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1 ===============================
2 FS-CACHE NETWORK FILESYSTEM API
3 ===============================
4
5There's an API by which a network filesystem can make use of the FS-Cache
6facilities. This is based around a number of principles:
7
8 (1) Caches can store a number of different object types. There are two main
9 object types: indices and files. The first is a special type used by
10 FS-Cache to make finding objects faster and to make retiring of groups of
11 objects easier.
12
13 (2) Every index, file or other object is represented by a cookie. This cookie
14 may or may not have anything associated with it, but the netfs doesn't
15 need to care.
16
17 (3) Barring the top-level index (one entry per cached netfs), the index
18 hierarchy for each netfs is structured according the whim of the netfs.
19
20This API is declared in <linux/fscache.h>.
21
22This document contains the following sections:
23
24 (1) Network filesystem definition
25 (2) Index definition
26 (3) Object definition
27 (4) Network filesystem (un)registration
28 (5) Cache tag lookup
29 (6) Index registration
30 (7) Data file registration
31 (8) Miscellaneous object registration
32 (9) Setting the data file size
33 (10) Page alloc/read/write
34 (11) Page uncaching
35 (12) Index and data file update
36 (13) Miscellaneous cookie operations
37 (14) Cookie unregistration
38 (15) Index and data file invalidation
39 (16) FS-Cache specific page flags.
40
41
42=============================
43NETWORK FILESYSTEM DEFINITION
44=============================
45
46FS-Cache needs a description of the network filesystem. This is specified
47using a record of the following structure:
48
49 struct fscache_netfs {
50 uint32_t version;
51 const char *name;
52 struct fscache_cookie *primary_index;
53 ...
54 };
55
56This first two fields should be filled in before registration, and the third
57will be filled in by the registration function; any other fields should just be
58ignored and are for internal use only.
59
60The fields are:
61
62 (1) The name of the netfs (used as the key in the toplevel index).
63
64 (2) The version of the netfs (if the name matches but the version doesn't, the
65 entire in-cache hierarchy for this netfs will be scrapped and begun
66 afresh).
67
68 (3) The cookie representing the primary index will be allocated according to
69 another parameter passed into the registration function.
70
71For example, kAFS (linux/fs/afs/) uses the following definitions to describe
72itself:
73
74 struct fscache_netfs afs_cache_netfs = {
75 .version = 0,
76 .name = "afs",
77 };
78
79
80================
81INDEX DEFINITION
82================
83
84Indices are used for two purposes:
85
86 (1) To aid the finding of a file based on a series of keys (such as AFS's
87 "cell", "volume ID", "vnode ID").
88
89 (2) To make it easier to discard a subset of all the files cached based around
90 a particular key - for instance to mirror the removal of an AFS volume.
91
92However, since it's unlikely that any two netfs's are going to want to define
93their index hierarchies in quite the same way, FS-Cache tries to impose as few
94restraints as possible on how an index is structured and where it is placed in
95the tree. The netfs can even mix indices and data files at the same level, but
96it's not recommended.
97
98Each index entry consists of a key of indeterminate length plus some auxiliary
99data, also of indeterminate length.
100
101There are some limits on indices:
102
103 (1) Any index containing non-index objects should be restricted to a single
104 cache. Any such objects created within an index will be created in the
105 first cache only. The cache in which an index is created can be
106 controlled by cache tags (see below).
107
108 (2) The entry data must be atomically journallable, so it is limited to about
109 400 bytes at present. At least 400 bytes will be available.
110
111 (3) The depth of the index tree should be judged with care as the search
112 function is recursive. Too many layers will run the kernel out of stack.
113
114
115=================
116OBJECT DEFINITION
117=================
118
119To define an object, a structure of the following type should be filled out:
120
121 struct fscache_cookie_def
122 {
123 uint8_t name[16];
124 uint8_t type;
125
126 struct fscache_cache_tag *(*select_cache)(
127 const void *parent_netfs_data,
128 const void *cookie_netfs_data);
129
130 uint16_t (*get_key)(const void *cookie_netfs_data,
131 void *buffer,
132 uint16_t bufmax);
133
134 void (*get_attr)(const void *cookie_netfs_data,
135 uint64_t *size);
136
137 uint16_t (*get_aux)(const void *cookie_netfs_data,
138 void *buffer,
139 uint16_t bufmax);
140
141 enum fscache_checkaux (*check_aux)(void *cookie_netfs_data,
142 const void *data,
143 uint16_t datalen);
144
145 void (*get_context)(void *cookie_netfs_data, void *context);
146
147 void (*put_context)(void *cookie_netfs_data, void *context);
148
149 void (*mark_pages_cached)(void *cookie_netfs_data,
150 struct address_space *mapping,
151 struct pagevec *cached_pvec);
152
153 void (*now_uncached)(void *cookie_netfs_data);
154 };
155
156This has the following fields:
157
158 (1) The type of the object [mandatory].
159
160 This is one of the following values:
161
162 (*) FSCACHE_COOKIE_TYPE_INDEX
163
164 This defines an index, which is a special FS-Cache type.
165
166 (*) FSCACHE_COOKIE_TYPE_DATAFILE
167
168 This defines an ordinary data file.
169
170 (*) Any other value between 2 and 255
171
172 This defines an extraordinary object such as an XATTR.
173
174 (2) The name of the object type (NUL terminated unless all 16 chars are used)
175 [optional].
176
177 (3) A function to select the cache in which to store an index [optional].
178
179 This function is invoked when an index needs to be instantiated in a cache
180 during the instantiation of a non-index object. Only the immediate index
181 parent for the non-index object will be queried. Any indices above that
182 in the hierarchy may be stored in multiple caches. This function does not
183 need to be supplied for any non-index object or any index that will only
184 have index children.
185
186 If this function is not supplied or if it returns NULL then the first
187 cache in the parent's list will be chosen, or failing that, the first
188 cache in the master list.
189
190 (4) A function to retrieve an object's key from the netfs [mandatory].
191
192 This function will be called with the netfs data that was passed to the
193 cookie acquisition function and the maximum length of key data that it may
194 provide. It should write the required key data into the given buffer and
195 return the quantity it wrote.
196
197 (5) A function to retrieve attribute data from the netfs [optional].
198
199 This function will be called with the netfs data that was passed to the
200 cookie acquisition function. It should return the size of the file if
201 this is a data file. The size may be used to govern how much cache must
202 be reserved for this file in the cache.
203
204 If the function is absent, a file size of 0 is assumed.
205
206 (6) A function to retrieve auxiliary data from the netfs [optional].
207
208 This function will be called with the netfs data that was passed to the
209 cookie acquisition function and the maximum length of auxiliary data that
210 it may provide. It should write the auxiliary data into the given buffer
211 and return the quantity it wrote.
212
213 If this function is absent, the auxiliary data length will be set to 0.
214
215 The length of the auxiliary data buffer may be dependent on the key
216 length. A netfs mustn't rely on being able to provide more than 400 bytes
217 for both.
218
219 (7) A function to check the auxiliary data [optional].
