+		key.offset = found_key.offset + length;
+		btrfs_release_path(path);
+	}
+
+	btrfs_free_path(path);
+
+	/*
+	 * ret can still be 1 from search_slot or next_leaf,
+	 * that's not an error
+	 */
+	return ret < 0 ? ret : 0;
+}
+
+static noinline_for_stack int scrub_supers(struct scrub_ctx *sctx,
+					   struct btrfs_device *scrub_dev)
+{
+	int	i;
+	u64	bytenr;
+	u64	gen;
+	int	ret;
+	struct btrfs_root *root = sctx->dev_root;
+
+	if (test_bit(BTRFS_FS_STATE_ERROR, &root->fs_info->fs_state))
+		return -EIO;
+
+	/* Seed devices of a new filesystem has their own generation. */
+	if (scrub_dev->fs_devices != root->fs_info->fs_devices)
+		gen = scrub_dev->generation;
+	else
+		gen = root->fs_info->last_trans_committed;
+
+	for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
+		bytenr = btrfs_sb_offset(i);
+		if (bytenr + BTRFS_SUPER_INFO_SIZE >
+		    scrub_dev->commit_total_bytes)
+			break;
+
+		ret = scrub_pages(sctx, bytenr, BTRFS_SUPER_INFO_SIZE, bytenr,
+				  scrub_dev, BTRFS_EXTENT_FLAG_SUPER, gen, i,
+				  NULL, 1, bytenr);
+		if (ret)
+			return ret;
+	}
+	wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);
+
+	return 0;
+}
+
+/*
+ * get a reference count on fs_info->scrub_workers. start worker if necessary
+ */
+static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
+						int is_dev_replace)
+{
+	int ret = 0;
+	unsigned int flags = WQ_FREEZABLE | WQ_UNBOUND;
+	int max_active = fs_info->thread_pool_size;
+
+	if (fs_info->scrub_workers_refcnt == 0) {
+		if (is_dev_replace)
+			fs_info->scrub_workers =
+				btrfs_alloc_workqueue("btrfs-scrub", flags,
+						      1, 4);
+		else
+			fs_info->scrub_workers =
+				btrfs_alloc_workqueue("btrfs-scrub", flags,
+						      max_active, 4);
+		if (!fs_info->scrub_workers) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		fs_info->scrub_wr_completion_workers =
+			btrfs_alloc_workqueue("btrfs-scrubwrc", flags,
+					      max_active, 2);
+		if (!fs_info->scrub_wr_completion_workers) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		fs_info->scrub_nocow_workers =
+			btrfs_alloc_workqueue("btrfs-scrubnc", flags, 1, 0);
+		if (!fs_info->scrub_nocow_workers) {
+			ret = -ENOMEM;
+			goto out;
+		}
+	}
+	++fs_info->scrub_workers_refcnt;
+out:
+	return ret;
+}
+
+static noinline_for_stack void scrub_workers_put(struct btrfs_fs_info *fs_info)
+{
+	if (--fs_info->scrub_workers_refcnt == 0) {
+		btrfs_destroy_workqueue(fs_info->scrub_workers);
+		btrfs_destroy_workqueue(fs_info->scrub_wr_completion_workers);
+		btrfs_destroy_workqueue(fs_info->scrub_nocow_workers);
+	}
+	WARN_ON(fs_info->scrub_workers_refcnt < 0);
+}
+
+int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
+		    u64 end, struct btrfs_scrub_progress *progress,
+		    int readonly, int is_dev_replace)
+{
+	struct scrub_ctx *sctx;
+	int ret;
+	struct btrfs_device *dev;
+	struct rcu_string *name;
+
+	if (btrfs_fs_closing(fs_info))
+		return -EINVAL;
+
+	if (fs_info->chunk_root->nodesize > BTRFS_STRIPE_LEN) {
+		/*
+		 * in this case scrub is unable to calculate the checksum
+		 * the way scrub is implemented. Do not handle this
+		 * situation at all because it won't ever happen.
