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+MOTIVATION
+
+Cleancache is a new optional feature provided by the VFS layer that
+potentially dramatically increases page cache effectiveness for
+many workloads in many environments at a negligible cost.
+
+Cleancache can be thought of as a page-granularity victim cache for clean
+pages that the kernel's pageframe replacement algorithm (PFRA) would like
+to keep around, but can't since there isn't enough memory.  So when the
+PFRA "evicts" a page, it first attempts to use cleancache code to
+put the data contained in that page into "transcendent memory", memory
+that is not directly accessible or addressable by the kernel and is
+of unknown and possibly time-varying size.
+
+Later, when a cleancache-enabled filesystem wishes to access a page
+in a file on disk, it first checks cleancache to see if it already
+contains it; if it does, the page of data is copied into the kernel
+and a disk access is avoided.
+
+Transcendent memory "drivers" for cleancache are currently implemented
+in Xen (using hypervisor memory) and zcache (using in-kernel compressed
+memory) and other implementations are in development.
+
+FAQs are included below.
+
+IMPLEMENTATION OVERVIEW
+
+A cleancache "backend" that provides transcendent memory registers itself
+to the kernel's cleancache "frontend" by calling cleancache_register_ops,
+passing a pointer to a cleancache_ops structure with funcs set appropriately.
+The functions provided must conform to certain semantics as follows:
+
+Most important, cleancache is "ephemeral".  Pages which are copied into
+cleancache have an indefinite lifetime which is completely unknowable
+by the kernel and so may or may not still be in cleancache at any later time.
+Thus, as its name implies, cleancache is not suitable for dirty pages.
+Cleancache has complete discretion over what pages to preserve and what
+pages to discard and when.
+
+Mounting a cleancache-enabled filesystem should call "init_fs" to obtain a
+pool id which, if positive, must be saved in the filesystem's superblock;
+a negative return value indicates failure.  A "put_page" will copy a
+(presumably about-to-be-evicted) page into cleancache and associate it with
+the pool id, a file key, and a page index into the file.  (The combination
+of a pool id, a file key, and an index is sometimes called a "handle".)
+A "get_page" will copy the page, if found, from cleancache into kernel memory.
+An "invalidate_page" will ensure the page no longer is present in cleancache;
+an "invalidate_inode" will invalidate all pages associated with the specified
+file; and, when a filesystem is unmounted, an "invalidate_fs" will invalidate
+all pages in all files specified by the given pool id and also surrender
+the pool id.
+
+An "init_shared_fs", like init_fs, obtains a pool id but tells cleancache
+to treat the pool as shared using a 128-bit UUID as a key.  On systems
+that may run multiple kernels (such as hard partitioned or virtualized
+systems) that may share a clustered filesystem, and where cleancache
+may be shared among those kernels, calls to init_shared_fs that specify the
+same UUID will receive the same pool id, thus allowing the pages to
+be shared.  Note that any security requirements must be imposed outside
+of the kernel (e.g. by "tools" that control cleancache).  Or a
+cleancache implementation can simply disable shared_init by always
+returning a negative value.
+
+If a get_page is successful on a non-shared pool, the page is invalidated
+(thus making cleancache an "exclusive" cache).  On a shared pool, the page
+is NOT invalidated on a successful get_page so that it remains accessible to
+other sharers.  The kernel is responsible for ensuring coherency between
+cleancache (shared or not), the page cache, and the filesystem, using
+cleancache invalidate operations as required.
+
+Note that cleancache must enforce put-put-get coherency and get-get
+coherency.  For the former, if two puts are made to the same handle but
+with different data, say AAA by the first put and BBB by the second, a
+subsequent get can never return the stale data (AAA).  For get-get coherency,
+if a get for a given handle fails, subsequent gets for that handle will
+never succeed unless preceded by a successful put with that handle.
+
+Last, cleancache provides no SMP serialization guarantees; if two
+different Linux threads are simultaneously putting and invalidating a page
+with the same handle, the results are indeterminate.  Callers must
+lock the page to ensure serial behavior.
+
+CLEANCACHE PERFORMANCE METRICS
+
+If properly configured, monitoring of cleancache is done via debugfs in
+the /sys/kernel/debug/cleancache directory.  The effectiveness of cleancache
+can be measured (across all filesystems) with:
+
+succ_gets	- number of gets that were successful
+failed_gets	- number of gets that failed
+puts		- number of puts attempted (all "succeed")
+invalidates	- number of invalidates attempted
+
+A backend implementation may provide additional metrics.
