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- ===============================
- 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 auxiliary
-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 chosen, 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 auxiliary 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 auxiliary data that
- it may provide. It should write the auxiliary data into the given buffer
- and return the quantity it wrote.
-
- If this function is absent, the auxiliary data length will be set to 0.
-
- The length of the auxiliary 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 auxiliary 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 auxiliary
- 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 auxiliary 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 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.
-
-
-When releasepage() is being implemented, a special FS-Cache function exists to
-manage the heuristics of coping with vmscan trying to eject pages, which may
-conflict with the cache trying to write pages to the cache (which may itself
-need to allocate memory):
-
- bool fscache_maybe_release_page(struct fscache_cookie *cookie,
- struct page *page,
- gfp_t gfp);
-
-This takes the netfs cookie, and the page and gfp arguments as supplied to
-releasepage(). It will return false if the page cannot be released yet for
-some reason and if it returns true, the page has been uncached and can now be
-released.
-
-To make a page available for release, this function may wait for an outstanding
-storage request to complete, or it may attempt to cancel the storage request -
-in which case the page will not be stored in the cache this time.
-
-
-BULK INODE PAGE UNCACHE
------------------------
-
-A convenience routine is provided to perform an uncache on all the pages
-attached to an inode. This assumes that the pages on the inode correspond on a
-1:1 basis with the pages in the cache.
-
- void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
- struct inode *inode);
-
-This takes the netfs cookie that the pages were cached with and the inode that
-the pages are attached to. This function will wait for pages to finish being
-written to the cache and for the cache to finish with the page generally. No
-error is returned.
-
-
-==========================
-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.