+		 */
+		btrfs_err(fs_info,
+			   "scrub: size assumption nodesize <= BTRFS_STRIPE_LEN (%d <= %d) fails",
+		       fs_info->chunk_root->nodesize, BTRFS_STRIPE_LEN);
+		return -EINVAL;
+	}
+
+	if (fs_info->chunk_root->sectorsize != PAGE_SIZE) {
+		/* not supported for data w/o checksums */
+		btrfs_err(fs_info,
+			   "scrub: size assumption sectorsize != PAGE_SIZE "
+			   "(%d != %lu) fails",
+		       fs_info->chunk_root->sectorsize, PAGE_SIZE);
+		return -EINVAL;
+	}
+
+	if (fs_info->chunk_root->nodesize >
+	    PAGE_SIZE * SCRUB_MAX_PAGES_PER_BLOCK ||
+	    fs_info->chunk_root->sectorsize >
+	    PAGE_SIZE * SCRUB_MAX_PAGES_PER_BLOCK) {
+		/*
+		 * would exhaust the array bounds of pagev member in
+		 * struct scrub_block
+		 */
+		btrfs_err(fs_info, "scrub: size assumption nodesize and sectorsize "
+			   "<= SCRUB_MAX_PAGES_PER_BLOCK (%d <= %d && %d <= %d) fails",
+		       fs_info->chunk_root->nodesize,
+		       SCRUB_MAX_PAGES_PER_BLOCK,
+		       fs_info->chunk_root->sectorsize,
+		       SCRUB_MAX_PAGES_PER_BLOCK);
+		return -EINVAL;
+	}
+
+
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	dev = btrfs_find_device(fs_info, devid, NULL, NULL);
+	if (!dev || (dev->missing && !is_dev_replace)) {
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		return -ENODEV;
+	}
+
+	if (!is_dev_replace && !readonly && !dev->writeable) {
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		rcu_read_lock();
+		name = rcu_dereference(dev->name);
+		btrfs_err(fs_info, "scrub: device %s is not writable",
+			  name->str);
+		rcu_read_unlock();
+		return -EROFS;
+	}
+
+	mutex_lock(&fs_info->scrub_lock);
+	if (!dev->in_fs_metadata || dev->is_tgtdev_for_dev_replace) {
+		mutex_unlock(&fs_info->scrub_lock);
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		return -EIO;
+	}
+
+	btrfs_dev_replace_lock(&fs_info->dev_replace);
+	if (dev->scrub_device ||
+	    (!is_dev_replace &&
+	     btrfs_dev_replace_is_ongoing(&fs_info->dev_replace))) {
+		btrfs_dev_replace_unlock(&fs_info->dev_replace);
+		mutex_unlock(&fs_info->scrub_lock);
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		return -EINPROGRESS;
+	}
+	btrfs_dev_replace_unlock(&fs_info->dev_replace);
+
+	ret = scrub_workers_get(fs_info, is_dev_replace);
+	if (ret) {
+		mutex_unlock(&fs_info->scrub_lock);
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		return ret;
+	}
+
+	sctx = scrub_setup_ctx(dev, is_dev_replace);
+	if (IS_ERR(sctx)) {
+		mutex_unlock(&fs_info->scrub_lock);
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		scrub_workers_put(fs_info);
+		return PTR_ERR(sctx);
+	}
+	sctx->readonly = readonly;
+	dev->scrub_device = sctx;
+	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+
+	/*
+	 * checking @scrub_pause_req here, we can avoid
+	 * race between committing transaction and scrubbing.
+	 */
+	__scrub_blocked_if_needed(fs_info);
+	atomic_inc(&fs_info->scrubs_running);
+	mutex_unlock(&fs_info->scrub_lock);
+
+	if (!is_dev_replace) {
+		/*
+		 * by holding device list mutex, we can
+		 * kick off writing super in log tree sync.
