summaryrefslogtreecommitdiff
path: root/Documentation/filesystems/porting
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/filesystems/porting')
-rw-r--r--Documentation/filesystems/porting485
1 files changed, 485 insertions, 0 deletions
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
new file mode 100644
index 000000000..e69274de8
--- /dev/null
+++ b/Documentation/filesystems/porting
@@ -0,0 +1,485 @@
+Changes since 2.5.0:
+
+---
+[recommended]
+
+New helpers: sb_bread(), sb_getblk(), sb_find_get_block(), set_bh(),
+ sb_set_blocksize() and sb_min_blocksize().
+
+Use them.
+
+(sb_find_get_block() replaces 2.4's get_hash_table())
+
+---
+[recommended]
+
+New methods: ->alloc_inode() and ->destroy_inode().
+
+Remove inode->u.foo_inode_i
+Declare
+ struct foo_inode_info {
+ /* fs-private stuff */
+ struct inode vfs_inode;
+ };
+ static inline struct foo_inode_info *FOO_I(struct inode *inode)
+ {
+ return list_entry(inode, struct foo_inode_info, vfs_inode);
+ }
+
+Use FOO_I(inode) instead of &inode->u.foo_inode_i;
+
+Add foo_alloc_inode() and foo_destroy_inode() - the former should allocate
+foo_inode_info and return the address of ->vfs_inode, the latter should free
+FOO_I(inode) (see in-tree filesystems for examples).
+
+Make them ->alloc_inode and ->destroy_inode in your super_operations.
+
+Keep in mind that now you need explicit initialization of private data
+typically between calling iget_locked() and unlocking the inode.
+
+At some point that will become mandatory.
+
+---
+[mandatory]
+
+Change of file_system_type method (->read_super to ->get_sb)
+
+->read_super() is no more. Ditto for DECLARE_FSTYPE and DECLARE_FSTYPE_DEV.
+
+Turn your foo_read_super() into a function that would return 0 in case of
+success and negative number in case of error (-EINVAL unless you have more
+informative error value to report). Call it foo_fill_super(). Now declare
+
+int foo_get_sb(struct file_system_type *fs_type,
+ int flags, const char *dev_name, void *data, struct vfsmount *mnt)
+{
+ return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
+ mnt);
+}
+
+(or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of
+filesystem).
+
+Replace DECLARE_FSTYPE... with explicit initializer and have ->get_sb set as
+foo_get_sb.
+
+---
+[mandatory]
+
+Locking change: ->s_vfs_rename_sem is taken only by cross-directory renames.
+Most likely there is no need to change anything, but if you relied on
+global exclusion between renames for some internal purpose - you need to
+change your internal locking. Otherwise exclusion warranties remain the
+same (i.e. parents and victim are locked, etc.).
+
+---
+[informational]
+
+Now we have the exclusion between ->lookup() and directory removal (by
+->rmdir() and ->rename()). If you used to need that exclusion and do
+it by internal locking (most of filesystems couldn't care less) - you
+can relax your locking.
+
+---
+[mandatory]
+
+->lookup(), ->truncate(), ->create(), ->unlink(), ->mknod(), ->mkdir(),
+->rmdir(), ->link(), ->lseek(), ->symlink(), ->rename()
+and ->readdir() are called without BKL now. Grab it on entry, drop upon return
+- that will guarantee the same locking you used to have. If your method or its
+parts do not need BKL - better yet, now you can shift lock_kernel() and
+unlock_kernel() so that they would protect exactly what needs to be
+protected.
+
+---
+[mandatory]
+
+BKL is also moved from around sb operations. BKL should have been shifted into
+individual fs sb_op functions. If you don't need it, remove it.
+
+---
+[informational]
+
+check for ->link() target not being a directory is done by callers. Feel
+free to drop it...
+
+---
+[informational]
+
+->link() callers hold ->i_mutex on the object we are linking to. Some of your
+problems might be over...
+
+---
+[mandatory]
+
+new file_system_type method - kill_sb(superblock). If you are converting
+an existing filesystem, set it according to ->fs_flags:
+ FS_REQUIRES_DEV - kill_block_super
+ FS_LITTER - kill_litter_super
+ neither - kill_anon_super
+FS_LITTER is gone - just remove it from fs_flags.
+
+---
+[mandatory]
+
+ FS_SINGLE is gone (actually, that had happened back when ->get_sb()
+went in - and hadn't been documented ;-/). Just remove it from fs_flags
+(and see ->get_sb() entry for other actions).
+
+---
+[mandatory]
+
+->setattr() is called without BKL now. Caller _always_ holds ->i_mutex, so
+watch for ->i_mutex-grabbing code that might be used by your ->setattr().