+
+FAQ
+
+1) Where's the value? (Andrew Morton)
+
+Cleancache provides a significant performance benefit to many workloads
+in many environments with negligible overhead by improving the
+effectiveness of the pagecache.  Clean pagecache pages are
+saved in transcendent memory (RAM that is otherwise not directly
+addressable to the kernel); fetching those pages later avoids "refaults"
+and thus disk reads.
+
+Cleancache (and its sister code "frontswap") provide interfaces for
+this transcendent memory (aka "tmem"), which conceptually lies between
+fast kernel-directly-addressable RAM and slower DMA/asynchronous devices.
+Disallowing direct kernel or userland reads/writes to tmem
+is ideal when data is transformed to a different form and size (such
+as with compression) or secretly moved (as might be useful for write-
+balancing for some RAM-like devices).  Evicted page-cache pages (and
+swap pages) are a great use for this kind of slower-than-RAM-but-much-
+faster-than-disk transcendent memory, and the cleancache (and frontswap)
+"page-object-oriented" specification provides a nice way to read and
+write -- and indirectly "name" -- the pages.
+
+In the virtual case, the whole point of virtualization is to statistically
+multiplex physical resources across the varying demands of multiple
+virtual machines.  This is really hard to do with RAM and efforts to
+do it well with no kernel change have essentially failed (except in some
+well-publicized special-case workloads).  Cleancache -- and frontswap --
+with a fairly small impact on the kernel, provide a huge amount
+of flexibility for more dynamic, flexible RAM multiplexing.
+Specifically, the Xen Transcendent Memory backend allows otherwise
+"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
+virtual machines, but the pages can be compressed and deduplicated to
+optimize RAM utilization.  And when guest OS's are induced to surrender
+underutilized RAM (e.g. with "self-ballooning"), page cache pages
+are the first to go, and cleancache allows those pages to be
+saved and reclaimed if overall host system memory conditions allow.
+
+And the identical interface used for cleancache can be used in
+physical systems as well.  The zcache driver acts as a memory-hungry
+device that stores pages of data in a compressed state.  And
+the proposed "RAMster" driver shares RAM across multiple physical
+systems.
+
+2) Why does cleancache have its sticky fingers so deep inside the
+   filesystems and VFS? (Andrew Morton and Christoph Hellwig)
+
+The core hooks for cleancache in VFS are in most cases a single line
+and the minimum set are placed precisely where needed to maintain
+coherency (via cleancache_invalidate operations) between cleancache,
+the page cache, and disk.  All hooks compile into nothingness if
+cleancache is config'ed off and turn into a function-pointer-
+compare-to-NULL if config'ed on but no backend claims the ops
+functions, or to a compare-struct-element-to-negative if a
+backend claims the ops functions but a filesystem doesn't enable
+cleancache.
+
+Some filesystems are built entirely on top of VFS and the hooks
+in VFS are sufficient, so don't require an "init_fs" hook; the
+initial implementation of cleancache didn't provide this hook.
+But for some filesystems (such as btrfs), the VFS hooks are
+incomplete and one or more hooks in fs-specific code are required.
+And for some other filesystems, such as tmpfs, cleancache may
+be counterproductive.  So it seemed prudent to require a filesystem
+to "opt in" to use cleancache, which requires adding a hook in
+each filesystem.  Not all filesystems are supported by cleancache
+only because they haven't been tested.  The existing set should
+be sufficient to validate the concept, the opt-in approach means
+that untested filesystems are not affected, and the hooks in the
+existing filesystems should make it very easy to add more
+filesystems in the future.
+
+The total impact of the hooks to existing fs and mm files is only
+about 40 lines added (not counting comments and blank lines).
+
+3) Why not make cleancache asynchronous and batched so it can
+   more easily interface with real devices with DMA instead
+   of copying each individual page? (Minchan Kim)
+
+The one-page-at-a-time copy semantics simplifies the implementation
+on both the frontend and backend and also allows the backend to
+do fancy things on-the-fly like page compression and
+page deduplication.  And since the data is "gone" (copied into/out
+of the pageframe) before the cleancache get/put call returns,
+a great deal of race conditions and potential coherency issues
+are avoided.  While the interface seems odd for a "real device"
+or for real kernel-addressable RAM, it makes perfect sense for
+transcendent memory.
