Merge git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache

* git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache: (41 commits)
  NFS: Add mount options to enable local caching on NFS
  NFS: Display local caching state
  NFS: Store pages from an NFS inode into a local cache
  NFS: Read pages from FS-Cache into an NFS inode
  NFS: nfs_readpage_async() needs to be accessible as a fallback for local caching
  NFS: Add read context retention for FS-Cache to call back with
  NFS: FS-Cache page management
  NFS: Add some new I/O counters for FS-Cache doing things for NFS
  NFS: Invalidate FsCache page flags when cache removed
  NFS: Use local disk inode cache
  NFS: Define and create inode-level cache objects
  NFS: Define and create superblock-level objects
  NFS: Define and create server-level objects
  NFS: Register NFS for caching and retrieve the top-level index
  NFS: Permit local filesystem caching to be enabled for NFS
  NFS: Add FS-Cache option bit and debug bit
  NFS: Add comment banners to some NFS functions
  FS-Cache: Make kAFS use FS-Cache
  CacheFiles: A cache that backs onto a mounted filesystem
  CacheFiles: Export things for CacheFiles
  ...

1 files changed, 778 insertions, 0 deletions

diff --git a/Documentation/filesystems/caching/netfs-api.txt b/Documentation/filesystems/caching/netfs-api.txt
new file mode 100644
index 00000000000..4db125b3a5c
--- /dev/null
+++ b/Documentation/filesystems/caching/netfs-api.txt
@@ -0,0 +1,778 @@
+			===============================
+			FS-CACHE NETWORK FILESYSTEM API
+			===============================
+
+There's an API by which a network filesystem can make use of the FS-Cache
+facilities.  This is based around a number of principles:
+
+ (1) Caches can store a number of different object types.  There are two main
+     object types: indices and files.  The first is a special type used by
+     FS-Cache to make finding objects faster and to make retiring of groups of
+     objects easier.
+
+ (2) Every index, file or other object is represented by a cookie.  This cookie
+     may or may not have anything associated with it, but the netfs doesn't
+     need to care.
+
+ (3) Barring the top-level index (one entry per cached netfs), the index
+     hierarchy for each netfs is structured according the whim of the netfs.
+
+This API is declared in <linux/fscache.h>.
+
+This document contains the following sections:
+
+	 (1) Network filesystem definition
+	 (2) Index definition
+	 (3) Object definition
+	 (4) Network filesystem (un)registration
+	 (5) Cache tag lookup
+	 (6) Index registration
+	 (7) Data file registration
+	 (8) Miscellaneous object registration
+	 (9) Setting the data file size
+	(10) Page alloc/read/write
+	(11) Page uncaching
+	(12) Index and data file update
+	(13) Miscellaneous cookie operations
+	(14) Cookie unregistration
+	(15) Index and data file invalidation
+	(16) FS-Cache specific page flags.
+
+
+=============================
+NETWORK FILESYSTEM DEFINITION
+=============================
+
+FS-Cache needs a description of the network filesystem.  This is specified
+using a record of the following structure:
+
+	struct fscache_netfs {
+		uint32_t			version;
+		const char			*name;
+		struct fscache_cookie		*primary_index;
+		...
+	};
+
+This first two fields should be filled in before registration, and the third
+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 in-cache hierarchy for this netfs will be scrapped and begun
+     afresh).
+
+ (3) 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:
+
+	struct fscache_netfs afs_cache_netfs = {
+		.version	= 0,
+		.name		= "afs",
+	};
+
+
+================
+INDEX DEFINITION
+================
+
+Indices are used for two purposes:
+
+ (1) To aid 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, FS-Cache 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 indices and data files at the same level, but
+it's not recommended.
+
+Each index entry consists of a key of indeterminate length plus some auxilliary
+data, also of indeterminate length.
+
+There are some limits on indices:
+
+ (1) Any index containing non-index objects should be restricted to a single
+     cache.  Any such objects created within an index will be created in the
+     first cache only.  The cache in which an index is created can be
+     controlled by cache tags (see below).