+Callers of notify_change() need ->i_mutex now.
+
+---
+[recommended]
+
+New super_block field "struct export_operations *s_export_op" for
+explicit support for exporting, e.g. via NFS. The structure is fully
+documented at its declaration in include/linux/fs.h, and in
+Documentation/filesystems/nfs/Exporting.
+
+Briefly it allows for the definition of decode_fh and encode_fh operations
+to encode and decode filehandles, and allows the filesystem to use
+a standard helper function for decode_fh, and provide file-system specific
+support for this helper, particularly get_parent.
+
+It is planned that this will be required for exporting once the code
+settles down a bit.
+
+[mandatory]
+
+s_export_op is now required for exporting a filesystem.
+isofs, ext2, ext3, resierfs, fat
+can be used as examples of very different filesystems.
+
+---
+[mandatory]
+
+iget4() and the read_inode2 callback have been superseded by iget5_locked()
+which has the following prototype,
+
+ struct inode *iget5_locked(struct super_block *sb, unsigned long ino,
+ int (*test)(struct inode *, void *),
+ int (*set)(struct inode *, void *),
+ void *data);
+
+'test' is an additional function that can be used when the inode
+number is not sufficient to identify the actual file object. 'set'
+should be a non-blocking function that initializes those parts of a
+newly created inode to allow the test function to succeed. 'data' is
+passed as an opaque value to both test and set functions.
+
+When the inode has been created by iget5_locked(), it will be returned with the
+I_NEW flag set and will still be locked. The filesystem then needs to finalize
+the initialization. Once the inode is initialized it must be unlocked by
+calling unlock_new_inode().
+
+The filesystem is responsible for setting (and possibly testing) i_ino
+when appropriate. There is also a simpler iget_locked function that
+just takes the superblock and inode number as arguments and does the
+test and set for you.
+
+e.g.
+ inode = iget_locked(sb, ino);
+ if (inode->i_state & I_NEW) {
+ err = read_inode_from_disk(inode);
+ if (err < 0) {
+ iget_failed(inode);
+ return err;
+ }
+ unlock_new_inode(inode);
+ }
+
+Note that if the process of setting up a new inode fails, then iget_failed()
+should be called on the inode to render it dead, and an appropriate error
+should be passed back to the caller.
+
+---
+[recommended]
+
+->getattr() finally getting used. See instances in nfs, minix, etc.
+
+---
+[mandatory]
+
+->revalidate() is gone. If your filesystem had it - provide ->getattr()
+and let it call whatever you had as ->revlidate() + (for symlinks that
+had ->revalidate()) add calls in ->follow_link()/->readlink().
+
+---
+[mandatory]
+
+->d_parent changes are not protected by BKL anymore. Read access is safe
+if at least one of the following is true:
+ * filesystem has no cross-directory rename()
+ * we know that parent had been locked (e.g. we are looking at
+->d_parent of ->lookup() argument).
+ * we are called from ->rename().
+ * the child's ->d_lock is held
+Audit your code and add locking if needed. Notice that any place that is
+not protected by the conditions above is risky even in the old tree - you
+had been relying on BKL and that's prone to screwups. Old tree had quite
+a few holes of that kind - unprotected access to ->d_parent leading to
+anything from oops to silent memory corruption.
+
+---
+[mandatory]
+
+ FS_NOMOUNT is gone. If you use it - just set MS_NOUSER in flags
+(see rootfs for one kind of solution and bdev/socket/pipe for another).
+
+---
+[recommended]
+
+ Use bdev_read_only(bdev) instead of is_read_only(kdev). The latter
+is still alive, but only because of the mess in drivers/s390/block/dasd.c.
+As soon as it gets fixed is_read_only() will die.
+
+---
+[mandatory]
+
+->permission() is called without BKL now. Grab it on entry, drop upon
+return - that will guarantee the same locking you used to have. If
+your method or its parts do not need BKL - better yet, now you can
+shift lock_kernel() and unlock_kernel() so that they would protect
+exactly what needs to be protected.
+
+---
+[mandatory]
+
+->statfs() is now called without BKL held. BKL should have been
+shifted into individual fs sb_op functions where it's not clear that
+it's safe to remove it. If you don't need it, remove it.
+
+---
+[mandatory]
+
+ is_read_only() is gone; use bdev_read_only() instead.
+
+---
+[mandatory]
+
+ destroy_buffers() is gone; use invalidate_bdev().
+
+---
+[mandatory]
+
+ fsync_dev() is gone; use fsync_bdev(). NOTE: lvm breakage is
+deliberate; as soon as struct block_device * is propagated in a reasonable
+way by that code fixing will become trivial; until then nothing can be
+done.
