Rework the CacheFS documentation to reflect FS-Cache split

diff -uNrp linux-2.6.9-rc3-mm2/Documentation/filesystems/cachefs.txt linux-2.6.9-rc3-mm2-fscache/Documentation/filesystems/cachefs.txt
@@ -1,892 +0,0 @@
-			  ===========================
-			  CacheFS: Caching Filesystem
-			  ===========================
-
-========
-OVERVIEW
-========
-
-CacheFS is a general purpose cache for network filesystems, though it could be
-used for caching other things such as ISO9660 filesystems too.
-
-CacheFS uses a block device directly rather than a bunch of files under an
-already mounted filesystem. For why this is so, see further on. If necessary,
-however, a file can be loopback mounted as a cache.
-
-CacheFS does not follow the idea of completely loading every netfs file opened
-into the cache before it can be operated upon, and then serving the pages out
-of CacheFS rather than the netfs 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.
-
-It rather serves the cache out in PAGE_SIZE chunks as and when requested by
-the netfs('s) using it.
-
-
-CacheFS provides the following facilities:
-
- (1) More than one block device can be mounted as a cache.
-
- (2) Caches can be mounted / unmounted at any time.
-
- (3) The netfs is provided with an interface that allows either party to
-     withdraw caching facilities from a file (required for (2)).
-
- (4) The interface to the netfs returns as few errors as possible, preferring
-     rather to let the netfs remain oblivious.
-
- (5) Cookies are used to represent files and indexes to the netfs. The simplest
-     cookie is just a NULL pointer - indicating nothing cached there.
-
- (6) The netfs is allowed to propose - dynamically - any index hierarchy it
-     desires, though it must be aware that the index search function is
-     recursive and stack space is limited.
-
- (7) Data I/O is done direct to and from the netfs's pages. The netfs indicates
-     that page A is at index B of the data-file represented by cookie C, and
-     that it should be read or written. CacheFS may or may not start I/O on
-     that page, but if it does, a netfs callback will be invoked to indicate
-     completion.
-
- (8) Cookies can be "retired" upon release. At this point CacheFS will mark
-     them as obsolete and the index hierarchy rooted at that point will get
-     recycled.
-
- (9) The netfs provides a "match" function for index searches. In addition to
-     saying whether a match was made or not, this can also specify that an
-     entry should be updated or deleted.
-
-(10) All metadata modifications (this includes index contents) are performed
-     as journalled transactions. These are replayed on mounting.
-
-
-=============================================
-WHY A BLOCK DEVICE? WHY NOT A BUNCH OF FILES?
-=============================================
-
-CacheFS is backed by a block device rather than being backed by a bunch of
-files on a filesystem. This confers several advantages:
-
- (1) Performance.
-
-     Going directly to a block device means that we can DMA directly to/from
-     the the netfs's pages. If another filesystem was managing the backing
-     store, everything would have to be copied between pages. Whilst DirectIO
-     does exist, it doesn't appear easy to make use of in this situation.
-
-     New address space or file operations could be added to make it possible to
-     persuade a backing discfs to generate block I/O directly to/from disc
-     blocks under its control, but that then means the discfs has to keep track
-     of I/O requests to pages not under its control.
-
-     Furthermore, we only have to do one lot of readahead calculations, not
-     two; in the discfs backing case, the netfs would do one and the discfs
-     would do one.
-
- (2) Memory.
-
-     Using a block device means that we have a lower memory usage - all data
-     pages belong to the netfs we're backing. If we used a filesystem, we would
-     have twice as many pages at certain points - one from the netfs and one
-     from the backing discfs. In the backing discfs model, under situations of
-     memory pressure, we'd have to allocate or keep around a discfs page to be
-     able to write out a netfs page; or else we'd need to be able to punch a
-     hole in the backing file.
