1 ==========================
2 FS-CACHE CACHE BACKEND API
3 ==========================
5 The FS-Cache system provides an API by which actual caches can be supplied to
6 FS-Cache for it to then serve out to network filesystems and other interested
9 This API is declared in <linux/fscache-cache.h>.
12 ====================================
13 INITIALISING AND REGISTERING A CACHE
14 ====================================
16 To start off, a cache definition must be initialised and registered for each
17 cache the backend wants to make available. For instance, CacheFS does this in
18 the fill_super() operation on mounting.
20 The cache definition (struct fscache_cache) should be initialised by calling:
22 void fscache_init_cache(struct fscache_cache *cache,
23 struct fscache_cache_ops *ops,
29 (*) "cache" is a pointer to the cache definition;
31 (*) "ops" is a pointer to the table of operations that the backend supports on
34 (*) "idfmt" is a format and printf-style arguments for constructing a label
38 The cache should then be registered with FS-Cache by passing a pointer to the
39 previously initialised cache definition to:
41 int fscache_add_cache(struct fscache_cache *cache,
42 struct fscache_object *fsdef,
45 Two extra arguments should also be supplied:
47 (*) "fsdef" which should point to the object representation for the FS-Cache
48 master index in this cache. Netfs primary index entries will be created
49 here. FS-Cache keeps the caller's reference to the index object if
50 successful and will release it upon withdrawal of the cache.
52 (*) "tagname" which, if given, should be a text string naming this cache. If
53 this is NULL, the identifier will be used instead. For CacheFS, the
54 identifier is set to name the underlying block device and the tag can be
57 This function may return -ENOMEM if it ran out of memory or -EEXIST if the tag
58 is already in use. 0 will be returned on success.
65 A cache can be withdrawn from the system by calling this function with a
66 pointer to the cache definition:
68 void fscache_withdraw_cache(struct fscache_cache *cache);
70 In CacheFS's case, this is called by put_super().
77 The cache methods are executed one of two contexts:
79 (1) that of the userspace process that issued the netfs operation that caused
80 the cache method to be invoked, or
82 (2) that of one of the processes in the FS-Cache thread pool.
84 In either case, this may not be an appropriate context in which to access the
87 The calling process's fsuid, fsgid and SELinux security identities may need to
88 be masqueraded for the duration of the cache driver's access to the cache.
89 This is left to the cache to handle; FS-Cache makes no effort in this regard.
92 ===================================
93 CONTROL AND STATISTICS PRESENTATION
94 ===================================
96 The cache may present data to the outside world through FS-Cache's interfaces
97 in sysfs and procfs - the former for control and the latter for statistics.
99 A sysfs directory called /sys/fs/fscache/<cachetag>/ is created if CONFIG_SYSFS
100 is enabled. This is accessible through the kobject struct fscache_cache::kobj
101 and is for use by the cache as it sees fit.
104 ========================
105 RELEVANT DATA STRUCTURES
106 ========================
108 (*) Index/Data file FS-Cache representation cookie:
110 struct fscache_cookie {
111 struct fscache_object_def *def;
112 struct fscache_netfs *netfs;
117 The fields that might be of use to the backend describe the object
118 definition, the netfs definition and the netfs's data for this cookie.
119 The object definition contain functions supplied by the netfs for loading
120 and matching index entries; these are required to provide some of the
124 (*) In-cache object representation:
126 struct fscache_object {
129 FSCACHE_OBJECT_RECYCLING,
133 struct fscache_cache *cache;
134 struct fscache_cookie *cookie;
138 Structures of this type should be allocated by the cache backend and
139 passed to FS-Cache when requested by the appropriate cache operation. In
140 the case of CacheFS, they're embedded in CacheFS's internal object
143 The debug_id is a simple integer that can be used in debugging messages
144 that refer to a particular object. In such a case it should be printed
145 using "OBJ%x" to be consistent with FS-Cache.
147 Each object contains a pointer to the cookie that represents the object it
148 is backing. An object should retired when put_object() is called if it is
149 in state FSCACHE_OBJECT_RECYCLING. The fscache_object struct should be
150 initialised by calling fscache_object_init(object).