+
+[mandatory]
+
+ block truncatation on error exit from ->write_begin, and ->direct_IO
+moved from generic methods (block_write_begin, cont_write_begin,
+nobh_write_begin, blockdev_direct_IO*) to callers. Take a look at
+ext2_write_failed and callers for an example.
+
+[mandatory]
+
+ ->truncate is gone. The whole truncate sequence needs to be
+implemented in ->setattr, which is now mandatory for filesystems
+implementing on-disk size changes. Start with a copy of the old inode_setattr
+and vmtruncate, and the reorder the vmtruncate + foofs_vmtruncate sequence to
+be in order of zeroing blocks using block_truncate_page or similar helpers,
+size update and on finally on-disk truncation which should not fail.
+inode_change_ok now includes the size checks for ATTR_SIZE and must be called
+in the beginning of ->setattr unconditionally.
+
+[mandatory]
+
+ ->clear_inode() and ->delete_inode() are gone; ->evict_inode() should
+be used instead. It gets called whenever the inode is evicted, whether it has
+remaining links or not. Caller does *not* evict the pagecache or inode-associated
+metadata buffers; the method has to use truncate_inode_pages_final() to get rid
+of those. Caller makes sure async writeback cannot be running for the inode while
+(or after) ->evict_inode() is called.
+
+ ->drop_inode() returns int now; it's called on final iput() with
+inode->i_lock held and it returns true if filesystems wants the inode to be
+dropped. As before, generic_drop_inode() is still the default and it's been
+updated appropriately. generic_delete_inode() is also alive and it consists
+simply of return 1. Note that all actual eviction work is done by caller after
+->drop_inode() returns.
+
+ As before, clear_inode() must be called exactly once on each call of
+->evict_inode() (as it used to be for each call of ->delete_inode()). Unlike
+before, if you are using inode-associated metadata buffers (i.e.
+mark_buffer_dirty_inode()), it's your responsibility to call
+invalidate_inode_buffers() before clear_inode().
+
+ NOTE: checking i_nlink in the beginning of ->write_inode() and bailing out
+if it's zero is not *and* *never* *had* *been* enough. Final unlink() and iput()
+may happen while the inode is in the middle of ->write_inode(); e.g. if you blindly
+free the on-disk inode, you may end up doing that while ->write_inode() is writing
+to it.
+
+---
+[mandatory]
+
+ .d_delete() now only advises the dcache as to whether or not to cache
+unreferenced dentries, and is now only called when the dentry refcount goes to
+0. Even on 0 refcount transition, it must be able to tolerate being called 0,
+1, or more times (eg. constant, idempotent).
+
+---
+[mandatory]
+
+ .d_compare() calling convention and locking rules are significantly
+changed. Read updated documentation in Documentation/filesystems/vfs.txt (and
+look at examples of other filesystems) for guidance.
+
+---
+[mandatory]
+
+ .d_hash() calling convention and locking rules are significantly
+changed. Read updated documentation in Documentation/filesystems/vfs.txt (and
+look at examples of other filesystems) for guidance.
+
+---
+[mandatory]
+ dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c
+for details of what locks to replace dcache_lock with in order to protect
+particular things. Most of the time, a filesystem only needs ->d_lock, which
+protects *all* the dcache state of a given dentry.
+
+--
+[mandatory]
+
+ Filesystems must RCU-free their inodes, if they can have been accessed
+via rcu-walk path walk (basically, if the file can have had a path name in the
+vfs namespace).
+
+ Even though i_dentry and i_rcu share storage in a union, we will
+initialize the former in inode_init_always(), so just leave it alone in
+the callback. It used to be necessary to clean it there, but not anymore
+(starting at 3.2).
+
+--
+[recommended]
+ vfs now tries to do path walking in "rcu-walk mode", which avoids
+atomic operations and scalability hazards on dentries and inodes (see
+Documentation/filesystems/path-lookup.txt). d_hash and d_compare changes
+(above) are examples of the changes required to support this. For more complex
+filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so
+no changes are required to the filesystem. However, this is costly and loses
+the benefits of rcu-walk mode. We will begin to add filesystem callbacks that
+are rcu-walk aware, shown below. Filesystems should take advantage of this
+where possible.
+
+--
+[mandatory]
+ d_revalidate is a callback that is made on every path element (if
+the filesystem provides it), which requires dropping out of rcu-walk mode. This
+may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be
+returned if the filesystem cannot handle rcu-walk. See
+Documentation/filesystems/vfs.txt for more details.
+
+ permission and check_acl are inode permission checks that are called
+on many or all directory inodes on the way down a path walk (to check for
+exec permission). These must now be rcu-walk aware (flags & IPERM_FLAG_RCU).