-
-     Furthermore, whilst we have to keep a CacheFS inode around in memory for
-     every netfs inode we're backing, a backing discfs would have to keep the
-     dentry and possibly a file struct too.
-
- (3) Holes.
-
-     The cache uses holes to indicate to the netfs that it hasn't yet
-     downloaded the data for that page.
-
-     Since CacheFS is its own filesystem, it can support holes in files
-     trivially. Running on top of another discfs would limit us to using ones
-     that can support holes.
-
-     Furthermore, it would have to be made possible to detect holes in a discfs
-     file, rather than just seeing zero filled blocks.
-
- (4) Data Consistency.
-
-     Cachefs uses a pair of journals to keep track of the state of the cache
-     and all the pages contained therein. This means that it doesn't get into
-     an inconsistent state in the on-disc cache and it doesn't lose disc space.
-
-     CacheFS takes especial care between the allocation of a block and its
-     splicing into the on-disc pointer tree, and the data having been written
-     to disc. If power is interrupted and then restored, the journals are
-     replayed and if it is seen that a block was allocated but not written it
-     is then punched out. Being backed by a discfs, I'm not certain what will
-     happen. It may well be possible to mark a discfs's journal, if it has one,
-     but how does the discfs deal with those marks? This also limits consistent
-     caching to running on journalled discfs's where there's a function to
-     write extraordinary marks into the journal.
-
-     The alternative would be to keep flags in the superblock, and to
-     re-initialise the cache if it wasn't cleanly unmounted.
-
-     Knowing that your cache is in a good state is vitally important if you,
-     say, put /usr on AFS. Some organisations put everything barring /etc,
-     /sbin, /lib and /var on AFS and have an enormous cache on every
-     computer. Imagine if the power goes out and renders every cache
-     inconsistent, requiring all the computers to re-initialise their caches
-     when the power comes back on...
-
- (5) Recycling.
-
-     Recycling is simple on CacheFS. It can just scan the metadata index to
-     look for inodes that require reclamation/recycling; and it can also build
-     up a list of the least recently used inodes so that they can be reclaimed
-     later to make space.
-
-     Doing this on a discfs would require a search going down through a nest
-     of directories, and would probably have to be done in userspace.
-
- (6) Disc Space.
-
-     Whilst the block device does set a hard ceiling on the amount of space
-     available, CacheFS can guarantee that all that space will be available to
-     the cache. On a discfs-backed cache, the administrator would probably want
-     to set a cache size limit, but the system wouldn't be able guarantee that
-     all that space would be available to the cache - not unless that cache was
-     on a partition of its own.
-
-     Furthermore, with a discfs-backed cache, if the recycler starts to reclaim
-     cache files to make space, the freed blocks may just be eaten directly by
-     userspace programs, potentially resulting in the entire cache being
-     consumed. Alternatively, netfs operations may end up being held up because
-     the cache can't get blocks on which to store the data.
-
- (7) Users.
-
-     Users can't so easily go into CacheFS and run amok. The worst they can do
-     is cause bits of the cache to be recycled early. With a discfs-backed
-     cache, they can do all sorts of bad things to the files belonging to the
-     cache, and they can do this quite by accident.
-
-
-On the other hand, there would be some advantages to using a file-based cache
-rather than a blockdev-based cache:
-
- (1) Having to copy to a discfs's page would mean that a netfs could just make
-     the copy and then assume its own page is ready to go.
-
- (2) Backing onto a discfs wouldn't require a committed block device. You would
-     just nominate a directory and go from there. With CacheFS you have to
-     repartition or install an extra drive to make use of it in an existing
-     system (though the loopback device offers a way out).
-
- (3) CacheFS requires the netfs to store a key in any pertinent index entry,
-     and it also permits a limited amount arbitrary data to be stored there.
-
-     A discfs could be requested to store the netfs's data in xattrs, and the
-     filename could be used to store the key, though the key would have to be
-     rendered as text not binary. Likewise indexes could be rendered as
-     directories with xattrs.