+		 */
+		mutex_lock(&fs_info->fs_devices->device_list_mutex);
+		ret = scrub_supers(sctx, dev);
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+	}
+
+	if (!ret)
+		ret = scrub_enumerate_chunks(sctx, dev, start, end,
+					     is_dev_replace);
+
+	wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);
+	atomic_dec(&fs_info->scrubs_running);
+	wake_up(&fs_info->scrub_pause_wait);
+
+	wait_event(sctx->list_wait, atomic_read(&sctx->workers_pending) == 0);
+
+	if (progress)
+		memcpy(progress, &sctx->stat, sizeof(*progress));
+
+	mutex_lock(&fs_info->scrub_lock);
+	dev->scrub_device = NULL;
+	scrub_workers_put(fs_info);
+	mutex_unlock(&fs_info->scrub_lock);
+
+	scrub_put_ctx(sctx);
+
+	return ret;
+}
+
+void btrfs_scrub_pause(struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+
+	mutex_lock(&fs_info->scrub_lock);
+	atomic_inc(&fs_info->scrub_pause_req);
+	while (atomic_read(&fs_info->scrubs_paused) !=
+	       atomic_read(&fs_info->scrubs_running)) {
+		mutex_unlock(&fs_info->scrub_lock);
+		wait_event(fs_info->scrub_pause_wait,
+			   atomic_read(&fs_info->scrubs_paused) ==
+			   atomic_read(&fs_info->scrubs_running));
+		mutex_lock(&fs_info->scrub_lock);
+	}
+	mutex_unlock(&fs_info->scrub_lock);
+}
+
+void btrfs_scrub_continue(struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+
+	atomic_dec(&fs_info->scrub_pause_req);
+	wake_up(&fs_info->scrub_pause_wait);
+}
+
+int btrfs_scrub_cancel(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->scrub_lock);
+	if (!atomic_read(&fs_info->scrubs_running)) {
+		mutex_unlock(&fs_info->scrub_lock);
+		return -ENOTCONN;
+	}
+
+	atomic_inc(&fs_info->scrub_cancel_req);
+	while (atomic_read(&fs_info->scrubs_running)) {
+		mutex_unlock(&fs_info->scrub_lock);
+		wait_event(fs_info->scrub_pause_wait,
+			   atomic_read(&fs_info->scrubs_running) == 0);
+		mutex_lock(&fs_info->scrub_lock);
+	}
+	atomic_dec(&fs_info->scrub_cancel_req);
+	mutex_unlock(&fs_info->scrub_lock);
+
+	return 0;
+}
+
+int btrfs_scrub_cancel_dev(struct btrfs_fs_info *fs_info,
+			   struct btrfs_device *dev)
+{
+	struct scrub_ctx *sctx;
+
+	mutex_lock(&fs_info->scrub_lock);
+	sctx = dev->scrub_device;
+	if (!sctx) {
+		mutex_unlock(&fs_info->scrub_lock);
+		return -ENOTCONN;
+	}
+	atomic_inc(&sctx->cancel_req);
+	while (dev->scrub_device) {
+		mutex_unlock(&fs_info->scrub_lock);
+		wait_event(fs_info->scrub_pause_wait,
+			   dev->scrub_device == NULL);
+		mutex_lock(&fs_info->scrub_lock);
+	}
+	mutex_unlock(&fs_info->scrub_lock);
+
+	return 0;
+}
+
+int btrfs_scrub_progress(struct btrfs_root *root, u64 devid,
+			 struct btrfs_scrub_progress *progress)
+{
+	struct btrfs_device *dev;
+	struct scrub_ctx *sctx = NULL;
+
+	mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
+	dev = btrfs_find_device(root->fs_info, devid, NULL, NULL);
+	if (dev)
+		sctx = dev->scrub_device;
+	if (sctx)
+		memcpy(progress, &sctx->stat, sizeof(*progress));
+	mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
+
+	return dev ? (sctx ? 0 : -ENOTCONN) : -ENODEV;
+}
+
+static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
+			       u64 extent_logical, u64 extent_len,
+			       u64 *extent_physical,
+			       struct btrfs_device **extent_dev,
+			       int *extent_mirror_num)