220
221 This function will be called to check that a match found in the cache for
222 this object is valid. For instance with AFS it could check the auxiliary
223 data against the data version number returned by the server to determine
224 whether the index entry in a cache is still valid.
225
226 If this function is absent, it will be assumed that matching objects in a
227 cache are always valid.
228
229 If present, the function should return one of the following values:
230
231 (*) FSCACHE_CHECKAUX_OKAY - the entry is okay as is
232 (*) FSCACHE_CHECKAUX_NEEDS_UPDATE - the entry requires update
233 (*) FSCACHE_CHECKAUX_OBSOLETE - the entry should be deleted
234
235 This function can also be used to extract data from the auxiliary data in
236 the cache and copy it into the netfs's structures.
237
238 (8) A pair of functions to manage contexts for the completion callback
239 [optional].
240
241 The cache read/write functions are passed a context which is then passed
242 to the I/O completion callback function. To ensure this context remains
243 valid until after the I/O completion is called, two functions may be
244 provided: one to get an extra reference on the context, and one to drop a
245 reference to it.
246
247 If the context is not used or is a type of object that won't go out of
248 scope, then these functions are not required. These functions are not
249 required for indices as indices may not contain data. These functions may
250 be called in interrupt context and so may not sleep.
251
252 (9) A function to mark a page as retaining cache metadata [optional].
253
254 This is called by the cache to indicate that it is retaining in-memory
255 information for this page and that the netfs should uncache the page when
256 it has finished. This does not indicate whether there's data on the disk
257 or not. Note that several pages at once may be presented for marking.
258
259 The PG_fscache bit is set on the pages before this function would be
260 called, so the function need not be provided if this is sufficient.
261
262 This function is not required for indices as they're not permitted data.
263
264(10) A function to unmark all the pages retaining cache metadata [mandatory].
265
266 This is called by FS-Cache to indicate that a backing store is being
267 unbound from a cookie and that all the marks on the pages should be
268 cleared to prevent confusion. Note that the cache will have torn down all
269 its tracking information so that the pages don't need to be explicitly
270 uncached.
271
272 This function is not required for indices as they're not permitted data.
273
274
275===================================
276NETWORK FILESYSTEM (UN)REGISTRATION
277===================================
278
279The first step is to declare the network filesystem to the cache. This also
280involves specifying the layout of the primary index (for AFS, this would be the
281"cell" level).
282
283The registration function is:
284
285 int fscache_register_netfs(struct fscache_netfs *netfs);
286
287It just takes a pointer to the netfs definition. It returns 0 or an error as
288appropriate.
289
290For kAFS, registration is done as follows:
291
292 ret = fscache_register_netfs(&afs_cache_netfs);
293
294The last step is, of course, unregistration:
295
296 void fscache_unregister_netfs(struct fscache_netfs *netfs);
297
298
299================
300CACHE TAG LOOKUP
301================
302
303FS-Cache permits the use of more than one cache. To permit particular index
304subtrees to be bound to particular caches, the second step is to look up cache
305representation tags. This step is optional; it can be left entirely up to
306FS-Cache as to which cache should be used. The problem with doing that is that
307FS-Cache will always pick the first cache that was registered.
308
309To get the representation for a named tag:
310
311 struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name);
312
313This takes a text string as the name and returns a representation of a tag. It
314will never return an error. It may return a dummy tag, however, if it runs out
315of memory; this will inhibit caching with this tag.
316
317Any representation so obtained must be released by passing it to this function:
318
319 void fscache_release_cache_tag(struct fscache_cache_tag *tag);
320
321The tag will be retrieved by FS-Cache when it calls the object definition
322operation select_cache().
323
324
325==================
326INDEX REGISTRATION
327==================
328
329The third step is to inform FS-Cache about part of an index hierarchy that can
330be used to locate files. This is done by requesting a cookie for each index in
331the path to the file:
332
333 struct fscache_cookie *
334 fscache_acquire_cookie(struct fscache_cookie *parent,
335 const struct fscache_object_def *def,
336 void *netfs_data);
337
338This function creates an index entry in the index represented by parent,
339filling in the index entry by calling the operations pointed to by def.
340
341Note that this function never returns an error - all errors are handled
342internally. It may, however, return NULL to indicate no cookie. It is quite
343acceptable to pass this token back to this function as the parent to another
344acquisition (or even to the relinquish cookie, read page and write page
345functions - see below).
346
347Note also that no indices are actually created in a cache until a non-index
348object needs to be created somewhere down the hierarchy. Furthermore, an index
349may be created in several different caches independently at different times.
350This is all handled transparently, and the netfs doesn't see any of it.
351
352For example, with AFS, a cell would be added to the primary index. This index
353entry would have a dependent inode containing a volume location index for the
354volume mappings within this cell:
355
356 cell->cache =
357 fscache_acquire_cookie(afs_cache_netfs.primary_index,
358 &afs_cell_cache_index_def,
359 cell);
360
361Then when a volume location was accessed, it would be entered into the cell's
362index and an inode would be allocated that acts as a volume type and hash chain
363combination:
364
365 vlocation->cache =
366 fscache_acquire_cookie(cell->cache,
367 &afs_vlocation_cache_index_def,
368 vlocation);
369
370And then a particular flavour of volume (R/O for example) could be added to
371that index, creating another index for vnodes (AFS inode equivalents):
372
373 volume->cache =
374 fscache_acquire_cookie(vlocation->cache,
375 &afs_volume_cache_index_def,
376 volume);
377
378
379======================
380DATA FILE REGISTRATION
381======================
382
383The fourth step is to request a data file be created in the cache. This is
384identical to index cookie acquisition. The only difference is that the type in
385the object definition should be something other than index type.
386
387 vnode->cache =
388 fscache_acquire_cookie(volume->cache,
389 &afs_vnode_cache_object_def,
390 vnode);
391
392
393=================================
394MISCELLANEOUS OBJECT REGISTRATION
395=================================
396
397An optional step is to request an object of miscellaneous type be created in
398the cache. This is almost identical to index cookie acquisition. The only
399difference is that the type in the object definition should be something other
400than index type. Whilst the parent object could be an index, it's more likely
401it would be some other type of object such as a data file.
402
403 xattr->cache =
404 fscache_acquire_cookie(vnode->cache,
405 &afs_xattr_cache_object_def,
406 xattr);
407
408Miscellaneous objects might be used to store extended attributes or directory
409entries for example.
410
411
412==========================
413SETTING THE DATA FILE SIZE
414==========================
415
416The fifth step is to set the physical attributes of the file, such as its size.
417This doesn't automatically reserve any space in the cache, but permits the
418cache to adjust its metadata for data tracking appropriately:
419
420 int fscache_attr_changed(struct fscache_cookie *cookie);
421
422The cache will return -ENOBUFS if there is no backing cache or if there is no
423space to allocate any extra metadata required in the cache. The attributes
424will be accessed with the get_attr() cookie definition operation.
425
426Note that attempts to read or write data pages in the cache over this size may
427be rebuffed with -ENOBUFS.