-
- (4) You could easily make your cache bigger if the discfs has plenty of space,
-     you could even go across multiple mountpoints.
-
-
-======================
-GENERAL ON-DISC LAYOUT
-======================
-
-The filesystem is divided into a number of parts:
-
-  0	+---------------------------+
-	|        Superblock         |
-  1	+---------------------------+
-	|      Update Journal       |
-	+---------------------------+
-	|     Validity Journal      |
-	+---------------------------+
-	|    Write-Back Journal     |
-	+---------------------------+
-	|                           |
-	|           Data            |
-	|                           |
- END	+---------------------------+
-
-The superblock contains the filesystem ID tags and pointers to all the other
-regions.
-
-The update journal consists of a set of entries of sector size that keep track
-of what changes have been made to the on-disc filesystem, but not yet
-committed.
-
-The validity journal contains records of data blocks that have been allocated
-but not yet written. Upon journal replay, all these blocks will be detached
-from their pointers and recycled.
-
-The writeback journal keeps track of changes that have been made locally to
-data blocks, but that have not yet been committed back to the server. This is
-not yet implemented.
-
-The journals are replayed upon mounting to make sure that the cache is in a
-reasonable state.
-
-The data region holds a number of things:
-
-  (1) Index Files
-
-      These are files of entries used by CacheFS internally and by filesystems
-      that wish to cache data here (such as AFS) to keep track of what's in
-      the cache at any given time.
-
-      The first index file (inode 1) is special. It holds the CacheFS-specific
-      metadata for every file in the cache (including direct, single-indirect
-      and double-indirect block pointers).
-
-      The second index file (inode 2) is also special. It has an entry for
-      each filesystem that's currently holding data in this cache.
-
-      Every allocated entry in an index has an inode bound to it. This inode is
-      either another index file or it is a data file.
-
-  (2) Cached Data Files
-
-      These are caches of files from remote servers. Holes in these files
-      represent blocks not yet obtained from the server.
-
-  (3) Indirection Blocks
-
-      Should a file have more blocks than can be pointed to by the few
-      pointers in its storage management record, then indirection blocks will
-      be used to point to further data or indirection blocks.
-
-      Three levels of indirection are currently supported:
-
-	- single indirection
-	- double indirection
-
-  (4) Allocation Nodes and Free Blocks
-
-      The free blocks of the filesystem are kept in two single-branched
-      "trees". One tree is the blocks that are ready to be allocated, and the
-      other is the blocks that have just been recycled. When the former tree
-      becomes empty, the latter tree is decanted across.
-
-      Each tree is arranged as a chain of "nodes", each node points to the next
-      node in the chain (unless it's at the end) and also up to 1022 free
-      blocks.
-
-Note that all blocks are PAGE_SIZE in size. The blocks are numbered starting
-with the superblock at 0. Using 32-bit block pointers, a maximum number of
-0xffffffff blocks can be accessed, meaning that the maximum cache size is ~16TB
-for 4KB pages.
-
-
-========
-MOUNTING
-========
-
-Since CacheFS is actually a quasi-filesystem, it requires a block device behind
-it. The way to give it one is to mount it as cachefs type on a directory
-somewhere. The mounted filesystem will then present the user with a set of
-directories outlining the index structure resident in the cache. Indexes
-(directories) and files can be turfed out of the cache by the sysadmin through
-the use of rmdir and unlink.
-
-For instance, if a cache contains AFS data, the user might see the following:
-
-	root>mount -t cachefs /dev/hdg9 /cache-hdg9
-	root>ls -1 /cache-hdg9
-	afs
-	root>ls -1 /cache-hdg9/afs
-	cambridge.redhat.com
-	root>ls -1 /cache-hdg9/afs/cambridge.redhat.com
-	root.afs
-	root.cell
-
-However, a block device that's going to be used for a cache must be prepared
-before it can be mounted initially. This is done very simply by:
-
-	echo "cachefs___" >/dev/hdg9
-
-During the initial mount, the basic structure will be scribed into the cache,
-and then a background thread will "recycle" the as-yet unused data blocks.