+
+ (2) The entry data must be atomically journallable, so it is limited to about
+     400 bytes at present.  At least 400 bytes will be available.
+
+ (3) 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.
+
+
+=================
+OBJECT DEFINITION
+=================
+
+To define an object, a structure of the following type should be filled out:
+
+	struct fscache_cookie_def
+	{
+		uint8_t name[16];
+		uint8_t type;
+
+		struct fscache_cache_tag *(*select_cache)(
+			const void *parent_netfs_data,
+			const void *cookie_netfs_data);
+
+		uint16_t (*get_key)(const void *cookie_netfs_data,
+				    void *buffer,
+				    uint16_t bufmax);
+
+		void (*get_attr)(const void *cookie_netfs_data,
+				 uint64_t *size);
+
+		uint16_t (*get_aux)(const void *cookie_netfs_data,
+				    void *buffer,
+				    uint16_t bufmax);
+
+		enum fscache_checkaux (*check_aux)(void *cookie_netfs_data,
+						   const void *data,
+						   uint16_t datalen);
+
+		void (*get_context)(void *cookie_netfs_data, void *context);
+
+		void (*put_context)(void *cookie_netfs_data, void *context);
+
+		void (*mark_pages_cached)(void *cookie_netfs_data,
+					  struct address_space *mapping,
+					  struct pagevec *cached_pvec);
+
+		void (*now_uncached)(void *cookie_netfs_data);
+	};
+
+This has the following fields:
+
+ (1) The type of the object [mandatory].
+
+     This is one of the following values:
+
+	(*) FSCACHE_COOKIE_TYPE_INDEX
+
+	    This defines an index, which is a special FS-Cache type.
+
+	(*) FSCACHE_COOKIE_TYPE_DATAFILE
+
+	    This defines an ordinary data file.
+
+	(*) Any other value between 2 and 255
+
+	    This defines an extraordinary object such as an XATTR.
+
+ (2) The name of the object type (NUL terminated unless all 16 chars are used)
+     [optional].
+
+ (3) A function to select the cache in which to store an index [optional].
+
+     This function is invoked when an index needs to be instantiated in a cache
+     during the instantiation of a non-index object.  Only the immediate index
+     parent for the non-index object will be queried.  Any indices above that
+     in the hierarchy may be stored in multiple caches.  This function does not
+     need to be supplied for any non-index object or any index that will only
+     have index children.
+
+     If this function is not supplied or if it returns NULL then the first
+     cache in the parent's list will be chosed, or failing that, the first
+     cache in the master list.
+
+ (4) A function to retrieve an object's key from the netfs [mandatory].
+
+     This function will be called with the netfs data that was passed to the
+     cookie acquisition function and the maximum length of key data that it may
+     provide.  It should write the required key data into the given buffer and
+     return the quantity it wrote.
+
+ (5) A function to retrieve attribute data from the netfs [optional].
+
+     This function will be called with the netfs data that was passed to the
+     cookie acquisition function.  It should return the size of the file if
+     this is a data file.  The size may be used to govern how much cache must
+     be reserved for this file in the cache.
+
+     If the function is absent, a file size of 0 is assumed.
+
+ (6) A function to retrieve auxilliary data from the netfs [optional].
+
+     This function will be called with the netfs data that was passed to the
+     cookie acquisition function and the maximum length of auxilliary data that
+     it may provide.  It should write the auxilliary data into the given buffer
+     and return the quantity it wrote.
+
+     If this function is absent, the auxilliary data length will be set to 0.
+
+     The length of the auxilliary data buffer may be dependent on the key
+     length.  A netfs mustn't rely on being able to provide more than 400 bytes
+     for both.
+
+ (7) A function to check the auxilliary data [optional].
+
+     This function will be called to check that a match found in the cache for
+     this object is valid.  For instance with AFS it could check the auxilliary
+     data against the data version number returned by the server to determine
+     whether the index entry in a cache is still valid.