+
+4) Why is non-shared cleancache "exclusive"?  And where is the
+   page "invalidated" after a "get"? (Minchan Kim)
+
+The main reason is to free up space in transcendent memory and
+to avoid unnecessary cleancache_invalidate calls.  If you want inclusive,
+the page can be "put" immediately following the "get".  If
+put-after-get for inclusive becomes common, the interface could
+be easily extended to add a "get_no_invalidate" call.
+
+The invalidate is done by the cleancache backend implementation.
+
+5) What's the performance impact?
+
+Performance analysis has been presented at OLS'09 and LCA'10.
+Briefly, performance gains can be significant on most workloads,
+especially when memory pressure is high (e.g. when RAM is
+overcommitted in a virtual workload); and because the hooks are
+invoked primarily in place of or in addition to a disk read/write,
+overhead is negligible even in worst case workloads.  Basically
+cleancache replaces I/O with memory-copy-CPU-overhead; on older
+single-core systems with slow memory-copy speeds, cleancache
+has little value, but in newer multicore machines, especially
+consolidated/virtualized machines, it has great value.
+
+6) How do I add cleancache support for filesystem X? (Boaz Harrash)
+
+Filesystems that are well-behaved and conform to certain
+restrictions can utilize cleancache simply by making a call to
+cleancache_init_fs at mount time.  Unusual, misbehaving, or
+poorly layered filesystems must either add additional hooks
+and/or undergo extensive additional testing... or should just
+not enable the optional cleancache.
+
+Some points for a filesystem to consider:
+
+- The FS should be block-device-based (e.g. a ram-based FS such
+  as tmpfs should not enable cleancache)
+- To ensure coherency/correctness, the FS must ensure that all
+  file removal or truncation operations either go through VFS or
+  add hooks to do the equivalent cleancache "invalidate" operations
+- To ensure coherency/correctness, either inode numbers must
+  be unique across the lifetime of the on-disk file OR the
+  FS must provide an "encode_fh" function.
+- The FS must call the VFS superblock alloc and deactivate routines
+  or add hooks to do the equivalent cleancache calls done there.
+- To maximize performance, all pages fetched from the FS should
+  go through the do_mpag_readpage routine or the FS should add
+  hooks to do the equivalent (cf. btrfs)
+- Currently, the FS blocksize must be the same as PAGESIZE.  This
+  is not an architectural restriction, but no backends currently
+  support anything different.
+- A clustered FS should invoke the "shared_init_fs" cleancache
+  hook to get best performance for some backends.
+
+7) Why not use the KVA of the inode as the key? (Christoph Hellwig)
+
+If cleancache would use the inode virtual address instead of
+inode/filehandle, the pool id could be eliminated.  But, this
+won't work because cleancache retains pagecache data pages
+persistently even when the inode has been pruned from the
+inode unused list, and only invalidates the data page if the file
+gets removed/truncated.  So if cleancache used the inode kva,
+there would be potential coherency issues if/when the inode
+kva is reused for a different file.  Alternately, if cleancache
+invalidated the pages when the inode kva was freed, much of the value
+of cleancache would be lost because the cache of pages in cleanache
+is potentially much larger than the kernel pagecache and is most
+useful if the pages survive inode cache removal.
+
+8) Why is a global variable required?
+
+The cleancache_enabled flag is checked in all of the frequently-used
+cleancache hooks.  The alternative is a function call to check a static
+variable. Since cleancache is enabled dynamically at runtime, systems
+that don't enable cleancache would suffer thousands (possibly
+tens-of-thousands) of unnecessary function calls per second.  So the
+global variable allows cleancache to be enabled by default at compile
+time, but have insignificant performance impact when cleancache remains
+disabled at runtime.
+
+9) Does cleanache work with KVM?
+
+The memory model of KVM is sufficiently different that a cleancache
+backend may have less value for KVM.  This remains to be tested,
+especially in an overcommitted system.
+
+10) Does cleancache work in userspace?  It sounds useful for
+   memory hungry caches like web browsers.  (Jamie Lokier)
+
+No plans yet, though we agree it sounds useful, at least for
+apps that bypass the page cache (e.g. O_DIRECT).
+
+Last updated: Dan Magenheimer, April 13 2011