428
429This operation schedules an attribute adjustment to happen asynchronously at
430some point in the future, and as such, it may happen after the function returns
431to the caller. The attribute adjustment excludes read and write operations.
432
433
434=====================
435PAGE READ/ALLOC/WRITE
436=====================
437
438And the sixth step is to store and retrieve pages in the cache. There are
439three functions that are used to do this.
440
441Note:
442
443 (1) A page should not be re-read or re-allocated without uncaching it first.
444
445 (2) A read or allocated page must be uncached when the netfs page is released
446 from the pagecache.
447
448 (3) A page should only be written to the cache if previous read or allocated.
449
450This permits the cache to maintain its page tracking in proper order.
451
452
453PAGE READ
454---------
455
456Firstly, the netfs should ask FS-Cache to examine the caches and read the
457contents cached for a particular page of a particular file if present, or else
458allocate space to store the contents if not:
459
460 typedef
461 void (*fscache_rw_complete_t)(struct page *page,
462 void *context,
463 int error);
464
465 int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
466 struct page *page,
467 fscache_rw_complete_t end_io_func,
468 void *context,
469 gfp_t gfp);
470
471The cookie argument must specify a cookie for an object that isn't an index,
472the page specified will have the data loaded into it (and is also used to
473specify the page number), and the gfp argument is used to control how any
474memory allocations made are satisfied.
475
476If the cookie indicates the inode is not cached:
477
478 (1) The function will return -ENOBUFS.
479
480Else if there's a copy of the page resident in the cache:
481
482 (1) The mark_pages_cached() cookie operation will be called on that page.
483
484 (2) The function will submit a request to read the data from the cache's
485 backing device directly into the page specified.
486
487 (3) The function will return 0.
488
489 (4) When the read is complete, end_io_func() will be invoked with:
490
491 (*) The netfs data supplied when the cookie was created.
492
493 (*) The page descriptor.
494
495 (*) The context argument passed to the above function. This will be
496 maintained with the get_context/put_context functions mentioned above.
497
498 (*) An argument that's 0 on success or negative for an error code.
499
500 If an error occurs, it should be assumed that the page contains no usable
501 data.
502
503 end_io_func() will be called in process context if the read is results in
504 an error, but it might be called in interrupt context if the read is
505 successful.
506
507Otherwise, if there's not a copy available in cache, but the cache may be able
508to store the page:
509
510 (1) The mark_pages_cached() cookie operation will be called on that page.
511
512 (2) A block may be reserved in the cache and attached to the object at the
513 appropriate place.
514
515 (3) The function will return -ENODATA.
516
517This function may also return -ENOMEM or -EINTR, in which case it won't have
518read any data from the cache.
519
520
521PAGE ALLOCATE
522-------------
523
524Alternatively, if there's not expected to be any data in the cache for a page
525because the file has been extended, a block can simply be allocated instead:
526
527 int fscache_alloc_page(struct fscache_cookie *cookie,
528 struct page *page,
529 gfp_t gfp);
530
531This is similar to the fscache_read_or_alloc_page() function, except that it
532never reads from the cache. It will return 0 if a block has been allocated,
533rather than -ENODATA as the other would. One or the other must be performed
534before writing to the cache.
535
536The mark_pages_cached() cookie operation will be called on the page if
537successful.
538
539
540PAGE WRITE
541----------
542
543Secondly, if the netfs changes the contents of the page (either due to an
544initial download or if a user performs a write), then the page should be
545written back to the cache:
546
547 int fscache_write_page(struct fscache_cookie *cookie,
548 struct page *page,
549 gfp_t gfp);
550
551The cookie argument must specify a data file cookie, the page specified should
552contain the data to be written (and is also used to specify the page number),
553and the gfp argument is used to control how any memory allocations made are
554satisfied.
555
556The page must have first been read or allocated successfully and must not have
557been uncached before writing is performed.
558
559If the cookie indicates the inode is not cached then:
560
561 (1) The function will return -ENOBUFS.
562
563Else if space can be allocated in the cache to hold this page:
564
565 (1) PG_fscache_write will be set on the page.
566
567 (2) The function will submit a request to write the data to cache's backing
568 device directly from the page specified.
569
570 (3) The function will return 0.
571
572 (4) When the write is complete PG_fscache_write is cleared on the page and
573 anyone waiting for that bit will be woken up.
574
575Else if there's no space available in the cache, -ENOBUFS will be returned. It
576is also possible for the PG_fscache_write bit to be cleared when no write took
577place if unforeseen circumstances arose (such as a disk error).
578
579Writing takes place asynchronously.
580
581
582MULTIPLE PAGE READ
583------------------
584
585A facility is provided to read several pages at once, as requested by the
586readpages() address space operation:
587
588 int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
589 struct address_space *mapping,
590 struct list_head *pages,
591 int *nr_pages,
592 fscache_rw_complete_t end_io_func,
593 void *context,
594 gfp_t gfp);
595
596This works in a similar way to fscache_read_or_alloc_page(), except:
597
598 (1) Any page it can retrieve data for is removed from pages and nr_pages and
599 dispatched for reading to the disk. Reads of adjacent pages on disk may
600 be merged for greater efficiency.
601
602 (2) The mark_pages_cached() cookie operation will be called on several pages
603 at once if they're being read or allocated.
604
605 (3) If there was an general error, then that error will be returned.
606
607 Else if some pages couldn't be allocated or read, then -ENOBUFS will be
608 returned.
609
610 Else if some pages couldn't be read but were allocated, then -ENODATA will
611 be returned.
612
613 Otherwise, if all pages had reads dispatched, then 0 will be returned, the
614 list will be empty and *nr_pages will be 0.
615
616 (4) end_io_func will be called once for each page being read as the reads
617 complete. It will be called in process context if error != 0, but it may
618 be called in interrupt context if there is no error.
619
620Note that a return of -ENODATA, -ENOBUFS or any other error does not preclude
621some of the pages being read and some being allocated. Those pages will have
622been marked appropriately and will need uncaching.
623
624
625==============
626PAGE UNCACHING
627==============
628
629To uncache a page, this function should be called:
630
631 void fscache_uncache_page(struct fscache_cookie *cookie,
632 struct page *page);
633
634This function permits the cache to release any in-memory representation it
635might be holding for this netfs page. This function must be called once for
636each page on which the read or write page functions above have been called to
637make sure the cache's in-memory tracking information gets torn down.
638
639Note that pages can't be explicitly deleted from the a data file. The whole
640data file must be retired (see the relinquish cookie function below).
641
642Furthermore, note that this does not cancel the asynchronous read or write
643operation started by the read/alloc and write functions, so the page
644invalidation functions must use:
645
646 bool fscache_check_page_write(struct fscache_cookie *cookie,
647 struct page *page);
648
649to see if a page is being written to the cache, and:
650
651 void fscache_wait_on_page_write(struct fscache_cookie *cookie,
652 struct page *page);
653
654to wait for it to finish if it is.