153 (*) FS-Cache operation record:
155 struct fscache_operation {
157 struct fscache_object *object;
159 #define FSCACHE_OP_EXCLUSIVE
160 void (*processor)(struct fscache_operation *op);
161 void (*release)(struct fscache_operation *op);
165 FS-Cache has a pool of threads that it uses to give CPU time to the
166 various asynchronous operations that need to be done as part of driving
167 the cache. These are represented by the above structure. The processor
168 method is called to give the op CPU time, and the release method to get
169 rid of it when its usage count reaches 0.
171 An operation can be made exclusive upon an object by setting the
172 appropriate flag before enqueuing it with fscache_enqueue_operation(). If
173 an operation needs more processing time, it should be enqueued again.
176 (*) FS-Cache retrieval operation record:
178 struct fscache_retrieval {
179 struct fscache_operation op;
180 struct address_space *mapping;
181 struct list_head *to_do;
185 A structure of this type is allocated by FS-Cache to record retrieval and
186 allocation requests made by the netfs. This struct is then passed to the
187 backend to do the operation. The backend may get extra refs to it by
188 calling fscache_get_retrieval() and refs may be discarded by calling
189 fscache_put_retrieval().
191 A retrieval operation can be used by the backend to do retrieval work. To
192 do this, the retrieval->op.processor method pointer should be set
193 appropriately by the backend and fscache_enqueue_retrieval() called to
194 submit it to the thread pool. CacheFiles, for example, uses this to queue
195 page examination when it detects PG_lock being cleared.
197 The to_do field is an empty list available for the cache backend to use as
201 (*) FS-Cache storage operation record:
203 struct fscache_storage {
204 struct fscache_operation op;
209 A structure of this type is allocated by FS-Cache to record outstanding
210 writes to be made. FS-Cache itself enqueues this operation and invokes
211 the write_page() method on the object at appropriate times to effect
219 The cache backend provides FS-Cache with a table of operations that can be
220 performed on the denizens of the cache. These are held in a structure of type:
222 struct fscache_cache_ops
224 (*) Name of cache provider [mandatory]:
228 This isn't strictly an operation, but should be pointed at a string naming
232 (*) Allocate a new object [mandatory]:
234 struct fscache_object *(*alloc_object)(struct fscache_cache *cache,
235 struct fscache_cookie *cookie)
237 This method is used to allocate a cache object representation to back a
238 cookie in a particular cache. fscache_object_init() should be called on
239 the object to initialise it prior to returning.
241 This function may also be used to parse the index key to be used for
242 multiple lookup calls to turn it into a more convenient form. FS-Cache
243 will call the lookup_complete() method to allow the cache to release the
244 form once lookup is complete or aborted.
247 (*) Look up and create object [mandatory]:
249 void (*lookup_object)(struct fscache_object *object)
251 This method is used to look up an object, given that the object is already
252 allocated and attached to the cookie. This should instantiate that object
253 in the cache if it can.
255 The method should call fscache_object_lookup_negative() as soon as
256 possible if it determines the object doesn't exist in the cache. If the
257 object is found to exist and the netfs indicates that it is valid then
258 fscache_obtained_object() should be called once the object is in a
259 position to have data stored in it. Similarly, fscache_obtained_object()
260 should also be called once a non-present object has been created.
262 If a lookup error occurs, fscache_object_lookup_error() should be called
263 to abort the lookup of that object.
266 (*) Release lookup data [mandatory]:
268 void (*lookup_complete)(struct fscache_object *object)
270 This method is called to ask the cache to release any resources it was
271 using to perform a lookup.
274 (*) Increment object refcount [mandatory]:
276 struct fscache_object *(*grab_object)(struct fscache_object *object)
278 This method is called to increment the reference count on an object. It
279 may fail (for instance if the cache is being withdrawn) by returning NULL.
