Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

5 files changed, 1774 insertions, 0 deletions

diff --git a/Documentation/filesystems/caching/backend-api.rst b/Documentation/filesystems/caching/backend-api.rst
new file mode 100644
index 000000000..3a199fc50
--- /dev/null
+++ b/Documentation/filesystems/caching/backend-api.rst
@@ -0,0 +1,479 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Cache Backend API
+=================
+
+The FS-Cache system provides an API by which actual caches can be supplied to
+FS-Cache for it to then serve out to network filesystems and other interested
+parties.  This API is used by::
+
+	#include <linux/fscache-cache.h>.
+
+
+Overview
+========
+
+Interaction with the API is handled on three levels: cache, volume and data
+storage, and each level has its own type of cookie object:
+
+	=======================	=======================
+	COOKIE			C TYPE
+	=======================	=======================
+	Cache cookie		struct fscache_cache
+	Volume cookie		struct fscache_volume
+	Data storage cookie	struct fscache_cookie
+	=======================	=======================
+
+Cookies are used to provide some filesystem data to the cache, manage state and
+pin the cache during access in addition to acting as reference points for the
+API functions.  Each cookie has a debugging ID that is included in trace points
+to make it easier to correlate traces.  Note, though, that debugging IDs are
+simply allocated from incrementing counters and will eventually wrap.
+
+The cache backend and the network filesystem can both ask for cache cookies -
+and if they ask for one of the same name, they'll get the same cookie.  Volume
+and data cookies, however, are created at the behest of the filesystem only.
+
+
+Cache Cookies
+=============
+
+Caches are represented in the API by cache cookies.  These are objects of
+type::
+
+	struct fscache_cache {
+		void		*cache_priv;
+		unsigned int	debug_id;
+		char		*name;
+		...
+	};
+
+There are a few fields that the cache backend might be interested in.  The
+``debug_id`` can be used in tracing to match lines referring to the same cache
+and ``name`` is the name the cache was registered with.  The ``cache_priv``
+member is private data provided by the cache when it is brought online.  The
+other fields are for internal use.
+
+
+Registering a Cache
+===================
+
+When a cache backend wants to bring a cache online, it should first register
+the cache name and that will get it a cache cookie.  This is done with::
+
+	struct fscache_cache *fscache_acquire_cache(const char *name);
+
+This will look up and potentially create a cache cookie.  The cache cookie may
+have already been created by a network filesystem looking for it, in which case
+that cache cookie will be used.  If the cache cookie is not in use by another
+cache, it will be moved into the preparing state, otherwise it will return
+busy.
+
+If successful, the cache backend can then start setting up the cache.  In the
+event that the initialisation fails, the cache backend should call::
+
+	void fscache_relinquish_cache(struct fscache_cache *cache);
+
+to reset and discard the cookie.
+
+
+Bringing a Cache Online
+=======================
+
+Once the cache is set up, it can be brought online by calling::
+
+	int fscache_add_cache(struct fscache_cache *cache,
+			      const struct fscache_cache_ops *ops,
+			      void *cache_priv);
+
+This stores the cache operations table pointer and cache private data into the
+cache cookie and moves the cache to the active state, thereby allowing accesses
+to take place.
+
+
+Withdrawing a Cache From Service
+================================
+
+The cache backend can withdraw a cache from service by calling this function::
+
+	void fscache_withdraw_cache(struct fscache_cache *cache);
+
+This moves the cache to the withdrawn state to prevent new cache- and
+volume-level accesses from starting and then waits for outstanding cache-level
+accesses to complete.
+
+The cache must then go through the data storage objects it has and tell fscache
+to withdraw them, calling::
+
+	void fscache_withdraw_cookie(struct fscache_cookie *cookie);
+
+on the cookie that each object belongs to.  This schedules the specified cookie
+for withdrawal.  This gets offloaded to a workqueue.  The cache backend can
+wait for completion by calling::
+
+	void fscache_wait_for_objects(struct fscache_cache *cache);
+
+Once all the cookies are withdrawn, a cache backend can withdraw all the
+volumes, calling::
+
+	void fscache_withdraw_volume(struct fscache_volume *volume);
+
+to tell fscache that a volume has been withdrawn.  This waits for all
+outstanding accesses on the volume to complete before returning.
+
+When the cache is completely withdrawn, fscache should be notified by
+calling::
+
+	void fscache_relinquish_cache(struct fscache_cache *cache);
+
+to clear fields in the cookie and discard the caller's ref on it.
+
+
+Volume Cookies
+==============
+
+Within a cache, the data storage objects are organised into logical volumes.
+These are represented in the API as objects of type::
+
+	struct fscache_volume {
+		struct fscache_cache		*cache;
+		void				*cache_priv;
+		unsigned int			debug_id;
+		char				*key;
+		unsigned int			key_hash;
+		...
+		u8				coherency_len;
+		u8				coherency[];
+	};
+
+There are a number of fields here that are of interest to the caching backend:
+
+   * ``cache`` - The parent cache cookie.
+
+   * ``cache_priv`` - A place for the cache to stash private data.
+
+   * ``debug_id`` - A debugging ID for logging in tracepoints.
+
+   * ``key`` - A printable string with no '/' characters in it that represents
+     the index key for the volume.  The key is NUL-terminated and padded out to
+     a multiple of 4 bytes.
+
+   * ``key_hash`` - A hash of the index key.  This should work out the same, no
+     matter the cpu arch and endianness.
+
+   * ``coherency`` - A piece of coherency data that should be checked when the
+     volume is bound to in the cache.
+
+   * ``coherency_len`` - The amount of data in the coherency buffer.
+
+
+Data Storage Cookies
+====================
+
+A volume is a logical group of data storage objects, each of which is
+represented to the network filesystem by a cookie.  Cookies are represented in
+the API as objects of type::
+
+	struct fscache_cookie {
+		struct fscache_volume		*volume;
+		void				*cache_priv;
+		unsigned long			flags;
+		unsigned int			debug_id;
+		unsigned int			inval_counter;
+		loff_t				object_size;
+		u8				advice;
+		u32				key_hash;
+		u8				key_len;
+		u8				aux_len;
+		...
+	};
+
+The fields in the cookie that are of interest to the cache backend are:
+
+   * ``volume`` - The parent volume cookie.
+
+   * ``cache_priv`` - A place for the cache to stash private data.
+
+   * ``flags`` - A collection of bit flags, including:
+
+      * FSCACHE_COOKIE_NO_DATA_TO_READ - There is no data available in the
+	cache to be read as the cookie has been created or invalidated.
+
+      * FSCACHE_COOKIE_NEEDS_UPDATE - The coherency data and/or object size has
+	been changed and needs committing.
+
+      * FSCACHE_COOKIE_LOCAL_WRITE - The netfs's data has been modified
+	locally, so the cache object may be in an incoherent state with respect
+	to the server.
+
+      * FSCACHE_COOKIE_HAVE_DATA - The backend should set this if it
+	successfully stores data into the cache.
+
+      * FSCACHE_COOKIE_RETIRED - The cookie was invalidated when it was
+	relinquished and the cached data should be discarded.
+
+   * ``debug_id`` - A debugging ID for logging in tracepoints.
+
+   * ``inval_counter`` - The number of invalidations done on the cookie.
+
+   * ``advice`` - Information about how the cookie is to be used.
+
+   * ``key_hash`` - A hash of the index key.  This should work out the same, no
+     matter the cpu arch and endianness.
+
+   * ``key_len`` - The length of the index key.
+
+   * ``aux_len`` - The length of the coherency data buffer.
+
+Each cookie has an index key, which may be stored inline to the cookie or
+elsewhere.  A pointer to this can be obtained by calling::
+
+	void *fscache_get_key(struct fscache_cookie *cookie);
+
+The index key is a binary blob, the storage for which is padded out to a
+multiple of 4 bytes.