-
-
-======================
-NETWORK FILESYSTEM API
-======================
-
-There is, of course, an API by which a network filesystem can make use of the
-CacheFS facilities. This is based around a number of principles:
-
- (1) Every file and index is represented by a cookie. This cookie may or may
-     not have anything associated with it, but the netfs doesn't need to care.
-
- (2) Barring the top-level index (one entry per cached netfs), the index
-     hierarchy for each netfs is structured according the whim of the netfs.
-
- (3) Any netfs page being backed by the cache must have a small token
-     associated with it (possibly pointed to by page->private) so that CacheFS
-     can keep track of it.
-
-This API is declared in <linux/cachefs.h>.
-
-
-NETWORK FILESYSTEM DEFINITION
------------------------------
-
-CacheFS needs a description of the network filesystem. This is specified using
-a record of the following structure:
-
-	struct cachefs_netfs {
-		const char			*name;
-		unsigned			version;
-		struct cachefs_netfs_operations	*ops;
-		struct cachefs_cookie		*primary_index;
-		...
-	};
-
-This first three fields should be filled in before registration, and the fourth
-will be filled in by the registration function; any other fields should just be
-ignored and are for internal use only.
-
-The fields are:
-
- (1) The name of the netfs (used as the key in the toplevel index).
-
- (2) The version of the netfs (if the name matches but the version doesn't, the
-     entire on-disc hierarchy for this netfs will be scrapped and begun
-     afresh).
-
- (3) The operations table is defined as follows:
-
-	struct cachefs_netfs_operations {
-		struct cachefs_page *(*get_page_cookie)(struct page *page);
-	};
-
-     The functions here must all be present. Currently the only one is:
-
-     (a) get_page_cookie(): Get the token used to bind a page to a block in a
-         cache. This function should allocate it if it doesn't exist.
-
-	 Return -ENOMEM if there's not enough memory and -ENODATA if the page
-	 just shouldn't be cached.
-
-	 Set *_page_cookie to point to the token and return 0 if there is now a
-	 cookie. Note that the netfs must keep track of the cookie itself (and
-	 free it later). page->private can be used for this (see below).
-
- (4) The cookie representing the primary index will be allocated according to
-     another parameter passed into the registration function.
-
-For example, kAFS (linux/fs/afs/) uses the following definitions to describe
-itself:
-
-	static struct cachefs_netfs_operations afs_cache_ops = {
-		.get_page_cookie	= afs_cache_get_page_cookie,
-	};
-
-	struct cachefs_netfs afs_cache_netfs = {
-		.name			= "afs",
-		.version		= 0,
-		.ops			= &afs_cache_ops,
-	};
-
-
-INDEX DEFINITION
-----------------
-
-Indexes are used for two purposes:
-
- (1) To speed up the finding of a file based on a series of keys (such as AFS's
-     "cell", "volume ID", "vnode ID").
-
- (2) To make it easier to discard a subset of all the files cached based around
-     a particular key - for instance to mirror the removal of an AFS volume.
-
-However, since it's unlikely that any two netfs's are going to want to define
-their index hierarchies in quite the same way, CacheFS tries to impose as few
-restraints as possible on how an index is structured and where it is placed in
-the tree. The netfs can even mix indexes and data files at the same level, but
-it's not recommended.
-
-There are some limits on indexes:
-
- (1) All entries in any given index must be the same size. An array of such
-     entries needn't fit exactly into a page, but they will be not laid across
-     a page boundary.
-
-     The netfs supplies a blob of data for each index entry, and CacheFS
-     provides an inode number and a flag.
-
- (2) The entries in one index can be of a different size to the entries in
-     another index.