655
656
657When releasepage() is being implemented, a special FS-Cache function exists to
658manage the heuristics of coping with vmscan trying to eject pages, which may
659conflict with the cache trying to write pages to the cache (which may itself
660need to allocate memory):
661
662 bool fscache_maybe_release_page(struct fscache_cookie *cookie,
663 struct page *page,
664 gfp_t gfp);
665
666This takes the netfs cookie, and the page and gfp arguments as supplied to
667releasepage(). It will return false if the page cannot be released yet for
668some reason and if it returns true, the page has been uncached and can now be
669released.
670
671To make a page available for release, this function may wait for an outstanding
672storage request to complete, or it may attempt to cancel the storage request -
673in which case the page will not be stored in the cache this time.
674
675
676BULK INODE PAGE UNCACHE
677-----------------------
678
679A convenience routine is provided to perform an uncache on all the pages
680attached to an inode. This assumes that the pages on the inode correspond on a
6811:1 basis with the pages in the cache.
682
683 void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
684 struct inode *inode);
685
686This takes the netfs cookie that the pages were cached with and the inode that
687the pages are attached to. This function will wait for pages to finish being
688written to the cache and for the cache to finish with the page generally. No
689error is returned.
690
691
692==========================
693INDEX AND DATA FILE UPDATE
694==========================
695
696To request an update of the index data for an index or other object, the
697following function should be called:
698
699 void fscache_update_cookie(struct fscache_cookie *cookie);
700
701This function will refer back to the netfs_data pointer stored in the cookie by
702the acquisition function to obtain the data to write into each revised index
703entry. The update method in the parent index definition will be called to
704transfer the data.
705
706Note that partial updates may happen automatically at other times, such as when
707data blocks are added to a data file object.
708
709
710===============================
711MISCELLANEOUS COOKIE OPERATIONS
712===============================
713
714There are a number of operations that can be used to control cookies:
715
716 (*) Cookie pinning:
717
718 int fscache_pin_cookie(struct fscache_cookie *cookie);
719 void fscache_unpin_cookie(struct fscache_cookie *cookie);
720
721 These operations permit data cookies to be pinned into the cache and to
722 have the pinning removed. They are not permitted on index cookies.
723
724 The pinning function will return 0 if successful, -ENOBUFS in the cookie
725 isn't backed by a cache, -EOPNOTSUPP if the cache doesn't support pinning,
726 -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
727 -EIO if there's any other problem.
728
729 (*) Data space reservation:
730
731 int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size);
732
733 This permits a netfs to request cache space be reserved to store up to the
734 given amount of a file. It is permitted to ask for more than the current
735 size of the file to allow for future file expansion.
736
737 If size is given as zero then the reservation will be cancelled.
738
739 The function will return 0 if successful, -ENOBUFS in the cookie isn't
740 backed by a cache, -EOPNOTSUPP if the cache doesn't support reservations,
741 -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
742 -EIO if there's any other problem.
743
744 Note that this doesn't pin an object in a cache; it can still be culled to
745 make space if it's not in use.
746
747
748=====================
749COOKIE UNREGISTRATION
750=====================
751
752To get rid of a cookie, this function should be called.
753
754 void fscache_relinquish_cookie(struct fscache_cookie *cookie,
755 int retire);
756
757If retire is non-zero, then the object will be marked for recycling, and all
758copies of it will be removed from all active caches in which it is present.
759Not only that but all child objects will also be retired.
760
761If retire is zero, then the object may be available again when next the
762acquisition function is called. Retirement here will overrule the pinning on a
763cookie.
764
765One very important note - relinquish must NOT be called for a cookie unless all
766the cookies for "child" indices, objects and pages have been relinquished
767first.
768
769
770================================
771INDEX AND DATA FILE INVALIDATION
772================================
773
774There is no direct way to invalidate an index subtree or a data file. To do
775this, the caller should relinquish and retire the cookie they have, and then
776acquire a new one.
777
778
779===========================
780FS-CACHE SPECIFIC PAGE FLAG
781===========================
782
783FS-Cache makes use of a page flag, PG_private_2, for its own purpose. This is
784given the alternative name PG_fscache.
785
786PG_fscache is used to indicate that the page is known by the cache, and that
787the cache must be informed if the page is going to go away. It's an indication
788to the netfs that the cache has an interest in this page, where an interest may
789be a pointer to it, resources allocated or reserved for it, or I/O in progress
790upon it.
791
792The netfs can use this information in methods such as releasepage() to
793determine whether it needs to uncache a page or update it.
794
795Furthermore, if this bit is set, releasepage() and invalidatepage() operations
796will be called on a page to get rid of it, even if PG_private is not set. This
797allows caching to attempted on a page before read_cache_pages() to be called
798after fscache_read_or_alloc_pages() as the former will try and release pages it
799was given under certain circumstances.
800
801This bit does not overlap with such as PG_private. This means that FS-Cache
802can be used with a filesystem that uses the block buffering code.
803
804There are a number of operations defined on this flag:
805
806 int PageFsCache(struct page *page);
807 void SetPageFsCache(struct page *page)
808 void ClearPageFsCache(struct page *page)
809 int TestSetPageFsCache(struct page *page)
810 int TestClearPageFsCache(struct page *page)
811
812These functions are bit test, bit set, bit clear, bit test and set and bit
813test and clear operations on PG_fscache.
1 ===============================
2 FS-CACHE NETWORK FILESYSTEM API
3 ===============================
4
5There's an API by which a network filesystem can make use of the FS-Cache
6facilities. This is based around a number of principles:
7
8 (1) Caches can store a number of different object types. There are two main
9 object types: indices and files. The first is a special type used by
10 FS-Cache to make finding objects faster and to make retiring of groups of
11 objects easier.
12
13 (2) Every index, file or other object is represented by a cookie. This cookie
14 may or may not have anything associated with it, but the netfs doesn't
15 need to care.
16
17 (3) Barring the top-level index (one entry per cached netfs), the index
18 hierarchy for each netfs is structured according the whim of the netfs.
19
20This API is declared in <linux/fscache.h>.
21
22This document contains the following sections:
23
24 (1) Network filesystem definition
25 (2) Index definition
26 (3) Object definition
27 (4) Network filesystem (un)registration
28 (5) Cache tag lookup
29 (6) Index registration
30 (7) Data file registration
31 (8) Miscellaneous object registration
32 (9) Setting the data file size
33 (10) Page alloc/read/write
34 (11) Page uncaching
35 (12) Index and data file consistency
36 (13) Cookie enablement
37 (14) Miscellaneous cookie operations
38 (15) Cookie unregistration
39 (16) Index invalidation
40 (17) Data file invalidation
41 (18) FS-Cache specific page flags.
42
43
44=============================
45NETWORK FILESYSTEM DEFINITION
46=============================
47
48FS-Cache needs a description of the network filesystem. This is specified
49using a record of the following structure:
50
51 struct fscache_netfs {
52 uint32_t version;
53 const char *name;
54 struct fscache_cookie *primary_index;
55 ...
56 };
57
58This first two fields should be filled in before registration, and the third
59will be filled in by the registration function; any other fields should just be
60ignored and are for internal use only.
61
62The fields are:
63
64 (1) The name of the netfs (used as the key in the toplevel index).