+
+Each cookie also has a buffer for coherency data.  This may also be inline or
+detached from the cookie and a pointer is obtained by calling::
+
+	void *fscache_get_aux(struct fscache_cookie *cookie);
+
+
+
+Cookie Accounting
+=================
+
+Data storage cookies are counted and this is used to block cache withdrawal
+completion until all objects have been destroyed.  The following functions are
+provided to the cache to deal with that::
+
+	void fscache_count_object(struct fscache_cache *cache);
+	void fscache_uncount_object(struct fscache_cache *cache);
+	void fscache_wait_for_objects(struct fscache_cache *cache);
+
+The count function records the allocation of an object in a cache and the
+uncount function records its destruction.  Warning: by the time the uncount
+function returns, the cache may have been destroyed.
+
+The wait function can be used during the withdrawal procedure to wait for
+fscache to finish withdrawing all the objects in the cache.  When it completes,
+there will be no remaining objects referring to the cache object or any volume
+objects.
+
+
+Cache Management API
+====================
+
+The cache backend implements the cache management API by providing a table of
+operations that fscache can use to manage various aspects of the cache.  These
+are held in a structure of type::
+
+	struct fscache_cache_ops {
+		const char *name;
+		...
+	};
+
+This contains a printable name for the cache backend driver plus a number of
+pointers to methods to allow fscache to request management of the cache:
+
+   * Set up a volume cookie [optional]::
+
+	void (*acquire_volume)(struct fscache_volume *volume);
+
+     This method is called when a volume cookie is being created.  The caller
+     holds a cache-level access pin to prevent the cache from going away for
+     the duration.  This method should set up the resources to access a volume
+     in the cache and should not return until it has done so.
+
+     If successful, it can set ``cache_priv`` to its own data.
+
+
+   * Clean up volume cookie [optional]::
+
+       void (*free_volume)(struct fscache_volume *volume);
+
+     This method is called when a volume cookie is being released if
+     ``cache_priv`` is set.
+
+
+   * Look up a cookie in the cache [mandatory]::
+
+	bool (*lookup_cookie)(struct fscache_cookie *cookie);
+
+     This method is called to look up/create the resources needed to access the
+     data storage for a cookie.  It is called from a worker thread with a
+     volume-level access pin in the cache to prevent it from being withdrawn.
+
+     True should be returned if successful and false otherwise.  If false is
+     returned, the withdraw_cookie op (see below) will be called.
+
+     If lookup fails, but the object could still be created (e.g. it hasn't
+     been cached before), then::
+
+		void fscache_cookie_lookup_negative(
+			struct fscache_cookie *cookie);
+
+     can be called to let the network filesystem proceed and start downloading
+     stuff whilst the cache backend gets on with the job of creating things.
+
+     If successful, ``cookie->cache_priv`` can be set.
+
+
+   * Withdraw an object without any cookie access counts held [mandatory]::
+
+	void (*withdraw_cookie)(struct fscache_cookie *cookie);
+
+     This method is called to withdraw a cookie from service.  It will be
+     called when the cookie is relinquished by the netfs, withdrawn or culled
+     by the cache backend or closed after a period of non-use by fscache.
+
+     The caller doesn't hold any access pins, but it is called from a
+     non-reentrant work item to manage races between the various ways
+     withdrawal can occur.
+
+     The cookie will have the ``FSCACHE_COOKIE_RETIRED`` flag set on it if the
+     associated data is to be removed from the cache.
+
+
+   * Change the size of a data storage object [mandatory]::
+
+	void (*resize_cookie)(struct netfs_cache_resources *cres,
+			      loff_t new_size);
+
+     This method is called to inform the cache backend of a change in size of
+     the netfs file due to local truncation.  The cache backend should make all
+     of the changes it needs to make before returning as this is done under the
+     netfs inode mutex.
+
+     The caller holds a cookie-level access pin to prevent a race with
+     withdrawal and the netfs must have the cookie marked in-use to prevent
+     garbage collection or culling from removing any resources.
+
+
+   * Invalidate a data storage object [mandatory]::
+
+	bool (*invalidate_cookie)(struct fscache_cookie *cookie);
+
+     This is called when the network filesystem detects a third-party
+     modification or when an O_DIRECT write is made locally.  This requests
+     that the cache backend should throw away all the data in the cache for
+     this object and start afresh.  It should return true if successful and
+     false otherwise.
+
+     On entry, new I O/operations are blocked.  Once the cache is in a position
+     to accept I/O again, the backend should release the block by calling::
+
+	void fscache_resume_after_invalidation(struct fscache_cookie *cookie);
+
+     If the method returns false, caching will be withdrawn for this cookie.
+
+
+   * Prepare to make local modifications to the cache [mandatory]::
+
+	void (*prepare_to_write)(struct fscache_cookie *cookie);
+
+     This method is called when the network filesystem finds that it is going
+     to need to modify the contents of the cache due to local writes or
+     truncations.  This gives the cache a chance to note that a cache object
+     may be incoherent with respect to the server and may need writing back
+     later.  This may also cause the cached data to be scrapped on later
+     rebinding if not properly committed.
+
+
+   * Begin an operation for the netfs lib [mandatory]::
+
+	bool (*begin_operation)(struct netfs_cache_resources *cres,
+				enum fscache_want_state want_state);
+
+     This method is called when an I/O operation is being set up (read, write
+     or resize).  The caller holds an access pin on the cookie and must have
+     marked the cookie as in-use.
+
+     If it can, the backend should attach any resources it needs to keep around
+     to the netfs_cache_resources object and return true.
+
+     If it can't complete the setup, it should return false.
+
+     The want_state parameter indicates the state the caller needs the cache
+     object to be in and what it wants to do during the operation:
+
+	* ``FSCACHE_WANT_PARAMS`` - The caller just wants to access cache
+	  object parameters; it doesn't need to do data I/O yet.
+
+	* ``FSCACHE_WANT_READ`` - The caller wants to read data.
+
+	* ``FSCACHE_WANT_WRITE`` - The caller wants to write to or resize the
+          cache object.
+
+     Note that there won't necessarily be anything attached to the cookie's
+     cache_priv yet if the cookie is still being created.
+
+
+Data I/O API
+============
+
+A cache backend provides a data I/O API by through the netfs library's ``struct
+netfs_cache_ops`` attached to a ``struct netfs_cache_resources`` by the
+``begin_operation`` method described above.
+
+See the Documentation/filesystems/netfs_library.rst for a description.
+
+
+Miscellaneous Functions
+=======================
+
+FS-Cache provides some utilities that a cache backend may make use of:
+
+   * Note occurrence of an I/O error in a cache::
+
+	void fscache_io_error(struct fscache_cache *cache);
+
+     This tells FS-Cache that an I/O error occurred in the cache.  This
+     prevents any new I/O from being started on the cache.
+
+     This does not actually withdraw the cache.  That must be done separately.
+
+   * Note cessation of caching on a cookie due to failure::
+
+	void fscache_caching_failed(struct fscache_cookie *cookie);
+
+     This notes that a the caching that was being done on a cookie failed in
+     some way, for instance the backing storage failed to be created or
+     invalidation failed and that no further I/O operations should take place
+     on it until the cache is reset.
+
+   * Count I/O requests::
+
+	void fscache_count_read(void);
+	void fscache_count_write(void);
+
+     These record reads and writes from/to the cache.  The numbers are
+     displayed in /proc/fs/fscache/stats.
+
+   * Count out-of-space errors::
+
+	void fscache_count_no_write_space(void);
+	void fscache_count_no_create_space(void);
+
+     These record ENOSPC errors in the cache, divided into failures of data
+     writes and failures of filesystem object creations (e.g. mkdir).