-
- (3) The entry data must be journallable, and thus must be able to fit into an
-     update journal entry - this limits the maximum size to a little over 400
-     bytes at present.
-
- (4) The index data must start with the key. The layout of the key is described
-     in the index definition, and this is used to display the key in some
-     appropriate way.
-
- (5) The depth of the index tree should be judged with care as the search
-     function is recursive. Too many layers will run the kernel out of stack.
-
-To define an index, a structure of the following type should be filled out:
-
-	struct cachefs_index_def
-	{
-		uint8_t name[8];
-		uint16_t data_size;
-		struct {
-			uint8_t type;
-			uint16_t len;
-		} keys[4];
-
-		cachefs_match_val_t (*match)(void *target_netfs_data,
-					     const void *entry);
-
-		void (*update)(void *source_netfs_data, void *entry);
-	};
-
-This has the following fields:
-
- (1) The name of the index (NUL terminated unless all 8 chars are used).
-
- (2) The size of the data blob provided by the netfs.
-
- (3) A definition of the key(s) at the beginning of the blob. The netfs is
-     permitted to specify up to four keys. The total length must not exceed the
-     data size. It is assumed that the keys will be laid end to end in order,
-     starting at the first byte of the data.
-
-     The type field specifies the way the data should be displayed. It can be
-     one of:
-
-	(*) CACHEFS_INDEX_KEYS_NOTUSED	- key field not used
-	(*) CACHEFS_INDEX_KEYS_BIN	- display byte-by-byte in hex
-	(*) CACHEFS_INDEX_KEYS_ASCIIZ	- NUL-terminated ASCII
-	(*) CACHEFS_INDEX_KEYS_IPV4ADDR	- display as IPv4 address
-	(*) CACHEFS_INDEX_KEYS_IPV6ADDR	- display as IPv6 address
-
- (4) A function to compare an in-page-cache index entry blob with the data
-     passed to the cookie acquisition function. This function can also be used
-     to extract data from the blob and copy it into the netfs's structures.
-
-     The values this function can return are:
-
-	(*) CACHEFS_MATCH_FAILED - failed to match
-	(*) CACHEFS_MATCH_SUCCESS - successful match
-	(*) CACHEFS_MATCH_SUCCESS_UPDATE - successful match, entry needs update
-	(*) CACHEFS_MATCH_SUCCESS_DELETE - entry should be deleted
-
-     For example, in linux/fs/afs/vnode.c:
-
-	static cachefs_match_val_t
-	afs_vnode_cache_match(void *target, const void *entry)
-	{
-		const struct afs_cache_vnode *cvnode = entry;
-		struct afs_vnode *vnode = target;
-
-		if (vnode->fid.vnode != cvnode->vnode_id)
-			return CACHEFS_MATCH_FAILED;
-
-		if (vnode->fid.unique != cvnode->vnode_unique ||
-		    vnode->status.version != cvnode->data_version)
-			return CACHEFS_MATCH_SUCCESS_DELETE;
-
-		return CACHEFS_MATCH_SUCCESS;
-	}
-
- (5) A function to initialise or update an in-page-cache index entry blob from
-     netfs data passed to CacheFS by the netfs. This function should not assume
-     that there's any data yet in the in-page-cache.
-
-     Continuing the above example:
-
-	static void afs_vnode_cache_update(void *source, void *entry)
-	{
-		struct afs_cache_vnode *cvnode = entry;
-		struct afs_vnode *vnode = source;
-
-		cvnode->vnode_id	= vnode->fid.vnode;
-		cvnode->vnode_unique	= vnode->fid.unique;
-		cvnode->data_version	= vnode->status.version;
-	}
-
-To finish the above example, the index definition for the "vnode" level is as
-follows:
-
-	struct cachefs_index_def afs_vnode_cache_index_def = {
-		.name		= "vnode",
-		.data_size	= sizeof(struct afs_cache_vnode),
-		.keys[0]	= { CACHEFS_INDEX_KEYS_BIN, 4 },
-		.match		= afs_vnode_cache_match,
-		.update		= afs_vnode_cache_update,
-	};
-
-The first element of struct afs_cache_vnode is the vnode ID.