+{
+	u64 mapped_length;
+	struct btrfs_bio *bbio = NULL;
+	int ret;
+
+	mapped_length = extent_len;
+	ret = btrfs_map_block(fs_info, READ, extent_logical,
+			      &mapped_length, &bbio, 0);
+	if (ret || !bbio || mapped_length < extent_len ||
+	    !bbio->stripes[0].dev->bdev) {
+		btrfs_put_bbio(bbio);
+		return;
+	}
+
+	*extent_physical = bbio->stripes[0].physical;
+	*extent_mirror_num = bbio->mirror_num;
+	*extent_dev = bbio->stripes[0].dev;
+	btrfs_put_bbio(bbio);
+}
+
+static int scrub_setup_wr_ctx(struct scrub_ctx *sctx,
+			      struct scrub_wr_ctx *wr_ctx,
+			      struct btrfs_fs_info *fs_info,
+			      struct btrfs_device *dev,
+			      int is_dev_replace)
+{
+	WARN_ON(wr_ctx->wr_curr_bio != NULL);
+
+	mutex_init(&wr_ctx->wr_lock);
+	wr_ctx->wr_curr_bio = NULL;
+	if (!is_dev_replace)
+		return 0;
+
+	WARN_ON(!dev->bdev);
+	wr_ctx->pages_per_wr_bio = min_t(int, SCRUB_PAGES_PER_WR_BIO,
+					 bio_get_nr_vecs(dev->bdev));
+	wr_ctx->tgtdev = dev;
+	atomic_set(&wr_ctx->flush_all_writes, 0);
+	return 0;
+}
+
+static void scrub_free_wr_ctx(struct scrub_wr_ctx *wr_ctx)
+{
+	mutex_lock(&wr_ctx->wr_lock);
+	kfree(wr_ctx->wr_curr_bio);
+	wr_ctx->wr_curr_bio = NULL;
+	mutex_unlock(&wr_ctx->wr_lock);
+}
+
+static int copy_nocow_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
+			    int mirror_num, u64 physical_for_dev_replace)
+{
+	struct scrub_copy_nocow_ctx *nocow_ctx;
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+
+	nocow_ctx = kzalloc(sizeof(*nocow_ctx), GFP_NOFS);
+	if (!nocow_ctx) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+
+	scrub_pending_trans_workers_inc(sctx);
+
+	nocow_ctx->sctx = sctx;
+	nocow_ctx->logical = logical;
+	nocow_ctx->len = len;
+	nocow_ctx->mirror_num = mirror_num;
+	nocow_ctx->physical_for_dev_replace = physical_for_dev_replace;
+	btrfs_init_work(&nocow_ctx->work, btrfs_scrubnc_helper,
+			copy_nocow_pages_worker, NULL, NULL);
+	INIT_LIST_HEAD(&nocow_ctx->inodes);
+	btrfs_queue_work(fs_info->scrub_nocow_workers,
+			 &nocow_ctx->work);
+
+	return 0;
+}
+
+static int record_inode_for_nocow(u64 inum, u64 offset, u64 root, void *ctx)
+{
+	struct scrub_copy_nocow_ctx *nocow_ctx = ctx;
+	struct scrub_nocow_inode *nocow_inode;
+
+	nocow_inode = kzalloc(sizeof(*nocow_inode), GFP_NOFS);
+	if (!nocow_inode)
+		return -ENOMEM;
+	nocow_inode->inum = inum;
+	nocow_inode->offset = offset;
+	nocow_inode->root = root;
+	list_add_tail(&nocow_inode->list, &nocow_ctx->inodes);
+	return 0;
+}
+
+#define COPY_COMPLETE 1
+
+static void copy_nocow_pages_worker(struct btrfs_work *work)
+{
+	struct scrub_copy_nocow_ctx *nocow_ctx =
+		container_of(work, struct scrub_copy_nocow_ctx, work);
+	struct scrub_ctx *sctx = nocow_ctx->sctx;
+	u64 logical = nocow_ctx->logical;
+	u64 len = nocow_ctx->len;
+	int mirror_num = nocow_ctx->mirror_num;
+	u64 physical_for_dev_replace = nocow_ctx->physical_for_dev_replace;
+	int ret;
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_path *path;
+	struct btrfs_root *root;
+	int not_written = 0;
+
+	fs_info = sctx->dev_root->fs_info;
+	root = fs_info->extent_root;
+
+	path = btrfs_alloc_path();
+	if (!path) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		not_written = 1;
+		goto out;
+	}
+