+
+   * Count objects culled::
+
+	void fscache_count_culled(void);
+
+     This records the culling of an object.
+
+   * Get the cookie from a set of cache resources::
+
+	struct fscache_cookie *fscache_cres_cookie(struct netfs_cache_resources *cres)
+
+     Pull a pointer to the cookie from the cache resources.  This may return a
+     NULL cookie if no cookie was set.
+
+
+API Function Reference
+======================
+
+.. kernel-doc:: include/linux/fscache-cache.h
diff --git a/Documentation/filesystems/caching/cachefiles.rst b/Documentation/filesystems/caching/cachefiles.rst
new file mode 100644
index 000000000..adfb7d079
--- /dev/null
+++ b/Documentation/filesystems/caching/cachefiles.rst
@@ -0,0 +1,483 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Cache on Already Mounted Filesystem
+===================================
+
+.. Contents:
+
+ (*) Overview.
+
+ (*) Requirements.
+
+ (*) Configuration.
+
+ (*) Starting the cache.
+
+ (*) Things to avoid.
+
+ (*) Cache culling.
+
+ (*) Cache structure.
+
+ (*) Security model and SELinux.
+
+ (*) A note on security.
+
+ (*) Statistical information.
+
+ (*) Debugging.
+
+
+Overview
+========
+
+CacheFiles is a caching backend that's meant to use as a cache a directory on
+an already mounted filesystem of a local type (such as Ext3).
+
+CacheFiles uses a userspace daemon to do some of the cache management - such as
+reaping stale nodes and culling.  This is called cachefilesd and lives in
+/sbin.
+
+The filesystem and data integrity of the cache are only as good as those of the
+filesystem providing the backing services.  Note that CacheFiles does not
+attempt to journal anything since the journalling interfaces of the various
+filesystems are very specific in nature.
+
+CacheFiles creates a misc character device - "/dev/cachefiles" - that is used
+to communication with the daemon.  Only one thing may have this open at once,
+and while it is open, a cache is at least partially in existence.  The daemon
+opens this and sends commands down it to control the cache.
+
+CacheFiles is currently limited to a single cache.
+
+CacheFiles attempts to maintain at least a certain percentage of free space on
+the filesystem, shrinking the cache by culling the objects it contains to make
+space if necessary - see the "Cache Culling" section.  This means it can be
+placed on the same medium as a live set of data, and will expand to make use of
+spare space and automatically contract when the set of data requires more
+space.
+
+
+
+Requirements
+============
+
+The use of CacheFiles and its daemon requires the following features to be
+available in the system and in the cache filesystem:
+
+	- dnotify.
+
+	- extended attributes (xattrs).
+
+	- openat() and friends.
+
+	- bmap() support on files in the filesystem (FIBMAP ioctl).
+
+	- The use of bmap() to detect a partial page at the end of the file.
+
+It is strongly recommended that the "dir_index" option is enabled on Ext3
+filesystems being used as a cache.
+
+
+Configuration
+=============
+
+The cache is configured by a script in /etc/cachefilesd.conf.  These commands
+set up cache ready for use.  The following script commands are available:
+
+ brun <N>%, bcull <N>%, bstop <N>%, frun <N>%, fcull <N>%, fstop <N>%
+	Configure the culling limits.  Optional.  See the section on culling
+	The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.
+
+	The commands beginning with a 'b' are file space (block) limits, those
+	beginning with an 'f' are file count limits.
+
+ dir <path>
+	Specify the directory containing the root of the cache.  Mandatory.
+
+ tag <name>
+	Specify a tag to FS-Cache to use in distinguishing multiple caches.
+	Optional.  The default is "CacheFiles".
+
+ debug <mask>
+	Specify a numeric bitmask to control debugging in the kernel module.
+	Optional.  The default is zero (all off).  The following values can be
+	OR'd into the mask to collect various information:
+
+		==	=================================================
+		1	Turn on trace of function entry (_enter() macros)
+		2	Turn on trace of function exit (_leave() macros)
+		4	Turn on trace of internal debug points (_debug())
+		==	=================================================
+
+	This mask can also be set through sysfs, eg::
+
+		echo 5 > /sys/module/cachefiles/parameters/debug
+
+
+Starting the Cache
+==================
+
+The cache is started by running the daemon.  The daemon opens the cache device,
+configures the cache and tells it to begin caching.  At that point the cache
+binds to fscache and the cache becomes live.
+
+The daemon is run as follows::
+
+	/sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]
+
+The flags are:
+
+ ``-d``
+	Increase the debugging level.  This can be specified multiple times and
+	is cumulative with itself.
+
+ ``-s``
+	Send messages to stderr instead of syslog.
+
+ ``-n``
+	Don't daemonise and go into background.
+
+ ``-f <configfile>``
+	Use an alternative configuration file rather than the default one.
+
+
+Things to Avoid
+===============
+
+Do not mount other things within the cache as this will cause problems.  The
+kernel module contains its own very cut-down path walking facility that ignores
+mountpoints, but the daemon can't avoid them.
+
+Do not create, rename or unlink files and directories in the cache while the
+cache is active, as this may cause the state to become uncertain.
+
+Renaming files in the cache might make objects appear to be other objects (the
+filename is part of the lookup key).
+
+Do not change or remove the extended attributes attached to cache files by the
+cache as this will cause the cache state management to get confused.
+
+Do not create files or directories in the cache, lest the cache get confused or
+serve incorrect data.
+
+Do not chmod files in the cache.  The module creates things with minimal
+permissions to prevent random users being able to access them directly.
+
+
+Cache Culling
+=============
+
+The cache may need culling occasionally to make space.  This involves
+discarding objects from the cache that have been used less recently than
+anything else.  Culling is based on the access time of data objects.  Empty
+directories are culled if not in use.
+
+Cache culling is done on the basis of the percentage of blocks and the
+percentage of files available in the underlying filesystem.  There are six
+"limits":
+
+ brun, frun
+     If the amount of free space and the number of available files in the cache
+     rises above both these limits, then culling is turned off.
+
+ bcull, fcull
+     If the amount of available space or the number of available files in the
+     cache falls below either of these limits, then culling is started.
+
+ bstop, fstop
+     If the amount of available space or the number of available files in the
+     cache falls below either of these limits, then no further allocation of
+     disk space or files is permitted until culling has raised things above
+     these limits again.
+
+These must be configured thusly::
+
+	0 <= bstop < bcull < brun < 100
+	0 <= fstop < fcull < frun < 100
+
+Note that these are percentages of available space and available files, and do
+_not_ appear as 100 minus the percentage displayed by the "df" program.
+
+The userspace daemon scans the cache to build up a table of cullable objects.
+These are then culled in least recently used order.  A new scan of the cache is
+started as soon as space is made in the table.  Objects will be skipped if
+their atimes have changed or if the kernel module says it is still using them.
+
+
+Cache Structure
+===============
+
+The CacheFiles module will create two directories in the directory it was
+given:
+
+ * cache/
+ * graveyard/
+
+The active cache objects all reside in the first directory.  The CacheFiles
+kernel module moves any retired or culled objects that it can't simply unlink
+to the graveyard from which the daemon will actually delete them.
+
+The daemon uses dnotify to monitor the graveyard directory, and will delete
+anything that appears therein.
+
+
+The module represents index objects as directories with the filename "I..." or
+"J...".  Note that the "cache/" directory is itself a special index.
+
+Data objects are represented as files if they have no children, or directories
+if they do.  Their filenames all begin "D..." or "E...".  If represented as a
+directory, data objects will have a file in the directory called "data" that
+actually holds the data.