65
66 (2) The version of the netfs (if the name matches but the version doesn't, the
67 entire in-cache hierarchy for this netfs will be scrapped and begun
68 afresh).
69
70 (3) The cookie representing the primary index will be allocated according to
71 another parameter passed into the registration function.
72
73For example, kAFS (linux/fs/afs/) uses the following definitions to describe
74itself:
75
76 struct fscache_netfs afs_cache_netfs = {
77 .version = 0,
78 .name = "afs",
79 };
80
81
82================
83INDEX DEFINITION
84================
85
86Indices are used for two purposes:
87
88 (1) To aid the finding of a file based on a series of keys (such as AFS's
89 "cell", "volume ID", "vnode ID").
90
91 (2) To make it easier to discard a subset of all the files cached based around
92 a particular key - for instance to mirror the removal of an AFS volume.
93
94However, since it's unlikely that any two netfs's are going to want to define
95their index hierarchies in quite the same way, FS-Cache tries to impose as few
96restraints as possible on how an index is structured and where it is placed in
97the tree. The netfs can even mix indices and data files at the same level, but
98it's not recommended.
99
100Each index entry consists of a key of indeterminate length plus some auxiliary
101data, also of indeterminate length.
102
103There are some limits on indices:
104
105 (1) Any index containing non-index objects should be restricted to a single
106 cache. Any such objects created within an index will be created in the
107 first cache only. The cache in which an index is created can be
108 controlled by cache tags (see below).
109
110 (2) The entry data must be atomically journallable, so it is limited to about
111 400 bytes at present. At least 400 bytes will be available.
112
113 (3) The depth of the index tree should be judged with care as the search
114 function is recursive. Too many layers will run the kernel out of stack.
115
116
117=================
118OBJECT DEFINITION
119=================
120
121To define an object, a structure of the following type should be filled out:
122
123 struct fscache_cookie_def
124 {
125 uint8_t name[16];
126 uint8_t type;
127
128 struct fscache_cache_tag *(*select_cache)(
129 const void *parent_netfs_data,
130 const void *cookie_netfs_data);
131
132 uint16_t (*get_key)(const void *cookie_netfs_data,
133 void *buffer,
134 uint16_t bufmax);
135
136 void (*get_attr)(const void *cookie_netfs_data,
137 uint64_t *size);
138
139 uint16_t (*get_aux)(const void *cookie_netfs_data,
140 void *buffer,
141 uint16_t bufmax);
142
143 enum fscache_checkaux (*check_aux)(void *cookie_netfs_data,
144 const void *data,
145 uint16_t datalen);
146
147 void (*get_context)(void *cookie_netfs_data, void *context);
148
149 void (*put_context)(void *cookie_netfs_data, void *context);
150
151 void (*mark_pages_cached)(void *cookie_netfs_data,
152 struct address_space *mapping,
153 struct pagevec *cached_pvec);
154
155 void (*now_uncached)(void *cookie_netfs_data);
156 };
157
158This has the following fields:
159
160 (1) The type of the object [mandatory].
161
162 This is one of the following values:
163
164 (*) FSCACHE_COOKIE_TYPE_INDEX
165
166 This defines an index, which is a special FS-Cache type.
167
168 (*) FSCACHE_COOKIE_TYPE_DATAFILE
169
170 This defines an ordinary data file.
171
172 (*) Any other value between 2 and 255
173
174 This defines an extraordinary object such as an XATTR.
175
176 (2) The name of the object type (NUL terminated unless all 16 chars are used)
177 [optional].
178
179 (3) A function to select the cache in which to store an index [optional].
180
181 This function is invoked when an index needs to be instantiated in a cache
182 during the instantiation of a non-index object. Only the immediate index
183 parent for the non-index object will be queried. Any indices above that
184 in the hierarchy may be stored in multiple caches. This function does not
185 need to be supplied for any non-index object or any index that will only
186 have index children.
187
188 If this function is not supplied or if it returns NULL then the first
189 cache in the parent's list will be chosen, or failing that, the first
190 cache in the master list.
191
192 (4) A function to retrieve an object's key from the netfs [mandatory].
193
194 This function will be called with the netfs data that was passed to the
195 cookie acquisition function and the maximum length of key data that it may
196 provide. It should write the required key data into the given buffer and
197 return the quantity it wrote.
198
199 (5) A function to retrieve attribute data from the netfs [optional].
200
201 This function will be called with the netfs data that was passed to the
202 cookie acquisition function. It should return the size of the file if
203 this is a data file. The size may be used to govern how much cache must
204 be reserved for this file in the cache.
205
206 If the function is absent, a file size of 0 is assumed.
207
208 (6) A function to retrieve auxiliary data from the netfs [optional].
209
210 This function will be called with the netfs data that was passed to the
211 cookie acquisition function and the maximum length of auxiliary data that
212 it may provide. It should write the auxiliary data into the given buffer
213 and return the quantity it wrote.
214
215 If this function is absent, the auxiliary data length will be set to 0.
216
217 The length of the auxiliary data buffer may be dependent on the key
218 length. A netfs mustn't rely on being able to provide more than 400 bytes
219 for both.
220
221 (7) A function to check the auxiliary data [optional].
222
223 This function will be called to check that a match found in the cache for
224 this object is valid. For instance with AFS it could check the auxiliary
225 data against the data version number returned by the server to determine
226 whether the index entry in a cache is still valid.
227
228 If this function is absent, it will be assumed that matching objects in a
229 cache are always valid.
230
231 If present, the function should return one of the following values:
232
233 (*) FSCACHE_CHECKAUX_OKAY - the entry is okay as is
234 (*) FSCACHE_CHECKAUX_NEEDS_UPDATE - the entry requires update
235 (*) FSCACHE_CHECKAUX_OBSOLETE - the entry should be deleted
236
237 This function can also be used to extract data from the auxiliary data in
238 the cache and copy it into the netfs's structures.
239
240 (8) A pair of functions to manage contexts for the completion callback
241 [optional].
242
243 The cache read/write functions are passed a context which is then passed
244 to the I/O completion callback function. To ensure this context remains
245 valid until after the I/O completion is called, two functions may be
246 provided: one to get an extra reference on the context, and one to drop a
247 reference to it.
248
249 If the context is not used or is a type of object that won't go out of
250 scope, then these functions are not required. These functions are not
251 required for indices as indices may not contain data. These functions may
252 be called in interrupt context and so may not sleep.
253
254 (9) A function to mark a page as retaining cache metadata [optional].
255
256 This is called by the cache to indicate that it is retaining in-memory
257 information for this page and that the netfs should uncache the page when
258 it has finished. This does not indicate whether there's data on the disk
259 or not. Note that several pages at once may be presented for marking.
260
261 The PG_fscache bit is set on the pages before this function would be
262 called, so the function need not be provided if this is sufficient.
263
264 This function is not required for indices as they're not permitted data.
265
266(10) A function to unmark all the pages retaining cache metadata [mandatory].