280 It should return the object pointer if successful.
283 (*) Lock/Unlock object [mandatory]:
285 void (*lock_object)(struct fscache_object *object)
286 void (*unlock_object)(struct fscache_object *object)
288 These methods are used to exclusively lock an object. It must be possible
289 to schedule with the lock held, so a spinlock isn't sufficient.
292 (*) Pin/Unpin object [optional]:
294 int (*pin_object)(struct fscache_object *object)
295 void (*unpin_object)(struct fscache_object *object)
297 These methods are used to pin an object into the cache. Once pinned an
298 object cannot be reclaimed to make space. Return -ENOSPC if there's not
299 enough space in the cache to permit this.
302 (*) Update object [mandatory]:
304 int (*update_object)(struct fscache_object *object)
306 This is called to update the index entry for the specified object. The
307 new information should be in object->cookie->netfs_data. This can be
308 obtained by calling object->cookie->def->get_aux()/get_attr().
311 (*) Discard object [mandatory]:
313 void (*drop_object)(struct fscache_object *object)
315 This method is called to indicate that an object has been unbound from its
316 cookie, and that the cache should release the object's resources and
317 retire it if it's in state FSCACHE_OBJECT_RECYCLING.
319 This method should not attempt to release any references held by the
320 caller. The caller will invoke the put_object() method as appropriate.
323 (*) Release object reference [mandatory]:
325 void (*put_object)(struct fscache_object *object)
327 This method is used to discard a reference to an object. The object may
328 be freed when all the references to it are released.
331 (*) Synchronise a cache [mandatory]:
333 void (*sync)(struct fscache_cache *cache)
335 This is called to ask the backend to synchronise a cache with its backing
339 (*) Dissociate a cache [mandatory]:
341 void (*dissociate_pages)(struct fscache_cache *cache)
343 This is called to ask a cache to perform any page dissociations as part of
347 (*) Notification that the attributes on a netfs file changed [mandatory]:
349 int (*attr_changed)(struct fscache_object *object);
351 This is called to indicate to the cache that certain attributes on a netfs
352 file have changed (for example the maximum size a file may reach). The
353 cache can read these from the netfs by calling the cookie's get_attr()
356 The cache may use the file size information to reserve space on the cache.
357 It should also call fscache_set_store_limit() to indicate to FS-Cache the
358 highest byte it's willing to store for an object.
360 This method may return -ve if an error occurred or the cache object cannot
361 be expanded. In such a case, the object will be withdrawn from service.
363 This operation is run asynchronously from FS-Cache's thread pool, and
364 storage and retrieval operations from the netfs are excluded during the
365 execution of this operation.
368 (*) Reserve cache space for an object's data [optional]:
370 int (*reserve_space)(struct fscache_object *object, loff_t size);
372 This is called to request that cache space be reserved to hold the data
373 for an object and the metadata used to track it. Zero size should be
374 taken as request to cancel a reservation.
376 This should return 0 if successful, -ENOSPC if there isn't enough space
377 available, or -ENOMEM or -EIO on other errors.
379 The reservation may exceed the current size of the object, thus permitting
380 future expansion. If the amount of space consumed by an object would
381 exceed the reservation, it's permitted to refuse requests to allocate
382 pages, but not required. An object may be pruned down to its reservation
383 size if larger than that already.
386 (*) Request page be read from cache [mandatory]:
388 int (*read_or_alloc_page)(struct fscache_retrieval *op,
392 This is called to attempt to read a netfs page from the cache, or to
393 reserve a backing block if not. FS-Cache will have done as much checking
394 as it can before calling, but most of the work belongs to the backend.
396 If there's no page in the cache, then -ENODATA should be returned if the
397 backend managed to reserve a backing block; -ENOBUFS or -ENOMEM if it
400 If there is suitable data in the cache, then a read operation should be
401 queued and 0 returned. When the read finishes, fscache_end_io() should be
404 The fscache_mark_pages_cached() should be called for the page if any cache
405 metadata is retained. This will indicate to the netfs that the page needs
406 explicit uncaching. This operation takes a pagevec, thus allowing several
407 pages to be marked at once.
409 The retrieval record pointed to by op should be retained for each page
410 queued and released when I/O on the page has been formally ended.