+
+Special objects are similar to data objects, except their filenames begin
+"S..." or "T...".
+
+
+If an object has children, then it will be represented as a directory.
+Immediately in the representative directory are a collection of directories
+named for hash values of the child object keys with an '@' prepended.  Into
+this directory, if possible, will be placed the representations of the child
+objects::
+
+	 /INDEX    /INDEX     /INDEX                            /DATA FILES
+	/=========/==========/=================================/================
+	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
+	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
+	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
+	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry
+
+
+If the key is so long that it exceeds NAME_MAX with the decorations added on to
+it, then it will be cut into pieces, the first few of which will be used to
+make a nest of directories, and the last one of which will be the objects
+inside the last directory.  The names of the intermediate directories will have
+'+' prepended::
+
+	J1223/@23/+xy...z/+kl...m/Epqr
+
+
+Note that keys are raw data, and not only may they exceed NAME_MAX in size,
+they may also contain things like '/' and NUL characters, and so they may not
+be suitable for turning directly into a filename.
+
+To handle this, CacheFiles will use a suitably printable filename directly and
+"base-64" encode ones that aren't directly suitable.  The two versions of
+object filenames indicate the encoding:
+
+	===============	===============	===============
+	OBJECT TYPE	PRINTABLE	ENCODED
+	===============	===============	===============
+	Index		"I..."		"J..."
+	Data		"D..."		"E..."
+	Special		"S..."		"T..."
+	===============	===============	===============
+
+Intermediate directories are always "@" or "+" as appropriate.
+
+
+Each object in the cache has an extended attribute label that holds the object
+type ID (required to distinguish special objects) and the auxiliary data from
+the netfs.  The latter is used to detect stale objects in the cache and update
+or retire them.
+
+
+Note that CacheFiles will erase from the cache any file it doesn't recognise or
+any file of an incorrect type (such as a FIFO file or a device file).
+
+
+Security Model and SELinux
+==========================
+
+CacheFiles is implemented to deal properly with the LSM security features of
+the Linux kernel and the SELinux facility.
+
+One of the problems that CacheFiles faces is that it is generally acting on
+behalf of a process, and running in that process's context, and that includes a
+security context that is not appropriate for accessing the cache - either
+because the files in the cache are inaccessible to that process, or because if
+the process creates a file in the cache, that file may be inaccessible to other
+processes.
+
+The way CacheFiles works is to temporarily change the security context (fsuid,
+fsgid and actor security label) that the process acts as - without changing the
+security context of the process when it the target of an operation performed by
+some other process (so signalling and suchlike still work correctly).
+
+
+When the CacheFiles module is asked to bind to its cache, it:
+
+ (1) Finds the security label attached to the root cache directory and uses
+     that as the security label with which it will create files.  By default,
+     this is::
+
+	cachefiles_var_t
+
+ (2) Finds the security label of the process which issued the bind request
+     (presumed to be the cachefilesd daemon), which by default will be::
+
+	cachefilesd_t
+
+     and asks LSM to supply a security ID as which it should act given the
+     daemon's label.  By default, this will be::
+
+	cachefiles_kernel_t
+
+     SELinux transitions the daemon's security ID to the module's security ID
+     based on a rule of this form in the policy::
+
+	type_transition <daemon's-ID> kernel_t : process <module's-ID>;
+
+     For instance::
+
+	type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;
+
+
+The module's security ID gives it permission to create, move and remove files
+and directories in the cache, to find and access directories and files in the
+cache, to set and access extended attributes on cache objects, and to read and
+write files in the cache.
+
+The daemon's security ID gives it only a very restricted set of permissions: it
+may scan directories, stat files and erase files and directories.  It may
+not read or write files in the cache, and so it is precluded from accessing the
+data cached therein; nor is it permitted to create new files in the cache.
+
+
+There are policy source files available in:
+
+	https://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2
+
+and later versions.  In that tarball, see the files::
+
+	cachefilesd.te
+	cachefilesd.fc
+	cachefilesd.if
+
+They are built and installed directly by the RPM.
+
+If a non-RPM based system is being used, then copy the above files to their own
+directory and run::
+
+	make -f /usr/share/selinux/devel/Makefile
+	semodule -i cachefilesd.pp
+
+You will need checkpolicy and selinux-policy-devel installed prior to the
+build.
+
+
+By default, the cache is located in /var/fscache, but if it is desirable that
+it should be elsewhere, than either the above policy files must be altered, or
+an auxiliary policy must be installed to label the alternate location of the
+cache.
+
+For instructions on how to add an auxiliary policy to enable the cache to be
+located elsewhere when SELinux is in enforcing mode, please see::
+
+	/usr/share/doc/cachefilesd-*/move-cache.txt
+
+When the cachefilesd rpm is installed; alternatively, the document can be found
+in the sources.
+
+
+A Note on Security
+==================
+
+CacheFiles makes use of the split security in the task_struct.  It allocates
+its own task_security structure, and redirects current->cred to point to it
+when it acts on behalf of another process, in that process's context.
+
+The reason it does this is that it calls vfs_mkdir() and suchlike rather than
+bypassing security and calling inode ops directly.  Therefore the VFS and LSM
+may deny the CacheFiles access to the cache data because under some
+circumstances the caching code is running in the security context of whatever
+process issued the original syscall on the netfs.
+
+Furthermore, should CacheFiles create a file or directory, the security
+parameters with that object is created (UID, GID, security label) would be
+derived from that process that issued the system call, thus potentially
+preventing other processes from accessing the cache - including CacheFiles's
+cache management daemon (cachefilesd).
+
+What is required is to temporarily override the security of the process that
+issued the system call.  We can't, however, just do an in-place change of the
+security data as that affects the process as an object, not just as a subject.
+This means it may lose signals or ptrace events for example, and affects what
+the process looks like in /proc.
+
+So CacheFiles makes use of a logical split in the security between the
+objective security (task->real_cred) and the subjective security (task->cred).
+The objective security holds the intrinsic security properties of a process and
+is never overridden.  This is what appears in /proc, and is what is used when a
+process is the target of an operation by some other process (SIGKILL for
+example).
+
+The subjective security holds the active security properties of a process, and
+may be overridden.  This is not seen externally, and is used when a process
+acts upon another object, for example SIGKILLing another process or opening a
+file.
+
+LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request
+for CacheFiles to run in a context of a specific security label, or to create
+files and directories with another security label.
+
+
+Statistical Information
+=======================
+
+If FS-Cache is compiled with the following option enabled::
+
+	CONFIG_CACHEFILES_HISTOGRAM=y
+
+then it will gather certain statistics and display them through a proc file.
+
+ /proc/fs/cachefiles/histogram
+
+     ::
+
+	cat /proc/fs/cachefiles/histogram
+	JIFS  SECS  LOOKUPS   MKDIRS    CREATES
+	===== ===== ========= ========= =========
+
+     This shows the breakdown of the number of times each amount of time
+     between 0 jiffies and HZ-1 jiffies a variety of tasks took to run.  The
+     columns are as follows:
+
+	=======		=======================================================
+	COLUMN		TIME MEASUREMENT
+	=======		=======================================================
+	LOOKUPS		Length of time to perform a lookup on the backing fs
+	MKDIRS		Length of time to perform a mkdir on the backing fs
+	CREATES		Length of time to perform a create on the backing fs
+	=======		=======================================================
+
+     Each row shows the number of events that took a particular range of times.
+     Each step is 1 jiffy in size.  The JIFS column indicates the particular
+     jiffy range covered, and the SECS field the equivalent number of seconds.