+See Documentation/filesystems/vfs.txt for more details.
+
+--
+[mandatory]
+ In ->fallocate() you must check the mode option passed in. If your
+filesystem does not support hole punching (deallocating space in the middle of a
+file) you must return -EOPNOTSUPP if FALLOC_FL_PUNCH_HOLE is set in mode.
+Currently you can only have FALLOC_FL_PUNCH_HOLE with FALLOC_FL_KEEP_SIZE set,
+so the i_size should not change when hole punching, even when puching the end of
+a file off.
+
+--
+[mandatory]
+ ->get_sb() is gone. Switch to use of ->mount(). Typically it's just
+a matter of switching from calling get_sb_... to mount_... and changing the
+function type. If you were doing it manually, just switch from setting ->mnt_root
+to some pointer to returning that pointer. On errors return ERR_PTR(...).
+
+--
+[mandatory]
+ ->permission() and generic_permission()have lost flags
+argument; instead of passing IPERM_FLAG_RCU we add MAY_NOT_BLOCK into mask.
+ generic_permission() has also lost the check_acl argument; ACL checking
+has been taken to VFS and filesystems need to provide a non-NULL ->i_op->get_acl
+to read an ACL from disk.
+
+--
+[mandatory]
+ If you implement your own ->llseek() you must handle SEEK_HOLE and
+SEEK_DATA. You can hanle this by returning -EINVAL, but it would be nicer to
+support it in some way. The generic handler assumes that the entire file is
+data and there is a virtual hole at the end of the file. So if the provided
+offset is less than i_size and SEEK_DATA is specified, return the same offset.
+If the above is true for the offset and you are given SEEK_HOLE, return the end
+of the file. If the offset is i_size or greater return -ENXIO in either case.
+
+[mandatory]
+ If you have your own ->fsync() you must make sure to call
+filemap_write_and_wait_range() so that all dirty pages are synced out properly.
+You must also keep in mind that ->fsync() is not called with i_mutex held
+anymore, so if you require i_mutex locking you must make sure to take it and
+release it yourself.
+
+--
+[mandatory]
+ d_alloc_root() is gone, along with a lot of bugs caused by code
+misusing it. Replacement: d_make_root(inode). The difference is,
+d_make_root() drops the reference to inode if dentry allocation fails.
+
+--
+[mandatory]
+ The witch is dead! Well, 2/3 of it, anyway. ->d_revalidate() and
+->lookup() do *not* take struct nameidata anymore; just the flags.
+--
+[mandatory]
+ ->create() doesn't take struct nameidata *; unlike the previous
+two, it gets "is it an O_EXCL or equivalent?" boolean argument. Note that
+local filesystems can ignore tha argument - they are guaranteed that the
+object doesn't exist. It's remote/distributed ones that might care...
+--
+[mandatory]
+ FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate()
+in your dentry operations instead.
+--
+[mandatory]
+ vfs_readdir() is gone; switch to iterate_dir() instead
+--
+[mandatory]
+ ->readdir() is gone now; switch to ->iterate()
+[mandatory]
+ vfs_follow_link has been removed. Filesystems must use nd_set_link
+ from ->follow_link for normal symlinks, or nd_jump_link for magic
+ /proc/<pid> style links.
+--
+[mandatory]
+ iget5_locked()/ilookup5()/ilookup5_nowait() test() callback used to be
+ called with both ->i_lock and inode_hash_lock held; the former is *not*
+ taken anymore, so verify that your callbacks do not rely on it (none
+ of the in-tree instances did). inode_hash_lock is still held,
+ of course, so they are still serialized wrt removal from inode hash,
+ as well as wrt set() callback of iget5_locked().
+--
+[mandatory]
+ d_materialise_unique() is gone; d_splice_alias() does everything you
+ need now. Remember that they have opposite orders of arguments ;-/
+--
+[mandatory]
+ f_dentry is gone; use f_path.dentry, or, better yet, see if you can avoid
+ it entirely.
+--
+[mandatory]
+ never call ->read() and ->write() directly; use __vfs_{read,write} or
+ wrappers; instead of checking for ->write or ->read being NULL, look for
+ FMODE_CAN_{WRITE,READ} in file->f_mode.
+--
+[mandatory]
+ do _not_ use new_sync_{read,write} for ->read/->write; leave it NULL
+ instead.
+--
+[mandatory]
+ ->aio_read/->aio_write are gone. Use ->read_iter/->write_iter.
generated by cgit v1.2.3-54-g00ecf (git 2.45.2) at 2024-11-19 05:04:52 +0000