+
+     If this function is absent, it will be assumed that matching objects in a
+     cache are always valid.
+
+     If present, the function should return one of the following values:
+
+	(*) FSCACHE_CHECKAUX_OKAY		- the entry is okay as is
+	(*) FSCACHE_CHECKAUX_NEEDS_UPDATE	- the entry requires update
+	(*) FSCACHE_CHECKAUX_OBSOLETE		- the entry should be deleted
+
+     This function can also be used to extract data from the auxilliary data in
+     the cache and copy it into the netfs's structures.
+
+ (8) A pair of functions to manage contexts for the completion callback
+     [optional].
+
+     The cache read/write functions are passed a context which is then passed
+     to the I/O completion callback function.  To ensure this context remains
+     valid until after the I/O completion is called, two functions may be
+     provided: one to get an extra reference on the context, and one to drop a
+     reference to it.
+
+     If the context is not used or is a type of object that won't go out of
+     scope, then these functions are not required.  These functions are not
+     required for indices as indices may not contain data.  These functions may
+     be called in interrupt context and so may not sleep.
+
+ (9) A function to mark a page as retaining cache metadata [optional].
+
+     This is called by the cache to indicate that it is retaining in-memory
+     information for this page and that the netfs should uncache the page when
+     it has finished.  This does not indicate whether there's data on the disk
+     or not.  Note that several pages at once may be presented for marking.
+
+     The PG_fscache bit is set on the pages before this function would be
+     called, so the function need not be provided if this is sufficient.
+
+     This function is not required for indices as they're not permitted data.
+
+(10) A function to unmark all the pages retaining cache metadata [mandatory].
+
+     This is called by FS-Cache to indicate that a backing store is being
+     unbound from a cookie and that all the marks on the pages should be
+     cleared to prevent confusion.  Note that the cache will have torn down all
+     its tracking information so that the pages don't need to be explicitly
+     uncached.
+
+     This function is not required for indices as they're not permitted data.
+
+
+===================================
+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 fscache_register_netfs(struct fscache_netfs *netfs);
+
+It just takes a pointer to the netfs definition.  It returns 0 or an error as
+appropriate.
+
+For kAFS, registration is done as follows:
+
+	ret = fscache_register_netfs(&afs_cache_netfs);
+
+The last step is, of course, unregistration:
+
+	void fscache_unregister_netfs(struct fscache_netfs *netfs);
+
+
+================
+CACHE TAG LOOKUP
+================
+
+FS-Cache permits the use of more than one cache.  To permit particular index
+subtrees to be bound to particular caches, the second step is to look up cache
+representation tags.  This step is optional; it can be left entirely up to
+FS-Cache as to which cache should be used.  The problem with doing that is that
+FS-Cache will always pick the first cache that was registered.
+
+To get the representation for a named tag:
+
+	struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name);
+
+This takes a text string as the name and returns a representation of a tag.  It
+will never return an error.  It may return a dummy tag, however, if it runs out
+of memory; this will inhibit caching with this tag.
+
+Any representation so obtained must be released by passing it to this function:
+
+	void fscache_release_cache_tag(struct fscache_cache_tag *tag);
+
+The tag will be retrieved by FS-Cache when it calls the object definition
+operation select_cache().
+
+
+==================
+INDEX REGISTRATION
+==================
+
+The third step is to inform FS-Cache 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 fscache_cookie *
+	fscache_acquire_cookie(struct fscache_cookie *parent,
+			       const struct fscache_object_def *def,
+			       void *netfs_data);
+
+This function creates an index entry in the index represented by parent,
+filling in the index entry by calling the operations pointed to by def.
+
+Note that this function never returns an error - all errors are handled
+internally.  It may, however, return NULL to indicate no cookie.  It is quite
+acceptable to pass this token back to this function as the parent to another
+acquisition (or even to the relinquish cookie, read page and write page
+functions - see below).