+
+
+Debugging
+=========
+
+If CONFIG_CACHEFILES_DEBUG is enabled, the CacheFiles facility can have runtime
+debugging enabled by adjusting the value in::
+
+	/sys/module/cachefiles/parameters/debug
+
+This is a bitmask of debugging streams to enable:
+
+	=======	=======	===============================	=======================
+	BIT	VALUE	STREAM				POINT
+	=======	=======	===============================	=======================
+	0	1	General				Function entry trace
+	1	2					Function exit trace
+	2	4					General
+	=======	=======	===============================	=======================
+
+The appropriate set of values should be OR'd together and the result written to
+the control file.  For example::
+
+	echo $((1|4|8)) >/sys/module/cachefiles/parameters/debug
+
+will turn on all function entry debugging.
diff --git a/Documentation/filesystems/caching/fscache.rst b/Documentation/filesystems/caching/fscache.rst
new file mode 100644
index 000000000..de1f32526
--- /dev/null
+++ b/Documentation/filesystems/caching/fscache.rst
@@ -0,0 +1,348 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+General Filesystem Caching
+==========================
+
+Overview
+========
+
+This facility is a general purpose cache for network filesystems, though it
+could be used for caching other things such as ISO9660 filesystems too.
+
+FS-Cache mediates between cache backends (such as CacheFiles) and network
+filesystems::
+
+	+---------+
+	|         |                                    +--------------+
+	|   NFS   |--+                                 |              |
+	|         |  |                             +-->|   CacheFS    |
+	+---------+  |               +----------+  |   |  /dev/hda5   |
+	             |               |          |  |   +--------------+
+	+---------+  +-------------->|          |  |
+	|         |      +-------+   |          |--+
+	|   AFS   |----->|       |   | FS-Cache |
+	|         |      | netfs |-->|          |--+
+	+---------+  +-->|  lib  |   |          |  |
+	             |   |       |   |          |  |   +--------------+
+	+---------+  |   +-------+   +----------+  |   |              |
+	|         |  |                             +-->|  CacheFiles  |
+	|   9P    |--+                                 |  /var/cache  |
+	|         |                                    +--------------+
+	+---------+
+
+Or to look at it another way, FS-Cache is a module that provides a caching
+facility to a network filesystem such that the cache is transparent to the
+user::
+
+	+---------+
+	|         |
+	| Server  |
+	|         |
+	+---------+
+	     |                  NETWORK
+	~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+	     |
+	     |           +----------+
+	     V           |          |
+	+---------+      |          |
+	|         |      |          |
+	|   NFS   |----->| FS-Cache |
+	|         |      |          |--+
+	+---------+      |          |  |   +--------------+   +--------------+
+	     |           |          |  |   |              |   |              |
+	     V           +----------+  +-->|  CacheFiles  |-->|  Ext3        |
+	+---------+                        |  /var/cache  |   |  /dev/sda6   |
+	|         |                        +--------------+   +--------------+
+	|   VFS   |                                ^                     ^
+	|         |                                |                     |
+	+---------+                                +--------------+      |
+	     |                  KERNEL SPACE                      |      |
+	~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
+	     |                  USER SPACE                        |      |
+	     V                                                    |      |
+	+---------+                                           +--------------+
+	|         |                                           |              |
+	| Process |                                           | cachefilesd  |
+	|         |                                           |              |
+	+---------+                                           +--------------+
+
+
+FS-Cache does not follow the idea of completely loading every netfs file
+opened in its entirety into a cache before permitting it to be accessed and
+then serving the pages out of that cache rather than the netfs inode because:
+
+ (1) It must be practical to operate without a cache.
+
+ (2) The size of any accessible file must not be limited to the size of the
+     cache.
+
+ (3) The combined size of all opened files (this includes mapped libraries)
+     must not be limited to the size of the cache.
+
+ (4) The user should not be forced to download an entire file just to do a
+     one-off access of a small portion of it (such as might be done with the
+     "file" program).
+
+It instead serves the cache out in chunks as and when requested by the netfs
+using it.
+
+
+FS-Cache provides the following facilities:
+
+   * More than one cache can be used at once.  Caches can be selected
+     explicitly by use of tags.
+
+   * Caches can be added / removed at any time, even whilst being accessed.
+
+   * The netfs is provided with an interface that allows either party to
+     withdraw caching facilities from a file (required for (2)).
+
+   * The interface to the netfs returns as few errors as possible, preferring
+     rather to let the netfs remain oblivious.
+
+   * There are three types of cookie: cache, volume and data file cookies.
+     Cache cookies represent the cache as a whole and are not normally visible
+     to the netfs; the netfs gets a volume cookie to represent a collection of
+     files (typically something that a netfs would get for a superblock); and
+     data file cookies are used to cache data (something that would be got for
+     an inode).
+
+   * Volumes are matched using a key.  This is a printable string that is used
+     to encode all the information that might be needed to distinguish one
+     superblock, say, from another.  This would be a compound of things like
+     cell name or server address, volume name or share path.  It must be a
+     valid pathname.
+
+   * Cookies are matched using a key.  This is a binary blob and is used to
+     represent the object within a volume (so the volume key need not form
+     part of the blob).  This might include things like an inode number and
+     uniquifier or a file handle.
+
+   * Cookie resources are set up and pinned by marking the cookie in-use.
+     This prevents the backing resources from being culled.  Timed garbage
+     collection is employed to eliminate cookies that haven't been used for a
+     short while, thereby reducing resource overload.  This is intended to be
+     used when a file is opened or closed.
+
+     A cookie can be marked in-use multiple times simultaneously; each mark
+     must be unused.
+
+   * Begin/end access functions are provided to delay cache withdrawal for the
+     duration of an operation and prevent structs from being freed whilst
+     we're looking at them.
+
+   * Data I/O is done by asynchronous DIO to/from a buffer described by the
+     netfs using an iov_iter.
+
+   * An invalidation facility is available to discard data from the cache and
+     to deal with I/O that's in progress that is accessing old data.
+
+   * Cookies can be "retired" upon release, thereby causing the object to be
+     removed from the cache.
+
+
+The netfs API to FS-Cache can be found in:
+
+	Documentation/filesystems/caching/netfs-api.rst
+
+The cache backend API to FS-Cache can be found in:
+
+	Documentation/filesystems/caching/backend-api.rst
+
+
+Statistical Information
+=======================
+
+If FS-Cache is compiled with the following options enabled::
+
+	CONFIG_FSCACHE_STATS=y
+
+then it will gather certain statistics and display them through:
+
+	/proc/fs/fscache/stats
+
+This shows counts of a number of events that can happen in FS-Cache:
+
++--------------+-------+-------------------------------------------------------+
+|CLASS         |EVENT  |MEANING                                                |
++==============+=======+=======================================================+
+|Cookies       |n=N    |Number of data storage cookies allocated               |
++              +-------+-------------------------------------------------------+
+|              |v=N    |Number of volume index cookies allocated               |
++              +-------+-------------------------------------------------------+
+|              |vcol=N |Number of volume index key collisions                  |
++              +-------+-------------------------------------------------------+
+|              |voom=N |Number of OOM events when allocating volume cookies    |
++--------------+-------+-------------------------------------------------------+
+|Acquire       |n=N    |Number of acquire cookie requests seen                 |
++              +-------+-------------------------------------------------------+
+|              |ok=N   |Number of acq reqs succeeded                           |
++              +-------+-------------------------------------------------------+
+|              |oom=N  |Number of acq reqs failed on ENOMEM                    |
++--------------+-------+-------------------------------------------------------+
+|LRU           |n=N    |Number of cookies currently on the LRU                 |
++              +-------+-------------------------------------------------------+
+|              |exp=N  |Number of cookies expired off of the LRU               |
++              +-------+-------------------------------------------------------+
+|              |rmv=N  |Number of cookies removed from the LRU                 |
++              +-------+-------------------------------------------------------+
+|              |drp=N  |Number of LRU'd cookies relinquished/withdrawn         |
++              +-------+-------------------------------------------------------+
+|              |at=N   |Time till next LRU cull (jiffies)                      |
++--------------+-------+-------------------------------------------------------+
+|Invals        |n=N    |Number of invalidations                                |
++--------------+-------+-------------------------------------------------------+
+|Updates       |n=N    |Number of update cookie requests seen                  |
++              +-------+-------------------------------------------------------+
+|              |rsz=N  |Number of resize requests                              |
++              +-------+-------------------------------------------------------+
+|              |rsn=N  |Number of skipped resize requests                      |
++--------------+-------+-------------------------------------------------------+
+|Relinqs       |n=N    |Number of relinquish cookie requests seen              |
++              +-------+-------------------------------------------------------+
+|              |rtr=N  |Number of rlq reqs with retire=true                    |
++              +-------+-------------------------------------------------------+
+|              |drop=N |Number of cookies no longer blocking re-acquisition    |
++--------------+-------+-------------------------------------------------------+
+|NoSpace       |nwr=N  |Number of write requests refused due to lack of space  |
++              +-------+-------------------------------------------------------+
+|              |ncr=N  |Number of create requests refused due to lack of space |
++              +-------+-------------------------------------------------------+
+|              |cull=N |Number of objects culled to make space                 |
++--------------+-------+-------------------------------------------------------+
+|IO            |rd=N   |Number of read operations in the cache                 |
++              +-------+-------------------------------------------------------+
+|              |wr=N   |Number of write operations in the cache                |
++--------------+-------+-------------------------------------------------------+
+
+Netfslib will also add some stats counters of its own.