-
-And for contrast, the cell index definition is:
-
-	struct cachefs_index_def afs_cache_cell_index_def = {
-		.name			= "cell_ix",
-		.data_size		= sizeof(afs_cell_t),
-		.keys[0]		= { CACHEFS_INDEX_KEYS_ASCIIZ, 64 },
-		.match			= afs_cell_cache_match,
-		.update			= afs_cell_cache_update,
-	};
-
-The cell index is the primary index for kAFS.
-
-
-NETWORK FILESYSTEM (UN)REGISTRATION
------------------------------------
-
-The first step is to declare the network filesystem to the cache. This also
-involves specifying the layout of the primary index (for AFS, this would be the
-"cell" level).
-
-The registration function is:
-
-	int cachefs_register_netfs(struct cachefs_netfs *netfs,
-				   struct cachefs_index_def *primary_idef);
-
-It just takes pointers to the netfs definition and the primary index
-definition. It returns 0 or an error as appropriate.
-
-For kAFS, registration is done as follows:
-
-	ret = cachefs_register_netfs(&afs_cache_netfs,
-				     &afs_cache_cell_index_def);
-
-The last step is, of course, unregistration:
-
-	void cachefs_unregister_netfs(struct cachefs_netfs *netfs);
-
-
-INDEX REGISTRATION
-------------------
-
-The second step is to inform cachefs about part of an index hierarchy that can
-be used to locate files. This is done by requesting a cookie for each index in
-the path to the file:
-
-	struct cachefs_cookie *
-	cachefs_acquire_cookie(struct cachefs_cookie *iparent,
-			       struct cachefs_index_def *idef,
-			       void *netfs_data);
-
-This function creates an index entry in the index represented by iparent,
-loading the associated blob by calling iparent's update method with the
-supplied netfs_data.
-
-It also creates a new index inode, formatted according to the definition
-supplied in idef. The new cookie is then returned in *_cookie.
-
-Note that this function never returns an error - all errors are handled
-internally. It may also return CACHEFS_NEGATIVE_COOKIE. It is quite acceptable
-to pass this token back to this function as iparent (or even to the relinquish
-cookie, read page and write page functions - see below).
-
-Note also that no indexes are actually created on disc until a data file needs
-to be created somewhere down the hierarchy. Furthermore, an index may be
-created in several different caches independently at different times. This is
-all handled transparently, and the netfs doesn't see any of it.
-
-For example, with AFS, a cell would be added to the primary index. This index
-entry would have a dependent inode containing a volume location index for the
-volume mappings within this cell:
-
-	cell->cache =
-		cachefs_acquire_cookie(afs_cache_netfs.primary_index,
-				       &afs_vlocation_cache_index_def,
-				       cell);
-
-Then when a volume location was accessed, it would be entered into the cell's
-index and an inode would be allocated that acts as a volume type and hash chain
-combination:
-
-	vlocation->cache =
-		cachefs_acquire_cookie(cell->cache,
-				       &afs_volume_cache_index_def,
-				       vlocation);
-
-And then a particular flavour of volume (R/O for example) could be added to
-that index, creating another index for vnodes (AFS inode equivalents):
-
-	volume->cache =
-		cachefs_acquire_cookie(vlocation->cache,
-				       &afs_vnode_cache_index_def,
-				       volume);
-
-
-DATA FILE REGISTRATION
-----------------------
-
-The third step is to request a data file be created in the cache. This is
-almost identical to index cookie acquisition. The only difference is that a
-NULL index definition is passed.