267
268 This is called by FS-Cache to indicate that a backing store is being
269 unbound from a cookie and that all the marks on the pages should be
270 cleared to prevent confusion. Note that the cache will have torn down all
271 its tracking information so that the pages don't need to be explicitly
272 uncached.
273
274 This function is not required for indices as they're not permitted data.
275
276
277===================================
278NETWORK FILESYSTEM (UN)REGISTRATION
279===================================
280
281The first step is to declare the network filesystem to the cache. This also
282involves specifying the layout of the primary index (for AFS, this would be the
283"cell" level).
284
285The registration function is:
286
287 int fscache_register_netfs(struct fscache_netfs *netfs);
288
289It just takes a pointer to the netfs definition. It returns 0 or an error as
290appropriate.
291
292For kAFS, registration is done as follows:
293
294 ret = fscache_register_netfs(&afs_cache_netfs);
295
296The last step is, of course, unregistration:
297
298 void fscache_unregister_netfs(struct fscache_netfs *netfs);
299
300
301================
302CACHE TAG LOOKUP
303================
304
305FS-Cache permits the use of more than one cache. To permit particular index
306subtrees to be bound to particular caches, the second step is to look up cache
307representation tags. This step is optional; it can be left entirely up to
308FS-Cache as to which cache should be used. The problem with doing that is that
309FS-Cache will always pick the first cache that was registered.
310
311To get the representation for a named tag:
312
313 struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name);
314
315This takes a text string as the name and returns a representation of a tag. It
316will never return an error. It may return a dummy tag, however, if it runs out
317of memory; this will inhibit caching with this tag.
318
319Any representation so obtained must be released by passing it to this function:
320
321 void fscache_release_cache_tag(struct fscache_cache_tag *tag);
322
323The tag will be retrieved by FS-Cache when it calls the object definition
324operation select_cache().
325
326
327==================
328INDEX REGISTRATION
329==================
330
331The third step is to inform FS-Cache about part of an index hierarchy that can
332be used to locate files. This is done by requesting a cookie for each index in
333the path to the file:
334
335 struct fscache_cookie *
336 fscache_acquire_cookie(struct fscache_cookie *parent,
337 const struct fscache_object_def *def,
338 void *netfs_data,
339 bool enable);
340
341This function creates an index entry in the index represented by parent,
342filling in the index entry by calling the operations pointed to by def.
343
344Note that this function never returns an error - all errors are handled
345internally. It may, however, return NULL to indicate no cookie. It is quite
346acceptable to pass this token back to this function as the parent to another
347acquisition (or even to the relinquish cookie, read page and write page
348functions - see below).
349
350Note also that no indices are actually created in a cache until a non-index
351object needs to be created somewhere down the hierarchy. Furthermore, an index
352may be created in several different caches independently at different times.
353This is all handled transparently, and the netfs doesn't see any of it.
354
355A cookie will be created in the disabled state if enabled is false. A cookie
356must be enabled to do anything with it. A disabled cookie can be enabled by
357calling fscache_enable_cookie() (see below).
358
359For example, with AFS, a cell would be added to the primary index. This index
360entry would have a dependent inode containing a volume location index for the
361volume mappings within this cell:
362
363 cell->cache =
364 fscache_acquire_cookie(afs_cache_netfs.primary_index,
365 &afs_cell_cache_index_def,
366 cell, true);
367
368Then when a volume location was accessed, it would be entered into the cell's
369index and an inode would be allocated that acts as a volume type and hash chain
370combination:
371
372 vlocation->cache =
373 fscache_acquire_cookie(cell->cache,
374 &afs_vlocation_cache_index_def,
375 vlocation, true);
376
377And then a particular flavour of volume (R/O for example) could be added to
378that index, creating another index for vnodes (AFS inode equivalents):
379
380 volume->cache =
381 fscache_acquire_cookie(vlocation->cache,
382 &afs_volume_cache_index_def,
383 volume, true);
384
385
386======================
387DATA FILE REGISTRATION
388======================
389
390The fourth step is to request a data file be created in the cache. This is
391identical to index cookie acquisition. The only difference is that the type in
392the object definition should be something other than index type.
393
394 vnode->cache =
395 fscache_acquire_cookie(volume->cache,
396 &afs_vnode_cache_object_def,
397 vnode, true);
398
399
400=================================
401MISCELLANEOUS OBJECT REGISTRATION
402=================================
403
404An optional step is to request an object of miscellaneous type be created in
405the cache. This is almost identical to index cookie acquisition. The only
406difference is that the type in the object definition should be something other
407than index type. Whilst the parent object could be an index, it's more likely
408it would be some other type of object such as a data file.
409
410 xattr->cache =
411 fscache_acquire_cookie(vnode->cache,
412 &afs_xattr_cache_object_def,
413 xattr, true);
414
415Miscellaneous objects might be used to store extended attributes or directory
416entries for example.
417
418
419==========================
420SETTING THE DATA FILE SIZE
421==========================
422
423The fifth step is to set the physical attributes of the file, such as its size.
424This doesn't automatically reserve any space in the cache, but permits the
425cache to adjust its metadata for data tracking appropriately:
426
427 int fscache_attr_changed(struct fscache_cookie *cookie);
428
429The cache will return -ENOBUFS if there is no backing cache or if there is no
430space to allocate any extra metadata required in the cache. The attributes
431will be accessed with the get_attr() cookie definition operation.
432
433Note that attempts to read or write data pages in the cache over this size may
434be rebuffed with -ENOBUFS.
435
436This operation schedules an attribute adjustment to happen asynchronously at
437some point in the future, and as such, it may happen after the function returns
438to the caller. The attribute adjustment excludes read and write operations.
439
440
441=====================
442PAGE ALLOC/READ/WRITE
443=====================
444
445And the sixth step is to store and retrieve pages in the cache. There are
446three functions that are used to do this.
447
448Note:
449
450 (1) A page should not be re-read or re-allocated without uncaching it first.
451
452 (2) A read or allocated page must be uncached when the netfs page is released
453 from the pagecache.
454
455 (3) A page should only be written to the cache if previous read or allocated.
456
457This permits the cache to maintain its page tracking in proper order.
458
459
460PAGE READ
461---------
462
463Firstly, the netfs should ask FS-Cache to examine the caches and read the
464contents cached for a particular page of a particular file if present, or else
465allocate space to store the contents if not:
466
467 typedef
468 void (*fscache_rw_complete_t)(struct page *page,
469 void *context,
470 int error);
471
472 int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
473 struct page *page,
474 fscache_rw_complete_t end_io_func,
475 void *context,
476 gfp_t gfp);
477
478The cookie argument must specify a cookie for an object that isn't an index,
479the page specified will have the data loaded into it (and is also used to
480specify the page number), and the gfp argument is used to control how any
481memory allocations made are satisfied.
482
483If the cookie indicates the inode is not cached:
484
485 (1) The function will return -ENOBUFS.
486
487Else if there's a copy of the page resident in the cache:
488
489 (1) The mark_pages_cached() cookie operation will be called on that page.
490
491 (2) The function will submit a request to read the data from the cache's
492 backing device directly into the page specified.
493
494 (3) The function will return 0.