+
+
+Cache List
+==========
+
+FS-Cache provides a list of cache cookies:
+
+	/proc/fs/fscache/cookies
+
+This will look something like::
+
+	# cat /proc/fs/fscache/caches
+	CACHE    REF   VOLS  OBJS  ACCES S NAME
+	======== ===== ===== ===== ===== = ===============
+	00000001     2     1  2123     1 A default
+
+where the columns are:
+
+	=======	===============================================================
+	COLUMN	DESCRIPTION
+	=======	===============================================================
+	CACHE	Cache cookie debug ID (also appears in traces)
+	REF	Number of references on the cache cookie
+	VOLS	Number of volumes cookies in this cache
+	OBJS	Number of cache objects in use
+	ACCES	Number of accesses pinning the cache
+	S	State
+	NAME	Name of the cache.
+	=======	===============================================================
+
+The state can be (-) Inactive, (P)reparing, (A)ctive, (E)rror or (W)ithdrawing.
+
+
+Volume List
+===========
+
+FS-Cache provides a list of volume cookies:
+
+	/proc/fs/fscache/volumes
+
+This will look something like::
+
+	VOLUME   REF   nCOOK ACC FL CACHE           KEY
+	======== ===== ===== === == =============== ================
+	00000001    55    54   1 00 default         afs,example.com,100058
+
+where the columns are:
+
+	=======	===============================================================
+	COLUMN	DESCRIPTION
+	=======	===============================================================
+	VOLUME	The volume cookie debug ID (also appears in traces)
+	REF	Number of references on the volume cookie
+	nCOOK	Number of cookies in the volume
+	ACC	Number of accesses pinning the cache
+	FL	Flags on the volume cookie
+	CACHE	Name of the cache or "-"
+	KEY	The indexing key for the volume
+	=======	===============================================================
+
+
+Cookie List
+===========
+
+FS-Cache provides a list of cookies:
+
+	/proc/fs/fscache/cookies
+
+This will look something like::
+
+	# head /proc/fs/fscache/cookies
+	COOKIE   VOLUME   REF ACT ACC S FL DEF
+	======== ======== === === === = == ================
+	00000435 00000001   1   0  -1 - 08 0000000201d080070000000000000000, 0000000000000000
+	00000436 00000001   1   0  -1 - 00 0000005601d080080000000000000000, 0000000000000051
+	00000437 00000001   1   0  -1 - 08 00023b3001d0823f0000000000000000, 0000000000000000
+	00000438 00000001   1   0  -1 - 08 0000005801d0807b0000000000000000, 0000000000000000
+	00000439 00000001   1   0  -1 - 08 00023b3201d080a10000000000000000, 0000000000000000
+	0000043a 00000001   1   0  -1 - 08 00023b3401d080a30000000000000000, 0000000000000000
+	0000043b 00000001   1   0  -1 - 08 00023b3601d080b30000000000000000, 0000000000000000
+	0000043c 00000001   1   0  -1 - 08 00023b3801d080b40000000000000000, 0000000000000000
+
+where the columns are:
+
+	=======	===============================================================
+	COLUMN	DESCRIPTION
+	=======	===============================================================
+	COOKIE	The cookie debug ID (also appears in traces)
+	VOLUME	The parent volume cookie debug ID
+	REF	Number of references on the volume cookie
+	ACT	Number of times the cookie is marked for in use
+	ACC	Number of access pins in the cookie
+	S	State of the cookie
+	FL	Flags on the cookie
+	DEF	Key, auxiliary data
+	=======	===============================================================
+
+
+Debugging
+=========
+
+If CONFIG_NETFS_DEBUG is enabled, the FS-Cache facility and NETFS support can
+have runtime debugging enabled by adjusting the value in::
+
+	/sys/module/netfs/parameters/debug
+
+This is a bitmask of debugging streams to enable:
+
+	=======	=======	===============================	=======================
+	BIT	VALUE	STREAM				POINT
+	=======	=======	===============================	=======================
+	0	1	Cache management		Function entry trace
+	1	2					Function exit trace
+	2	4					General
+	3	8	Cookie management		Function entry trace
+	4	16					Function exit trace
+	5	32					General
+	6-8						(Not used)
+	9	512	I/O operation management	Function entry trace
+	10	1024					Function exit trace
+	11	2048					General
+	=======	=======	===============================	=======================
+
+The appropriate set of values should be OR'd together and the result written to
+the control file.  For example::
+
+	echo $((1|8|512)) >/sys/module/netfs/parameters/debug
+
+will turn on all function entry debugging.
diff --git a/Documentation/filesystems/caching/index.rst b/Documentation/filesystems/caching/index.rst
new file mode 100644
index 000000000..df4307124
--- /dev/null
+++ b/Documentation/filesystems/caching/index.rst
@@ -0,0 +1,12 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Filesystem Caching
+==================
+
+.. toctree::
+   :maxdepth: 2
+
+   fscache
+   netfs-api
+   backend-api
+   cachefiles
diff --git a/Documentation/filesystems/caching/netfs-api.rst b/Documentation/filesystems/caching/netfs-api.rst
new file mode 100644
index 000000000..665b27f15
--- /dev/null
+++ b/Documentation/filesystems/caching/netfs-api.rst
@@ -0,0 +1,452 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Network Filesystem Caching API
+==============================
+
+Fscache provides an API by which a network filesystem can make use of local
+caching facilities.  The API is arranged around a number of principles:
+
+ (1) A cache is logically organised into volumes and data storage objects
+     within those volumes.
+
+ (2) Volumes and data storage objects are represented by various types of
+     cookie.
+
+ (3) Cookies have keys that distinguish them from their peers.
+
+ (4) Cookies have coherency data that allows a cache to determine if the
+     cached data is still valid.
+
+ (5) I/O is done asynchronously where possible.
+
+This API is used by::
+
+	#include <linux/fscache.h>.