411 fscache_get/put_retrieval() are available for this purpose.
413 The retrieval record may be used to get CPU time via the FS-Cache thread
414 pool. If this is desired, the op->op.processor should be set to point to
415 the appropriate processing routine, and fscache_enqueue_retrieval() should
416 be called at an appropriate point to request CPU time. For instance, the
417 retrieval routine could be enqueued upon the completion of a disk read.
418 The to_do field in the retrieval record is provided to aid in this.
420 If an I/O error occurs, fscache_io_error() should be called and -ENOBUFS
421 returned if possible or fscache_end_io() called with a suitable error
424 fscache_put_retrieval() should be called after a page or pages are dealt
425 with. This will complete the operation when all pages are dealt with.
428 (*) Request pages be read from cache [mandatory]:
430 int (*read_or_alloc_pages)(struct fscache_retrieval *op,
431 struct list_head *pages,
435 This is like the read_or_alloc_page() method, except it is handed a list
436 of pages instead of one page. Any pages on which a read operation is
437 started must be added to the page cache for the specified mapping and also
438 to the LRU. Such pages must also be removed from the pages list and
439 *nr_pages decremented per page.
441 If there was an error such as -ENOMEM, then that should be returned; else
442 if one or more pages couldn't be read or allocated, then -ENOBUFS should
443 be returned; else if one or more pages couldn't be read, then -ENODATA
444 should be returned. If all the pages are dispatched then 0 should be
448 (*) Request page be allocated in the cache [mandatory]:
450 int (*allocate_page)(struct fscache_retrieval *op,
454 This is like the read_or_alloc_page() method, except that it shouldn't
455 read from the cache, even if there's data there that could be retrieved.
456 It should, however, set up any internal metadata required such that
457 the write_page() method can write to the cache.
459 If there's no backing block available, then -ENOBUFS should be returned
460 (or -ENOMEM if there were other problems). If a block is successfully
461 allocated, then the netfs page should be marked and 0 returned.
464 (*) Request pages be allocated in the cache [mandatory]:
466 int (*allocate_pages)(struct fscache_retrieval *op,
467 struct list_head *pages,
471 This is an multiple page version of the allocate_page() method. pages and
472 nr_pages should be treated as for the read_or_alloc_pages() method.
475 (*) Request page be written to cache [mandatory]:
477 int (*write_page)(struct fscache_storage *op,
480 This is called to write from a page on which there was a previously
481 successful read_or_alloc_page() call or similar. FS-Cache filters out
482 pages that don't have mappings.
484 This method is called asynchronously from the FS-Cache thread pool. It is
485 not required to actually store anything, provided -ENODATA is then
486 returned to the next read of this page.
488 If an error occurred, then a negative error code should be returned,
489 otherwise zero should be returned. FS-Cache will take appropriate action
490 in response to an error, such as withdrawing this object.
492 If this method returns success then FS-Cache will inform the netfs
496 (*) Discard retained per-page metadata [mandatory]:
498 void (*uncache_page)(struct fscache_object *object, struct page *page)
500 This is called when a netfs page is being evicted from the pagecache. The
501 cache backend should tear down any internal representation or tracking it
502 maintains for this page.
509 FS-Cache provides some utilities that a cache backend may make use of:
511 (*) Note occurrence of an I/O error in a cache:
513 void fscache_io_error(struct fscache_cache *cache)
515 This tells FS-Cache that an I/O error occurred in the cache. After this
516 has been called, only resource dissociation operations (object and page
517 release) will be passed from the netfs to the cache backend for the
520 This does not actually withdraw the cache. That must be done separately.
523 (*) Invoke the retrieval I/O completion function:
525 void fscache_end_io(struct fscache_retrieval *op, struct page *page,
528 This is called to note the end of an attempt to retrieve a page. The
529 error value should be 0 if successful and an error otherwise.