+
+Note also that no indices are actually created in a cache until a non-index
+object 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 =
+		fscache_acquire_cookie(afs_cache_netfs.primary_index,
+				       &afs_cell_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 =
+		fscache_acquire_cookie(cell->cache,
+				       &afs_vlocation_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 =
+		fscache_acquire_cookie(vlocation->cache,
+				       &afs_volume_cache_index_def,
+				       volume);
+
+
+======================
+DATA FILE REGISTRATION
+======================
+
+The fourth step is to request a data file be created in the cache.  This is
+identical to index cookie acquisition.  The only difference is that the type in
+the object definition should be something other than index type.
+
+	vnode->cache =
+		fscache_acquire_cookie(volume->cache,
+				       &afs_vnode_cache_object_def,
+				       vnode);
+
+
+=================================
+MISCELLANEOUS OBJECT REGISTRATION
+=================================
+
+An optional step is to request an object of miscellaneous type be created in
+the cache.  This is almost identical to index cookie acquisition.  The only
+difference is that the type in the object definition should be something other
+than index type.  Whilst the parent object could be an index, it's more likely
+it would be some other type of object such as a data file.
+
+	xattr->cache =
+		fscache_acquire_cookie(vnode->cache,
+				       &afs_xattr_cache_object_def,
+				       xattr);
+
+Miscellaneous objects might be used to store extended attributes or directory
+entries for example.
+
+
+==========================
+SETTING THE DATA FILE SIZE
+==========================
+
+The fifth step is to set the physical attributes of the file, such as its size.
+This doesn't automatically reserve any space in the cache, but permits the
+cache to adjust its metadata for data tracking appropriately:
+
+	int fscache_attr_changed(struct fscache_cookie *cookie);
+
+The cache will return -ENOBUFS if there is no backing cache or if there is no
+space to allocate any extra metadata required in the cache.  The attributes
+will be accessed with the get_attr() cookie definition operation.
+
+Note that attempts to read or write data pages in the cache over this size may
+be rebuffed with -ENOBUFS.
+
+This operation schedules an attribute adjustment to happen asynchronously at
+some point in the future, and as such, it may happen after the function returns
+to the caller.  The attribute adjustment excludes read and write operations.
+
+
+=====================
+PAGE READ/ALLOC/WRITE
+=====================
+
+And the sixth step is to store and retrieve pages in the cache.  There are
+three functions that are used to do this.
+
+Note:
+
+ (1) A page should not be re-read or re-allocated without uncaching it first.
+
+ (2) A read or allocated page must be uncached when the netfs page is released
+     from the pagecache.
+
+ (3) A page should only be written to the cache if previous read or allocated.
+
+This permits the cache to maintain its page tracking in proper order.
+
+
+PAGE READ
+---------
+
+Firstly, the netfs should ask FS-Cache 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 (*fscache_rw_complete_t)(struct page *page,
+				      void *context,
+				      int error);
+
+	int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
+				       struct page *page,
+				       fscache_rw_complete_t end_io_func,
+				       void *context,
+				       gfp_t gfp);
+
+The cookie argument must specify a cookie for an object that isn't an index,
+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 in the cache:
+
+ (1) The mark_pages_cached() cookie operation will be called on that page.
+
+ (2) The function will submit a request to read the data from the cache's
+     backing device directly into the page specified.
+
+ (3) The function will return 0.
+
+ (4) 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 context argument passed to the above function.  This will be
+         maintained with the get_context/put_context functions mentioned above.
+
+     (*) An argument that's 0 on success or negative for an error code.
+
+     If an error occurs, it should be assumed that the page contains no usable
+     data.
+
+     end_io_func() will be called in process context if the read is results in
+     an error, but it might be called in interrupt context if the read is
+     successful.
+
+Otherwise, if there's not a copy available in cache, but the cache may be able
+to store the page:
+
+ (1) The mark_pages_cached() cookie operation will be called on that page.