-
-	vnode->cache =
-		cachefs_acquire_cookie(volume->cache,
-				       NULL,
-				       vnode);
-
-
-
-PAGE ALLOC/READ/WRITE
----------------------
-
-And the fourth step is to propose a page be cached. There are two functions
-that are used to do this.
-
-Firstly, the netfs should ask CacheFS to examine the caches and read the
-contents cached for a particular page of a particular file if present, or else
-allocate space to store the contents if not:
-
-	typedef
-	void (*cachefs_rw_complete_t)(void *cookie_data,
-				      struct page *page,
-				      void *end_io_data,
-				      int error);
-
-	int cachefs_read_or_alloc_page(struct cachefs_cookie *cookie,
-				       struct page *page,
-				       cachefs_rw_complete_t end_io_func,
-				       void *end_io_data,
-				       unsigned long gfp);
-
-The cookie argument must specify a data file cookie, the page specified will
-have the data loaded into it (and is also used to specify the page number), and
-the gfp argument is used to control how any memory allocations made are satisfied.
-
-If the cookie indicates the inode is not cached:
-
- (1) The function will return -ENOBUFS.
-
-Else if there's a copy of the page resident on disc:
-
- (1) The function will submit a request to read the data off the disc directly
-     into the page specified.
-
- (2) The function will return 0.
-
- (3) When the read is complete, end_io_func() will be invoked with:
-
-     (*) The netfs data supplied when the cookie was created.
-
-     (*) The page descriptor.
-
-     (*) The data passed to the above function.
-
-     (*) An argument that's 0 on success or negative for an error.
-
-     If an error occurs, it should be assumed that the page contains no usable
-     data.
-
-Otherwise, if there's not a copy available on disc:
-
- (1) A block may be allocated in the cache and attached to the inode at the
-     appropriate place.
-
- (2) The validity journal will be marked to indicate this page does not yet
-     contain valid data.
-
- (3) The function will return -ENODATA.
-
-
-Secondly, if the netfs changes the contents of the page (either due to an
-initial download or if a user performs a write), then the page should be
-written back to the cache:
-
-	int cachefs_write_page(struct cachefs_cookie *cookie,
-			       struct page *page,
-			       cachefs_rw_complete_t end_io_func,
-			       void *end_io_data,
-			       unsigned long gfp);
-
-The cookie argument must specify a data file cookie, the page specified should
-contain the data to be written (and is also used to specify the page number),
-and the gfp argument is used to control how any memory allocations made are
-satisfied.
-
-If the cookie indicates the inode is not cached then:
-
- (1) The function will return -ENOBUFS.
-
-Else if there's a block allocated on disc to hold this page:
-
- (1) The function will submit a request to write the data to the disc directly
-     from the page specified.
-
- (2) The function will return 0.
-
- (3) When the write is complete:
-
-     (a) Any associated validity journal entry will be cleared (the block now
-	 contains valid data as far as CacheFS is concerned).
-
-     (b) end_io_func() will be invoked with:
-
-	 (*) The netfs data supplied when the cookie was created.
-
-	 (*) The page descriptor.
-
-	 (*) The data passed to the above function.
-
-	 (*) An argument that's 0 on success or negative for an error.
-
-	 If an error happens, it can be assumed that the page has been
-	 discarded from the cache.
-
-
-PAGE UNCACHING
---------------
-
-To uncache a page, this function should be called:
-
-	void cachefs_uncache_page(struct cachefs_cookie *cookie,
-				  struct page *page);
-
-This detaches the page specified from the data file indicated by the cookie and
-unbinds it from the underlying block.
-
-Note that pages can't be explicitly detached from the a data file. The whole
-data file must be retired (see the relinquish cookie function below).
-
-Furthermore, note that this does not cancel the asynchronous read or write
-operation started by the read/alloc and write functions.