+	trans = btrfs_join_transaction(root);
+	if (IS_ERR(trans)) {
+		not_written = 1;
+		goto out;
+	}
+
+	ret = iterate_inodes_from_logical(logical, fs_info, path,
+					  record_inode_for_nocow, nocow_ctx);
+	if (ret != 0 && ret != -ENOENT) {
+		btrfs_warn(fs_info, "iterate_inodes_from_logical() failed: log %llu, "
+			"phys %llu, len %llu, mir %u, ret %d",
+			logical, physical_for_dev_replace, len, mirror_num,
+			ret);
+		not_written = 1;
+		goto out;
+	}
+
+	btrfs_end_transaction(trans, root);
+	trans = NULL;
+	while (!list_empty(&nocow_ctx->inodes)) {
+		struct scrub_nocow_inode *entry;
+		entry = list_first_entry(&nocow_ctx->inodes,
+					 struct scrub_nocow_inode,
+					 list);
+		list_del_init(&entry->list);
+		ret = copy_nocow_pages_for_inode(entry->inum, entry->offset,
+						 entry->root, nocow_ctx);
+		kfree(entry);
+		if (ret == COPY_COMPLETE) {
+			ret = 0;
+			break;
+		} else if (ret) {
+			break;
+		}
+	}
+out:
+	while (!list_empty(&nocow_ctx->inodes)) {
+		struct scrub_nocow_inode *entry;
+		entry = list_first_entry(&nocow_ctx->inodes,
+					 struct scrub_nocow_inode,
+					 list);
+		list_del_init(&entry->list);
+		kfree(entry);
+	}
+	if (trans && !IS_ERR(trans))
+		btrfs_end_transaction(trans, root);
+	if (not_written)
+		btrfs_dev_replace_stats_inc(&fs_info->dev_replace.
+					    num_uncorrectable_read_errors);
+
+	btrfs_free_path(path);
+	kfree(nocow_ctx);
+
+	scrub_pending_trans_workers_dec(sctx);
+}
+
+static int check_extent_to_block(struct inode *inode, u64 start, u64 len,
+				 u64 logical)
+{
+	struct extent_state *cached_state = NULL;
+	struct btrfs_ordered_extent *ordered;
+	struct extent_io_tree *io_tree;
+	struct extent_map *em;
+	u64 lockstart = start, lockend = start + len - 1;
+	int ret = 0;
+
+	io_tree = &BTRFS_I(inode)->io_tree;
+
+	lock_extent_bits(io_tree, lockstart, lockend, 0, &cached_state);
+	ordered = btrfs_lookup_ordered_range(inode, lockstart, len);
+	if (ordered) {
+		btrfs_put_ordered_extent(ordered);
+		ret = 1;
+		goto out_unlock;
+	}
+
+	em = btrfs_get_extent(inode, NULL, 0, start, len, 0);
+	if (IS_ERR(em)) {
+		ret = PTR_ERR(em);
+		goto out_unlock;
+	}
+
+	/*
+	 * This extent does not actually cover the logical extent anymore,
+	 * move on to the next inode.
+	 */
+	if (em->block_start > logical ||
+	    em->block_start + em->block_len < logical + len) {
+		free_extent_map(em);
+		ret = 1;
+		goto out_unlock;
+	}
+	free_extent_map(em);
+
+out_unlock:
+	unlock_extent_cached(io_tree, lockstart, lockend, &cached_state,
+			     GFP_NOFS);
+	return ret;
+}
+
+static int copy_nocow_pages_for_inode(u64 inum, u64 offset, u64 root,
+				      struct scrub_copy_nocow_ctx *nocow_ctx)
+{
+	struct btrfs_fs_info *fs_info = nocow_ctx->sctx->dev_root->fs_info;
+	struct btrfs_key key;
+	struct inode *inode;
+	struct page *page;
+	struct btrfs_root *local_root;
+	struct extent_io_tree *io_tree;
+	u64 physical_for_dev_replace;
+	u64 nocow_ctx_logical;
+	u64 len = nocow_ctx->len;
+	unsigned long index;
+	int srcu_index;
+	int ret = 0;
+	int err = 0;
+
+	key.objectid = root;
+	key.type = BTRFS_ROOT_ITEM_KEY;
+	key.offset = (u64)-1;
+
+	srcu_index = srcu_read_lock(&fs_info->subvol_srcu);
+
+	local_root = btrfs_read_fs_root_no_name(fs_info, &key);