495
496 (4) When the read is complete, end_io_func() will be invoked with:
497
498 (*) The netfs data supplied when the cookie was created.
499
500 (*) The page descriptor.
501
502 (*) The context argument passed to the above function. This will be
503 maintained with the get_context/put_context functions mentioned above.
504
505 (*) An argument that's 0 on success or negative for an error code.
506
507 If an error occurs, it should be assumed that the page contains no usable
508 data. fscache_readpages_cancel() may need to be called.
509
510 end_io_func() will be called in process context if the read is results in
511 an error, but it might be called in interrupt context if the read is
512 successful.
513
514Otherwise, if there's not a copy available in cache, but the cache may be able
515to store the page:
516
517 (1) The mark_pages_cached() cookie operation will be called on that page.
518
519 (2) A block may be reserved in the cache and attached to the object at the
520 appropriate place.
521
522 (3) The function will return -ENODATA.
523
524This function may also return -ENOMEM or -EINTR, in which case it won't have
525read any data from the cache.
526
527
528PAGE ALLOCATE
529-------------
530
531Alternatively, if there's not expected to be any data in the cache for a page
532because the file has been extended, a block can simply be allocated instead:
533
534 int fscache_alloc_page(struct fscache_cookie *cookie,
535 struct page *page,
536 gfp_t gfp);
537
538This is similar to the fscache_read_or_alloc_page() function, except that it
539never reads from the cache. It will return 0 if a block has been allocated,
540rather than -ENODATA as the other would. One or the other must be performed
541before writing to the cache.
542
543The mark_pages_cached() cookie operation will be called on the page if
544successful.
545
546
547PAGE WRITE
548----------
549
550Secondly, if the netfs changes the contents of the page (either due to an
551initial download or if a user performs a write), then the page should be
552written back to the cache:
553
554 int fscache_write_page(struct fscache_cookie *cookie,
555 struct page *page,
556 gfp_t gfp);
557
558The cookie argument must specify a data file cookie, the page specified should
559contain the data to be written (and is also used to specify the page number),
560and the gfp argument is used to control how any memory allocations made are
561satisfied.
562
563The page must have first been read or allocated successfully and must not have
564been uncached before writing is performed.
565
566If the cookie indicates the inode is not cached then:
567
568 (1) The function will return -ENOBUFS.
569
570Else if space can be allocated in the cache to hold this page:
571
572 (1) PG_fscache_write will be set on the page.
573
574 (2) The function will submit a request to write the data to cache's backing
575 device directly from the page specified.
576
577 (3) The function will return 0.
578
579 (4) When the write is complete PG_fscache_write is cleared on the page and
580 anyone waiting for that bit will be woken up.
581
582Else if there's no space available in the cache, -ENOBUFS will be returned. It
583is also possible for the PG_fscache_write bit to be cleared when no write took
584place if unforeseen circumstances arose (such as a disk error).
585
586Writing takes place asynchronously.
587
588
589MULTIPLE PAGE READ
590------------------
591
592A facility is provided to read several pages at once, as requested by the
593readpages() address space operation:
594
595 int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
596 struct address_space *mapping,
597 struct list_head *pages,
598 int *nr_pages,
599 fscache_rw_complete_t end_io_func,
600 void *context,
601 gfp_t gfp);
602
603This works in a similar way to fscache_read_or_alloc_page(), except:
604
605 (1) Any page it can retrieve data for is removed from pages and nr_pages and
606 dispatched for reading to the disk. Reads of adjacent pages on disk may
607 be merged for greater efficiency.
608
609 (2) The mark_pages_cached() cookie operation will be called on several pages
610 at once if they're being read or allocated.
611
612 (3) If there was an general error, then that error will be returned.
613
614 Else if some pages couldn't be allocated or read, then -ENOBUFS will be
615 returned.
616
617 Else if some pages couldn't be read but were allocated, then -ENODATA will
618 be returned.
619
620 Otherwise, if all pages had reads dispatched, then 0 will be returned, the
621 list will be empty and *nr_pages will be 0.
622
623 (4) end_io_func will be called once for each page being read as the reads
624 complete. It will be called in process context if error != 0, but it may
625 be called in interrupt context if there is no error.
626
627Note that a return of -ENODATA, -ENOBUFS or any other error does not preclude
628some of the pages being read and some being allocated. Those pages will have
629been marked appropriately and will need uncaching.
630
631
632CANCELLATION OF UNREAD PAGES
633----------------------------
634
635If one or more pages are passed to fscache_read_or_alloc_pages() but not then
636read from the cache and also not read from the underlying filesystem then
637those pages will need to have any marks and reservations removed. This can be
638done by calling:
639
640 void fscache_readpages_cancel(struct fscache_cookie *cookie,
641 struct list_head *pages);
642
643prior to returning to the caller. The cookie argument should be as passed to
644fscache_read_or_alloc_pages(). Every page in the pages list will be examined
645and any that have PG_fscache set will be uncached.
646
647
648==============
649PAGE UNCACHING
650==============
651
652To uncache a page, this function should be called:
653
654 void fscache_uncache_page(struct fscache_cookie *cookie,
655 struct page *page);
656
657This function permits the cache to release any in-memory representation it
658might be holding for this netfs page. This function must be called once for
659each page on which the read or write page functions above have been called to
660make sure the cache's in-memory tracking information gets torn down.
661
662Note that pages can't be explicitly deleted from the a data file. The whole
663data file must be retired (see the relinquish cookie function below).
664
665Furthermore, note that this does not cancel the asynchronous read or write
666operation started by the read/alloc and write functions, so the page
667invalidation functions must use:
668
669 bool fscache_check_page_write(struct fscache_cookie *cookie,
670 struct page *page);
671
672to see if a page is being written to the cache, and:
673
674 void fscache_wait_on_page_write(struct fscache_cookie *cookie,
675 struct page *page);
676
677to wait for it to finish if it is.
678
679
680When releasepage() is being implemented, a special FS-Cache function exists to
681manage the heuristics of coping with vmscan trying to eject pages, which may
682conflict with the cache trying to write pages to the cache (which may itself
683need to allocate memory):
684
685 bool fscache_maybe_release_page(struct fscache_cookie *cookie,
686 struct page *page,
687 gfp_t gfp);
688
689This takes the netfs cookie, and the page and gfp arguments as supplied to
690releasepage(). It will return false if the page cannot be released yet for
691some reason and if it returns true, the page has been uncached and can now be
692released.
693
694To make a page available for release, this function may wait for an outstanding
695storage request to complete, or it may attempt to cancel the storage request -
696in which case the page will not be stored in the cache this time.
697
698
699BULK INODE PAGE UNCACHE
700-----------------------
701
702A convenience routine is provided to perform an uncache on all the pages
703attached to an inode. This assumes that the pages on the inode correspond on a
7041:1 basis with the pages in the cache.
705
706 void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
707 struct inode *inode);
708
709This takes the netfs cookie that the pages were cached with and the inode that
710the pages are attached to. This function will wait for pages to finish being
711written to the cache and for the cache to finish with the page generally. No
712error is returned.