-
-
-INDEX AND DATA FILE UPDATE
---------------------------
-
-To request an update of the index data for an index or data file, the following
-function should be called:
-
-	void cachefs_update_cookie(struct cachefs_cookie *cookie);
-
-This function will refer back to the netfs_data pointer stored in the cookie by
-the acquisition function to obtain the data to write into each revised index
-entry. The update method in the parent index definition will be called to
-transfer the data.
-
-
-INDEX AND DATA FILE UNREGISTRATION
-----------------------------------
-
-To get rid of a cookie, this function should be called.
-
-	void cachefs_relinquish_cookie(struct cachefs_cookie *cookie,
-				       int retire);
-
-If retire is non-zero, then the index or file will be marked for recycling, and
-all copies of it will be removed from all active caches in which it is present.
-
-If retire is zero, then the inode may be available again next the the
-acquisition function is called.
-
-One very important note - relinquish must NOT be called unless all "child"
-indexes, files and pages have been relinquished first.
-
-
-PAGE TOKEN MANAGEMENT
----------------------
-
-As previously mentioned, the netfs must keep a token associated with each page
-currently actively backed by the cache. This is used by CacheFS to go from a
-page to the internal representation of the underlying block and back again. It
-is particularly important for managing the withdrawal of a cache whilst it is
-in active service (eg: it got unmounted).
-
-The token is this:
-
-	struct cachefs_page {
-		...
-	};
-
-Note that all fields are for internal CacheFS use only.
-
-The token only needs to be allocated when CacheFS asks for it. This it will do
-by calling the get_page_cookie() method in the netfs definition ops table. Once
-allocated, the same token should be presented every time the method is called
-again for a particular page.
-
-The token should be retained by the netfs, and should be deleted only after the
-page has been uncached.
-
-One way to achieve this is to attach the token to page->private (and set the
-PG_private bit on the page) once allocated. Shortcut routines are provided by
-CacheFS to do this. Firstly, to retrieve if present and allocate if not:
-
-	struct cachefs_page *cachefs_page_get_private(struct page *page,
-						      unsigned gfp);
-
-Secondly to retrieve if present and BUG if not:
-
-	static inline
-	struct cachefs_page *cachefs_page_grab_private(struct page *page);
-
-To clean up the tokens, the netfs inode hosting the page should be provided
-with address space operations that circumvent the buffer-head operations for a
-page. For instance:
-
-	struct address_space_operations afs_fs_aops = {
-		...
-		.sync_page	= block_sync_page,
-		.set_page_dirty	= __set_page_dirty_nobuffers,
-		.releasepage	= afs_file_releasepage,
-		.invalidatepage	= afs_file_invalidatepage,
-	};
-
-	static int afs_file_invalidatepage(struct page *page,
-					   unsigned long offset)
-	{
-		struct afs_vnode *vnode = AFS_FS_I(page->mapping->host);
-		int ret = 1;
-
-		BUG_ON(!PageLocked(page));
-		if (!PagePrivate(page))
-			return 1;
-		cachefs_uncache_page(vnode->cache,page);
-		if (offset == 0)
-			return 1;
-		BUG_ON(!PageLocked(page));
-		if (PageWriteback(page))
-			return 0;
-		return page->mapping->a_ops->releasepage(page, 0);
-	}
-
-	static int afs_file_releasepage(struct page *page, int gfp_flags)
-	{
-		struct cachefs_page *token;
-		struct afs_vnode *vnode = AFS_FS_I(page->mapping->host);
-
-		if (PagePrivate(page)) {
-			cachefs_uncache_page(vnode->cache, page);
-			token = (struct cachefs_page *) page->private;
-			page->private = 0;
-			ClearPagePrivate(page);
-			if (token)
-				kfree(token);
-		}
-		return 0;
-	}
-
-
-INDEX AND DATA FILE INVALIDATION
---------------------------------
-
-There is no direct way to invalidate an index subtree or a data file. To do
-this, the caller should relinquish and retire the cookie they have, and then
-acquire a new one.

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