+	if (IS_ERR(local_root)) {
+		srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
+		return PTR_ERR(local_root);
+	}
+
+	key.type = BTRFS_INODE_ITEM_KEY;
+	key.objectid = inum;
+	key.offset = 0;
+	inode = btrfs_iget(fs_info->sb, &key, local_root, NULL);
+	srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
+	if (IS_ERR(inode))
+		return PTR_ERR(inode);
+
+	/* Avoid truncate/dio/punch hole.. */
+	mutex_lock(&inode->i_mutex);
+	inode_dio_wait(inode);
+
+	physical_for_dev_replace = nocow_ctx->physical_for_dev_replace;
+	io_tree = &BTRFS_I(inode)->io_tree;
+	nocow_ctx_logical = nocow_ctx->logical;
+
+	ret = check_extent_to_block(inode, offset, len, nocow_ctx_logical);
+	if (ret) {
+		ret = ret > 0 ? 0 : ret;
+		goto out;
+	}
+
+	while (len >= PAGE_CACHE_SIZE) {
+		index = offset >> PAGE_CACHE_SHIFT;
+again:
+		page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
+		if (!page) {
+			btrfs_err(fs_info, "find_or_create_page() failed");
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		if (PageUptodate(page)) {
+			if (PageDirty(page))
+				goto next_page;
+		} else {
+			ClearPageError(page);
+			err = extent_read_full_page(io_tree, page,
+							   btrfs_get_extent,
+							   nocow_ctx->mirror_num);
+			if (err) {
+				ret = err;
+				goto next_page;
+			}
+
+			lock_page(page);
+			/*
+			 * If the page has been remove from the page cache,
+			 * the data on it is meaningless, because it may be
+			 * old one, the new data may be written into the new
+			 * page in the page cache.
+			 */
+			if (page->mapping != inode->i_mapping) {
+				unlock_page(page);
+				page_cache_release(page);
+				goto again;
+			}
+			if (!PageUptodate(page)) {
+				ret = -EIO;
+				goto next_page;
+			}
+		}
+
+		ret = check_extent_to_block(inode, offset, len,
+					    nocow_ctx_logical);
+		if (ret) {
+			ret = ret > 0 ? 0 : ret;
+			goto next_page;
+		}
+
+		err = write_page_nocow(nocow_ctx->sctx,
+				       physical_for_dev_replace, page);
+		if (err)
+			ret = err;
+next_page:
+		unlock_page(page);
+		page_cache_release(page);
+
+		if (ret)
+			break;
+
+		offset += PAGE_CACHE_SIZE;
+		physical_for_dev_replace += PAGE_CACHE_SIZE;
+		nocow_ctx_logical += PAGE_CACHE_SIZE;
+		len -= PAGE_CACHE_SIZE;
+	}
+	ret = COPY_COMPLETE;
+out:
+	mutex_unlock(&inode->i_mutex);
+	iput(inode);
+	return ret;
+}
+
+static int write_page_nocow(struct scrub_ctx *sctx,
+			    u64 physical_for_dev_replace, struct page *page)
+{
+	struct bio *bio;
+	struct btrfs_device *dev;
+	int ret;
+
+	dev = sctx->wr_ctx.tgtdev;
+	if (!dev)
+		return -EIO;
+	if (!dev->bdev) {
+		printk_ratelimited(KERN_WARNING
+			"BTRFS: scrub write_page_nocow(bdev == NULL) is unexpected!\n");
+		return -EIO;
+	}
+	bio = btrfs_io_bio_alloc(GFP_NOFS, 1);
+	if (!bio) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+	bio->bi_iter.bi_size = 0;
+	bio->bi_iter.bi_sector = physical_for_dev_replace >> 9;
+	bio->bi_bdev = dev->bdev;
+	ret = bio_add_page(bio, page, PAGE_CACHE_SIZE, 0);
+	if (ret != PAGE_CACHE_SIZE) {
+leave_with_eio:
+		bio_put(bio);
+		btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_WRITE_ERRS);
+		return -EIO;
+	}
+
+	if (btrfsic_submit_bio_wait(WRITE_SYNC, bio))
+		goto leave_with_eio;
+
+	bio_put(bio);
+	return 0;
+}