+
+ (2) A block may be reserved in the cache and attached to the object at the
+     appropriate place.
+
+ (3) The function will return -ENODATA.
+
+This function may also return -ENOMEM or -EINTR, in which case it won't have
+read any data from the cache.
+
+
+PAGE ALLOCATE
+-------------
+
+Alternatively, if there's not expected to be any data in the cache for a page
+because the file has been extended, a block can simply be allocated instead:
+
+	int fscache_alloc_page(struct fscache_cookie *cookie,
+			       struct page *page,
+			       gfp_t gfp);
+
+This is similar to the fscache_read_or_alloc_page() function, except that it
+never reads from the cache.  It will return 0 if a block has been allocated,
+rather than -ENODATA as the other would.  One or the other must be performed
+before writing to the cache.
+
+The mark_pages_cached() cookie operation will be called on the page if
+successful.
+
+
+PAGE WRITE
+----------
+
+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 fscache_write_page(struct fscache_cookie *cookie,
+			       struct page *page,
+			       gfp_t 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.
+
+The page must have first been read or allocated successfully and must not have
+been uncached before writing is performed.
+
+If the cookie indicates the inode is not cached then:
+
+ (1) The function will return -ENOBUFS.
+
+Else if space can be allocated in the cache to hold this page:
+
+ (1) PG_fscache_write will be set on the page.
+
+ (2) The function will submit a request to write the data to cache's backing
+     device directly from the page specified.
+
+ (3) The function will return 0.
+
+ (4) When the write is complete PG_fscache_write is cleared on the page and
+     anyone waiting for that bit will be woken up.
+
+Else if there's no space available in the cache, -ENOBUFS will be returned.  It
+is also possible for the PG_fscache_write bit to be cleared when no write took
+place if unforeseen circumstances arose (such as a disk error).
+
+Writing takes place asynchronously.
+
+
+MULTIPLE PAGE READ
+------------------
+
+A facility is provided to read several pages at once, as requested by the
+readpages() address space operation:
+
+	int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
+					struct address_space *mapping,
+					struct list_head *pages,
+					int *nr_pages,
+					fscache_rw_complete_t end_io_func,
+					void *context,
+					gfp_t gfp);
+
+This works in a similar way to fscache_read_or_alloc_page(), except:
+
+ (1) Any page it can retrieve data for is removed from pages and nr_pages and
+     dispatched for reading to the disk.  Reads of adjacent pages on disk may
+     be merged for greater efficiency.
+
+ (2) The mark_pages_cached() cookie operation will be called on several pages
+     at once if they're being read or allocated.
+
+ (3) If there was an general error, then that error will be returned.
+
+     Else if some pages couldn't be allocated or read, then -ENOBUFS will be
+     returned.
+
+     Else if some pages couldn't be read but were allocated, then -ENODATA will
+     be returned.
+
+     Otherwise, if all pages had reads dispatched, then 0 will be returned, the
+     list will be empty and *nr_pages will be 0.
+
+ (4) end_io_func will be called once for each page being read as the reads
+     complete.  It will be called in process context if error != 0, but it may
+     be called in interrupt context if there is no error.
+
+Note that a return of -ENODATA, -ENOBUFS or any other error does not preclude
+some of the pages being read and some being allocated.  Those pages will have
+been marked appropriately and will need uncaching.
+
+
+==============
+PAGE UNCACHING
+==============
+
+To uncache a page, this function should be called:
+
+	void fscache_uncache_page(struct fscache_cookie *cookie,
+				  struct page *page);
+
+This function permits the cache to release any in-memory representation it
+might be holding for this netfs page.  This function must be called once for
+each page on which the read or write page functions above have been called to
+make sure the cache's in-memory tracking information gets torn down.
+
+Note that pages can't be explicitly deleted 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, so the page
+invalidation and release functions must use:
+
+	bool fscache_check_page_write(struct fscache_cookie *cookie,
+				      struct page *page);
+
+to see if a page is being written to the cache, and:
+
+	void fscache_wait_on_page_write(struct fscache_cookie *cookie,
+					struct page *page);
+
+to wait for it to finish if it is.