713
714
715===============================
716INDEX AND DATA FILE CONSISTENCY
717===============================
718
719To find out whether auxiliary data for an object is up to data within the
720cache, the following function can be called:
721
722 int fscache_check_consistency(struct fscache_cookie *cookie)
723
724This will call back to the netfs to check whether the auxiliary data associated
725with a cookie is correct. It returns 0 if it is and -ESTALE if it isn't; it
726may also return -ENOMEM and -ERESTARTSYS.
727
728To request an update of the index data for an index or other object, the
729following function should be called:
730
731 void fscache_update_cookie(struct fscache_cookie *cookie);
732
733This function will refer back to the netfs_data pointer stored in the cookie by
734the acquisition function to obtain the data to write into each revised index
735entry. The update method in the parent index definition will be called to
736transfer the data.
737
738Note that partial updates may happen automatically at other times, such as when
739data blocks are added to a data file object.
740
741
742=================
743COOKIE ENABLEMENT
744=================
745
746Cookies exist in one of two states: enabled and disabled. If a cookie is
747disabled, it ignores all attempts to acquire child cookies; check, update or
748invalidate its state; allocate, read or write backing pages - though it is
749still possible to uncache pages and relinquish the cookie.
750
751The initial enablement state is set by fscache_acquire_cookie(), but the cookie
752can be enabled or disabled later. To disable a cookie, call:
753
754 void fscache_disable_cookie(struct fscache_cookie *cookie,
755 bool invalidate);
756
757If the cookie is not already disabled, this locks the cookie against other
758enable and disable ops, marks the cookie as being disabled, discards or
759invalidates any backing objects and waits for cessation of activity on any
760associated object before unlocking the cookie.
761
762All possible failures are handled internally. The caller should consider
763calling fscache_uncache_all_inode_pages() afterwards to make sure all page
764markings are cleared up.
765
766Cookies can be enabled or reenabled with:
767
768 void fscache_enable_cookie(struct fscache_cookie *cookie,
769 bool (*can_enable)(void *data),
770 void *data)
771
772If the cookie is not already enabled, this locks the cookie against other
773enable and disable ops, invokes can_enable() and, if the cookie is not an index
774cookie, will begin the procedure of acquiring backing objects.
775
776The optional can_enable() function is passed the data argument and returns a
777ruling as to whether or not enablement should actually be permitted to begin.
778
779All possible failures are handled internally. The cookie will only be marked
780as enabled if provisional backing objects are allocated.
781
782
783===============================
784MISCELLANEOUS COOKIE OPERATIONS
785===============================
786
787There are a number of operations that can be used to control cookies:
788
789 (*) Cookie pinning:
790
791 int fscache_pin_cookie(struct fscache_cookie *cookie);
792 void fscache_unpin_cookie(struct fscache_cookie *cookie);
793
794 These operations permit data cookies to be pinned into the cache and to
795 have the pinning removed. They are not permitted on index cookies.
796
797 The pinning function will return 0 if successful, -ENOBUFS in the cookie
798 isn't backed by a cache, -EOPNOTSUPP if the cache doesn't support pinning,
799 -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
800 -EIO if there's any other problem.
801
802 (*) Data space reservation:
803
804 int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size);
805
806 This permits a netfs to request cache space be reserved to store up to the
807 given amount of a file. It is permitted to ask for more than the current
808 size of the file to allow for future file expansion.
809
810 If size is given as zero then the reservation will be cancelled.
811
812 The function will return 0 if successful, -ENOBUFS in the cookie isn't
813 backed by a cache, -EOPNOTSUPP if the cache doesn't support reservations,
814 -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
815 -EIO if there's any other problem.
816
817 Note that this doesn't pin an object in a cache; it can still be culled to
818 make space if it's not in use.
819
820
821=====================
822COOKIE UNREGISTRATION
823=====================
824
825To get rid of a cookie, this function should be called.
826
827 void fscache_relinquish_cookie(struct fscache_cookie *cookie,
828 bool retire);
829
830If retire is non-zero, then the object will be marked for recycling, and all
831copies of it will be removed from all active caches in which it is present.
832Not only that but all child objects will also be retired.
833
834If retire is zero, then the object may be available again when next the
835acquisition function is called. Retirement here will overrule the pinning on a
836cookie.
837
838One very important note - relinquish must NOT be called for a cookie unless all
839the cookies for "child" indices, objects and pages have been relinquished
840first.
841
842
843==================
844INDEX INVALIDATION
845==================
846
847There is no direct way to invalidate an index subtree. To do this, the caller
848should relinquish and retire the cookie they have, and then acquire a new one.
849
850
851======================
852DATA FILE INVALIDATION
853======================
854
855Sometimes it will be necessary to invalidate an object that contains data.
856Typically this will be necessary when the server tells the netfs of a foreign
857change - at which point the netfs has to throw away all the state it had for an
858inode and reload from the server.
859
860To indicate that a cache object should be invalidated, the following function
861can be called:
862
863 void fscache_invalidate(struct fscache_cookie *cookie);
864
865This can be called with spinlocks held as it defers the work to a thread pool.
866All extant storage, retrieval and attribute change ops at this point are
867cancelled and discarded. Some future operations will be rejected until the
868cache has had a chance to insert a barrier in the operations queue. After
869that, operations will be queued again behind the invalidation operation.
870
871The invalidation operation will perform an attribute change operation and an
872auxiliary data update operation as it is very likely these will have changed.
873
874Using the following function, the netfs can wait for the invalidation operation
875to have reached a point at which it can start submitting ordinary operations
876once again:
877
878 void fscache_wait_on_invalidate(struct fscache_cookie *cookie);
879
880
881===========================
882FS-CACHE SPECIFIC PAGE FLAG
883===========================
884
885FS-Cache makes use of a page flag, PG_private_2, for its own purpose. This is
886given the alternative name PG_fscache.
887
888PG_fscache is used to indicate that the page is known by the cache, and that
889the cache must be informed if the page is going to go away. It's an indication
890to the netfs that the cache has an interest in this page, where an interest may
891be a pointer to it, resources allocated or reserved for it, or I/O in progress
892upon it.
893
894The netfs can use this information in methods such as releasepage() to
895determine whether it needs to uncache a page or update it.
896
897Furthermore, if this bit is set, releasepage() and invalidatepage() operations
898will be called on a page to get rid of it, even if PG_private is not set. This
899allows caching to attempted on a page before read_cache_pages() to be called
900after fscache_read_or_alloc_pages() as the former will try and release pages it
901was given under certain circumstances.
902
903This bit does not overlap with such as PG_private. This means that FS-Cache
904can be used with a filesystem that uses the block buffering code.
905
906There are a number of operations defined on this flag:
907
908 int PageFsCache(struct page *page);
909 void SetPageFsCache(struct page *page)
910 void ClearPageFsCache(struct page *page)
911 int TestSetPageFsCache(struct page *page)
912 int TestClearPageFsCache(struct page *page)
913
914These functions are bit test, bit set, bit clear, bit test and set and bit
915test and clear operations on PG_fscache.