+
+.. This document contains the following sections:
+
+	 (1) Overview
+	 (2) Volume registration
+	 (3) Data file registration
+	 (4) Declaring a cookie to be in use
+	 (5) Resizing a data file (truncation)
+	 (6) Data I/O API
+	 (7) Data file coherency
+	 (8) Data file invalidation
+	 (9) Write back resource management
+	(10) Caching of local modifications
+	(11) Page release and invalidation
+
+
+Overview
+========
+
+The fscache hierarchy is organised on two levels from a network filesystem's
+point of view.  The upper level represents "volumes" and the lower level
+represents "data storage objects".  These are represented by two types of
+cookie, hereafter referred to as "volume cookies" and "cookies".
+
+A network filesystem acquires a volume cookie for a volume using a volume key,
+which represents all the information that defines that volume (e.g. cell name
+or server address, volume ID or share name).  This must be rendered as a
+printable string that can be used as a directory name (ie. no '/' characters
+and shouldn't begin with a '.').  The maximum name length is one less than the
+maximum size of a filename component (allowing the cache backend one char for
+its own purposes).
+
+A filesystem would typically have a volume cookie for each superblock.
+
+The filesystem then acquires a cookie for each file within that volume using an
+object key.  Object keys are binary blobs and only need to be unique within
+their parent volume.  The cache backend is responsible for rendering the binary
+blob into something it can use and may employ hash tables, trees or whatever to
+improve its ability to find an object.  This is transparent to the network
+filesystem.
+
+A filesystem would typically have a cookie for each inode, and would acquire it
+in iget and relinquish it when evicting the cookie.
+
+Once it has a cookie, the filesystem needs to mark the cookie as being in use.
+This causes fscache to send the cache backend off to look up/create resources
+for the cookie in the background, to check its coherency and, if necessary, to
+mark the object as being under modification.
+
+A filesystem would typically "use" the cookie in its file open routine and
+unuse it in file release and it needs to use the cookie around calls to
+truncate the cookie locally.  It *also* needs to use the cookie when the
+pagecache becomes dirty and unuse it when writeback is complete.  This is
+slightly tricky, and provision is made for it.
+
+When performing a read, write or resize on a cookie, the filesystem must first
+begin an operation.  This copies the resources into a holding struct and puts
+extra pins into the cache to stop cache withdrawal from tearing down the
+structures being used.  The actual operation can then be issued and conflicting
+invalidations can be detected upon completion.
+
+The filesystem is expected to use netfslib to access the cache, but that's not
+actually required and it can use the fscache I/O API directly.
+
+
+Volume Registration
+===================
+
+The first step for a network filesystem is to acquire a volume cookie for the
+volume it wants to access::
+
+	struct fscache_volume *
+	fscache_acquire_volume(const char *volume_key,
+			       const char *cache_name,
+			       const void *coherency_data,
+			       size_t coherency_len);
+
+This function creates a volume cookie with the specified volume key as its name
+and notes the coherency data.
+
+The volume key must be a printable string with no '/' characters in it.  It
+should begin with the name of the filesystem and should be no longer than 254
+characters.  It should uniquely represent the volume and will be matched with
+what's stored in the cache.
+
+The caller may also specify the name of the cache to use.  If specified,
+fscache will look up or create a cache cookie of that name and will use a cache
+of that name if it is online or comes online.  If no cache name is specified,
+it will use the first cache that comes to hand and set the name to that.
+
+The specified coherency data is stored in the cookie and will be matched
+against coherency data stored on disk.  The data pointer may be NULL if no data
+is provided.  If the coherency data doesn't match, the entire cache volume will
+be invalidated.
+
+This function can return errors such as EBUSY if the volume key is already in
+use by an acquired volume or ENOMEM if an allocation failure occurred.  It may
+also return a NULL volume cookie if fscache is not enabled.  It is safe to
+pass a NULL cookie to any function that takes a volume cookie.  This will
+cause that function to do nothing.
+
+
+When the network filesystem has finished with a volume, it should relinquish it
+by calling::
+
+	void fscache_relinquish_volume(struct fscache_volume *volume,
+				       const void *coherency_data,
+				       bool invalidate);
+
+This will cause the volume to be committed or removed, and if sealed the
+coherency data will be set to the value supplied.  The amount of coherency data
+must match the length specified when the volume was acquired.  Note that all
+data cookies obtained in this volume must be relinquished before the volume is
+relinquished.
+
+
+Data File Registration
+======================
+
+Once it has a volume cookie, a network filesystem can use it to acquire a
+cookie for data storage::
+
+	struct fscache_cookie *
+	fscache_acquire_cookie(struct fscache_volume *volume,
+			       u8 advice,
+			       const void *index_key,
+			       size_t index_key_len,
+			       const void *aux_data,
+			       size_t aux_data_len,
+			       loff_t object_size)
+
+This creates the cookie in the volume using the specified index key.  The index
+key is a binary blob of the given length and must be unique for the volume.
+This is saved into the cookie.  There are no restrictions on the content, but
+its length shouldn't exceed about three quarters of the maximum filename length
+to allow for encoding.
+
+The caller should also pass in a piece of coherency data in aux_data.  A buffer
+of size aux_data_len will be allocated and the coherency data copied in.  It is
+assumed that the size is invariant over time.  The coherency data is used to
+check the validity of data in the cache.  Functions are provided by which the
+coherency data can be updated.
+
+The file size of the object being cached should also be provided.  This may be
+used to trim the data and will be stored with the coherency data.
+
+This function never returns an error, though it may return a NULL cookie on
+allocation failure or if fscache is not enabled.  It is safe to pass in a NULL
+volume cookie and pass the NULL cookie returned to any function that takes it.
+This will cause that function to do nothing.
+
+
+When the network filesystem has finished with a cookie, it should relinquish it
+by calling::
+
+	void fscache_relinquish_cookie(struct fscache_cookie *cookie,
+				       bool retire);
+
+This will cause fscache to either commit the storage backing the cookie or
+delete it.
+
+
+Marking A Cookie In-Use
+=======================
+
+Once a cookie has been acquired by a network filesystem, the filesystem should
+tell fscache when it intends to use the cookie (typically done on file open)
+and should say when it has finished with it (typically on file close)::
+
+	void fscache_use_cookie(struct fscache_cookie *cookie,
+				bool will_modify);
+	void fscache_unuse_cookie(struct fscache_cookie *cookie,
+				  const void *aux_data,
+				  const loff_t *object_size);
+
+The *use* function tells fscache that it will use the cookie and, additionally,
+indicate if the user is intending to modify the contents locally.  If not yet
+done, this will trigger the cache backend to go and gather the resources it
+needs to access/store data in the cache.  This is done in the background, and
+so may not be complete by the time the function returns.
+
+The *unuse* function indicates that a filesystem has finished using a cookie.
+It optionally updates the stored coherency data and object size and then
+decreases the in-use counter.  When the last user unuses the cookie, it is
+scheduled for garbage collection.  If not reused within a short time, the
+resources will be released to reduce system resource consumption.
+
+A cookie must be marked in-use before it can be accessed for read, write or
+resize - and an in-use mark must be kept whilst there is dirty data in the
+pagecache in order to avoid an oops due to trying to open a file during process
+exit.
+
+Note that in-use marks are cumulative.  For each time a cookie is marked
+in-use, it must be unused.
+
+
+Resizing A Data File (Truncation)
+=================================
+
+If a network filesystem file is resized locally by truncation, the following
+should be called to notify the cache::
+
+	void fscache_resize_cookie(struct fscache_cookie *cookie,
+				   loff_t new_size);
+
+The caller must have first marked the cookie in-use.  The cookie and the new
+size are passed in and the cache is synchronously resized.  This is expected to
+be called from ``->setattr()`` inode operation under the inode lock.