+
+
+==========================
+INDEX AND DATA FILE UPDATE
+==========================
+
+To request an update of the index data for an index or other object, the
+following function should be called:
+
+	void fscache_update_cookie(struct fscache_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.
+
+Note that partial updates may happen automatically at other times, such as when
+data blocks are added to a data file object.
+
+
+===============================
+MISCELLANEOUS COOKIE OPERATIONS
+===============================
+
+There are a number of operations that can be used to control cookies:
+
+ (*) Cookie pinning:
+
+	int fscache_pin_cookie(struct fscache_cookie *cookie);
+	void fscache_unpin_cookie(struct fscache_cookie *cookie);
+
+     These operations permit data cookies to be pinned into the cache and to
+     have the pinning removed.  They are not permitted on index cookies.
+
+     The pinning function will return 0 if successful, -ENOBUFS in the cookie
+     isn't backed by a cache, -EOPNOTSUPP if the cache doesn't support pinning,
+     -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
+     -EIO if there's any other problem.
+
+ (*) Data space reservation:
+
+	int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size);
+
+     This permits a netfs to request cache space be reserved to store up to the
+     given amount of a file.  It is permitted to ask for more than the current
+     size of the file to allow for future file expansion.
+
+     If size is given as zero then the reservation will be cancelled.
+
+     The function will return 0 if successful, -ENOBUFS in the cookie isn't
+     backed by a cache, -EOPNOTSUPP if the cache doesn't support reservations,
+     -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
+     -EIO if there's any other problem.
+
+     Note that this doesn't pin an object in a cache; it can still be culled to
+     make space if it's not in use.
+
+
+=====================
+COOKIE UNREGISTRATION
+=====================
+
+To get rid of a cookie, this function should be called.
+
+	void fscache_relinquish_cookie(struct fscache_cookie *cookie,
+				       int retire);
+
+If retire is non-zero, then the object will be marked for recycling, and all
+copies of it will be removed from all active caches in which it is present.
+Not only that but all child objects will also be retired.
+
+If retire is zero, then the object may be available again when next the
+acquisition function is called.  Retirement here will overrule the pinning on a
+cookie.
+
+One very important note - relinquish must NOT be called for a cookie unless all
+the cookies for "child" indices, objects and pages have been relinquished
+first.
+
+
+================================
+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.
+
+
+===========================
+FS-CACHE SPECIFIC PAGE FLAG
+===========================
+
+FS-Cache makes use of a page flag, PG_private_2, for its own purpose.  This is
+given the alternative name PG_fscache.
+
+PG_fscache is used to indicate that the page is known by the cache, and that
+the cache must be informed if the page is going to go away.  It's an indication
+to the netfs that the cache has an interest in this page, where an interest may
+be a pointer to it, resources allocated or reserved for it, or I/O in progress
+upon it.
+
+The netfs can use this information in methods such as releasepage() to
+determine whether it needs to uncache a page or update it.
+
+Furthermore, if this bit is set, releasepage() and invalidatepage() operations
+will be called on a page to get rid of it, even if PG_private is not set.  This
+allows caching to attempted on a page before read_cache_pages() to be called
+after fscache_read_or_alloc_pages() as the former will try and release pages it
+was given under certain circumstances.
+
+This bit does not overlap with such as PG_private.  This means that FS-Cache
+can be used with a filesystem that uses the block buffering code.
+
+There are a number of operations defined on this flag:
+
+	int PageFsCache(struct page *page);
+	void SetPageFsCache(struct page *page)
+	void ClearPageFsCache(struct page *page)
+	int TestSetPageFsCache(struct page *page)
+	int TestClearPageFsCache(struct page *page)
+
+These functions are bit test, bit set, bit clear, bit test and set and bit
+test and clear operations on PG_fscache.