532 (*) Record that one or more pages being retrieved or allocated have been dealt
535 void fscache_retrieval_complete(struct fscache_retrieval *op,
538 This is called to record the fact that one or more pages have been dealt
539 with and are no longer the concern of this operation. When the number of
540 pages remaining in the operation reaches 0, the operation will be
544 (*) Record operation completion:
546 void fscache_op_complete(struct fscache_operation *op);
548 This is called to record the completion of an operation. This deducts
549 this operation from the parent object's run state, potentially permitting
550 one or more pending operations to start running.
553 (*) Set highest store limit:
555 void fscache_set_store_limit(struct fscache_object *object,
558 This sets the limit FS-Cache imposes on the highest byte it's willing to
559 try and store for a netfs. Any page over this limit is automatically
560 rejected by fscache_read_alloc_page() and co with -ENOBUFS.
563 (*) Mark pages as being cached:
565 void fscache_mark_pages_cached(struct fscache_retrieval *op,
566 struct pagevec *pagevec);
568 This marks a set of pages as being cached. After this has been called,
569 the netfs must call fscache_uncache_page() to unmark the pages.
572 (*) Perform coherency check on an object:
574 enum fscache_checkaux fscache_check_aux(struct fscache_object *object,
578 This asks the netfs to perform a coherency check on an object that has
579 just been looked up. The cookie attached to the object will determine the
580 netfs to use. data and datalen should specify where the auxiliary data
581 retrieved from the cache can be found.
583 One of three values will be returned:
585 (*) FSCACHE_CHECKAUX_OKAY
587 The coherency data indicates the object is valid as is.
589 (*) FSCACHE_CHECKAUX_NEEDS_UPDATE
591 The coherency data needs updating, but otherwise the object is
594 (*) FSCACHE_CHECKAUX_OBSOLETE
596 The coherency data indicates that the object is obsolete and should
600 (*) Initialise a freshly allocated object:
602 void fscache_object_init(struct fscache_object *object);
604 This initialises all the fields in an object representation.
607 (*) Indicate the destruction of an object:
609 void fscache_object_destroyed(struct fscache_cache *cache);
611 This must be called to inform FS-Cache that an object that belonged to a
612 cache has been destroyed and deallocated. This will allow continuation
613 of the cache withdrawal process when it is stopped pending destruction of
617 (*) Indicate negative lookup on an object:
619 void fscache_object_lookup_negative(struct fscache_object *object);
621 This is called to indicate to FS-Cache that a lookup process for an object
622 found a negative result.
624 This changes the state of an object to permit reads pending on lookup
625 completion to go off and start fetching data from the netfs server as it's
626 known at this point that there can't be any data in the cache.
628 This may be called multiple times on an object. Only the first call is
629 significant - all subsequent calls are ignored.
632 (*) Indicate an object has been obtained:
634 void fscache_obtained_object(struct fscache_object *object);
636 This is called to indicate to FS-Cache that a lookup process for an object
637 produced a positive result, or that an object was created. This should
638 only be called once for any particular object.
640 This changes the state of an object to indicate:
642 (1) if no call to fscache_object_lookup_negative() has been made on
643 this object, that there may be data available, and that reads can
644 now go and look for it; and
646 (2) that writes may now proceed against this object.
649 (*) Indicate that object lookup failed:
651 void fscache_object_lookup_error(struct fscache_object *object);
653 This marks an object as having encountered a fatal error (usually EIO)
654 and causes it to move into a state whereby it will be withdrawn as soon
658 (*) Get and release references on a retrieval record:
660 void fscache_get_retrieval(struct fscache_retrieval *op);
661 void fscache_put_retrieval(struct fscache_retrieval *op);
663 These two functions are used to retain a retrieval record whilst doing
664 asynchronous data retrieval and block allocation.
667 (*) Enqueue a retrieval record for processing.
669 void fscache_enqueue_retrieval(struct fscache_retrieval *op);
671 This enqueues a retrieval record for processing by the FS-Cache thread
672 pool. One of the threads in the pool will invoke the retrieval record's
673 op->op.processor callback function. This function may be called from
674 within the callback function.
677 (*) List of object state names:
679 const char *fscache_object_states[];
681 For debugging purposes, this may be used to turn the state that an object
682 is in into a text string for display purposes.