+
+
+Data I/O API
+============
+
+To do data I/O operations directly through a cookie, the following functions
+are available::
+
+	int fscache_begin_read_operation(struct netfs_cache_resources *cres,
+					 struct fscache_cookie *cookie);
+	int fscache_read(struct netfs_cache_resources *cres,
+			 loff_t start_pos,
+			 struct iov_iter *iter,
+			 enum netfs_read_from_hole read_hole,
+			 netfs_io_terminated_t term_func,
+			 void *term_func_priv);
+	int fscache_write(struct netfs_cache_resources *cres,
+			  loff_t start_pos,
+			  struct iov_iter *iter,
+			  netfs_io_terminated_t term_func,
+			  void *term_func_priv);
+
+The *begin* function sets up an operation, attaching the resources required to
+the cache resources block from the cookie.  Assuming it doesn't return an error
+(for instance, it will return -ENOBUFS if given a NULL cookie, but otherwise do
+nothing), then one of the other two functions can be issued.
+
+The *read* and *write* functions initiate a direct-IO operation.  Both take the
+previously set up cache resources block, an indication of the start file
+position, and an I/O iterator that describes buffer and indicates the amount of
+data.
+
+The read function also takes a parameter to indicate how it should handle a
+partially populated region (a hole) in the disk content.  This may be to ignore
+it, skip over an initial hole and place zeros in the buffer or give an error.
+
+The read and write functions can be given an optional termination function that
+will be run on completion::
+
+	typedef
+	void (*netfs_io_terminated_t)(void *priv, ssize_t transferred_or_error,
+				      bool was_async);
+
+If a termination function is given, the operation will be run asynchronously
+and the termination function will be called upon completion.  If not given, the
+operation will be run synchronously.  Note that in the asynchronous case, it is
+possible for the operation to complete before the function returns.
+
+Both the read and write functions end the operation when they complete,
+detaching any pinned resources.
+
+The read operation will fail with ESTALE if invalidation occurred whilst the
+operation was ongoing.
+
+
+Data File Coherency
+===================
+
+To request an update of the coherency data and file size on a cookie, the
+following should be called::
+
+	void fscache_update_cookie(struct fscache_cookie *cookie,
+				   const void *aux_data,
+				   const loff_t *object_size);
+
+This will update the cookie's coherency data and/or file size.
+
+
+Data File Invalidation
+======================
+
+Sometimes it will be necessary to invalidate an object that contains data.
+Typically this will be necessary when the server informs the network filesystem
+of a remote third-party change - at which point the filesystem has to throw
+away the state and cached data that it had for an file and reload from the
+server.
+
+To indicate that a cache object should be invalidated, the following should be
+called::
+
+	void fscache_invalidate(struct fscache_cookie *cookie,
+				const void *aux_data,
+				loff_t size,
+				unsigned int flags);
+
+This increases the invalidation counter in the cookie to cause outstanding
+reads to fail with -ESTALE, sets the coherency data and file size from the
+information supplied, blocks new I/O on the cookie and dispatches the cache to
+go and get rid of the old data.
+
+Invalidation runs asynchronously in a worker thread so that it doesn't block
+too much.
+
+
+Write-Back Resource Management
+==============================
+
+To write data to the cache from network filesystem writeback, the cache
+resources required need to be pinned at the point the modification is made (for
+instance when the page is marked dirty) as it's not possible to open a file in
+a thread that's exiting.
+
+The following facilities are provided to manage this:
+
+ * An inode flag, ``I_PINNING_FSCACHE_WB``, is provided to indicate that an
+   in-use is held on the cookie for this inode.  It can only be changed if the
+   the inode lock is held.
+
+ * A flag, ``unpinned_fscache_wb`` is placed in the ``writeback_control``
+   struct that gets set if ``__writeback_single_inode()`` clears
+   ``I_PINNING_FSCACHE_WB`` because all the dirty pages were cleared.
+
+To support this, the following functions are provided::
+
+	bool fscache_dirty_folio(struct address_space *mapping,
+				 struct folio *folio,
+				 struct fscache_cookie *cookie);
+	void fscache_unpin_writeback(struct writeback_control *wbc,
+				     struct fscache_cookie *cookie);
+	void fscache_clear_inode_writeback(struct fscache_cookie *cookie,
+					   struct inode *inode,
+					   const void *aux);
+
+The *set* function is intended to be called from the filesystem's
+``dirty_folio`` address space operation.  If ``I_PINNING_FSCACHE_WB`` is not
+set, it sets that flag and increments the use count on the cookie (the caller
+must already have called ``fscache_use_cookie()``).
+
+The *unpin* function is intended to be called from the filesystem's
+``write_inode`` superblock operation.  It cleans up after writing by unusing
+the cookie if unpinned_fscache_wb is set in the writeback_control struct.
+
+The *clear* function is intended to be called from the netfs's ``evict_inode``
+superblock operation.  It must be called *after*
+``truncate_inode_pages_final()``, but *before* ``clear_inode()``.  This cleans
+up any hanging ``I_PINNING_FSCACHE_WB``.  It also allows the coherency data to
+be updated.
+
+
+Caching of Local Modifications
+==============================
+
+If a network filesystem has locally modified data that it wants to write to the
+cache, it needs to mark the pages to indicate that a write is in progress, and
+if the mark is already present, it needs to wait for it to be removed first
+(presumably due to an already in-progress operation).  This prevents multiple
+competing DIO writes to the same storage in the cache.
+
+Firstly, the netfs should determine if caching is available by doing something
+like::
+
+	bool caching = fscache_cookie_enabled(cookie);
+
+If caching is to be attempted, pages should be waited for and then marked using
+the following functions provided by the netfs helper library::
+
+	void set_page_fscache(struct page *page);
+	void wait_on_page_fscache(struct page *page);
+	int wait_on_page_fscache_killable(struct page *page);
+
+Once all the pages in the span are marked, the netfs can ask fscache to
+schedule a write of that region::
+
+	void fscache_write_to_cache(struct fscache_cookie *cookie,
+				    struct address_space *mapping,
+				    loff_t start, size_t len, loff_t i_size,
+				    netfs_io_terminated_t term_func,
+				    void *term_func_priv,
+				    bool caching)
+
+And if an error occurs before that point is reached, the marks can be removed
+by calling::
+
+	void fscache_clear_page_bits(struct address_space *mapping,
+				     loff_t start, size_t len,
+				     bool caching)
+
+In these functions, a pointer to the mapping to which the source pages are
+attached is passed in and start and len indicate the size of the region that's
+going to be written (it doesn't have to align to page boundaries necessarily,
+but it does have to align to DIO boundaries on the backing filesystem).  The
+caching parameter indicates if caching should be skipped, and if false, the
+functions do nothing.
+
+The write function takes some additional parameters: the cookie representing
+the cache object to be written to, i_size indicates the size of the netfs file
+and term_func indicates an optional completion function, to which
+term_func_priv will be passed, along with the error or amount written.
+
+Note that the write function will always run asynchronously and will unmark all
+the pages upon completion before calling term_func.
+
+
+Page Release and Invalidation
+=============================
+
+Fscache keeps track of whether we have any data in the cache yet for a cache
+object we've just created.  It knows it doesn't have to do any reading until it
+has done a write and then the page it wrote from has been released by the VM,
+after which it *has* to look in the cache.
+
+To inform fscache that a page might now be in the cache, the following function
+should be called from the ``release_folio`` address space op::
+
+	void fscache_note_page_release(struct fscache_cookie *cookie);
+
+if the page has been released (ie. release_folio returned true).
+
+Page release and page invalidation should also wait for any mark left on the
+page to say that a DIO write is underway from that page::
+
+	void wait_on_page_fscache(struct page *page);
+	int wait_on_page_fscache_killable(struct page *page);
+
+
+API Function Reference
+======================
+
+.. kernel-doc:: include/linux/fscache.h