1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================== 4 Network Filesystem Caching API 5 ============================== 6 7 Fscache provides an API by which a network filesystem can make use of local 8 caching facilities. The API is arranged around a number of principles: 9 10 (1) A cache is logically organised into volumes and data storage objects 11 within those volumes. 12 13 (2) Volumes and data storage objects are represented by various types of 14 cookie. 15 16 (3) Cookies have keys that distinguish them from their peers. 17 18 (4) Cookies have coherency data that allows a cache to determine if the 19 cached data is still valid. 20 21 (5) I/O is done asynchronously where possible. 22 23 This API is used by:: 24 25 #include <linux/fscache.h>. 26 27 .. This document contains the following sections: 28 29 (1) Overview 30 (2) Volume registration 31 (3) Data file registration 32 (4) Declaring a cookie to be in use 33 (5) Resizing a data file (truncation) 34 (6) Data I/O API 35 (7) Data file coherency 36 (8) Data file invalidation 37 (9) Write back resource management 38 (10) Caching of local modifications 39 (11) Page release and invalidation 40 41 42 Overview 43 ======== 44 45 The fscache hierarchy is organised on two levels from a network filesystem's 46 point of view. The upper level represents "volumes" and the lower level 47 represents "data storage objects". These are represented by two types of 48 cookie, hereafter referred to as "volume cookies" and "cookies". 49 50 A network filesystem acquires a volume cookie for a volume using a volume key, 51 which represents all the information that defines that volume (e.g. cell name 52 or server address, volume ID or share name). This must be rendered as a 53 printable string that can be used as a directory name (ie. no '/' characters 54 and shouldn't begin with a '.'). The maximum name length is one less than the 55 maximum size of a filename component (allowing the cache backend one char for 56 its own purposes). 57 58 A filesystem would typically have a volume cookie for each superblock. 59 60 The filesystem then acquires a cookie for each file within that volume using an 61 object key. Object keys are binary blobs and only need to be unique within 62 their parent volume. The cache backend is responsible for rendering the binary 63 blob into something it can use and may employ hash tables, trees or whatever to 64 improve its ability to find an object. This is transparent to the network 65 filesystem. 66 67 A filesystem would typically have a cookie for each inode, and would acquire it 68 in iget and relinquish it when evicting the cookie. 69 70 Once it has a cookie, the filesystem needs to mark the cookie as being in use. 71 This causes fscache to send the cache backend off to look up/create resources 72 for the cookie in the background, to check its coherency and, if necessary, to 73 mark the object as being under modification. 74 75 A filesystem would typically "use" the cookie in its file open routine and 76 unuse it in file release and it needs to use the cookie around calls to 77 truncate the cookie locally. It *also* needs to use the cookie when the 78 pagecache becomes dirty and unuse it when writeback is complete. This is 79 slightly tricky, and provision is made for it. 80 81 When performing a read, write or resize on a cookie, the filesystem must first 82 begin an operation. This copies the resources into a holding struct and puts 83 extra pins into the cache to stop cache withdrawal from tearing down the 84 structures being used. The actual operation can then be issued and conflicting 85 invalidations can be detected upon completion. 86 87 The filesystem is expected to use netfslib to access the cache, but that's not 88 actually required and it can use the fscache I/O API directly. 89 90 91 Volume Registration 92 =================== 93 94 The first step for a network filesystem is to acquire a volume cookie for the 95 volume it wants to access:: 96 97 struct fscache_volume * 98 fscache_acquire_volume(const char *volume_key, 99 const char *cache_name, 100 const void *coherency_data, 101 size_t coherency_len); 102 103 This function creates a volume cookie with the specified volume key as its name 104 and notes the coherency data. 105 106 The volume key must be a printable string with no '/' characters in it. It 107 should begin with the name of the filesystem and should be no longer than 254 108 characters. It should uniquely represent the volume and will be matched with 109 what's stored in the cache. 110 111 The caller may also specify the name of the cache to use. If specified, 112 fscache will look up or create a cache cookie of that name and will use a cache 113 of that name if it is online or comes online. If no cache name is specified, 114 it will use the first cache that comes to hand and set the name to that. 115 116 The specified coherency data is stored in the cookie and will be matched 117 against coherency data stored on disk. The data pointer may be NULL if no data 118 is provided. If the coherency data doesn't match, the entire cache volume will 119 be invalidated. 120 121 This function can return errors such as EBUSY if the volume key is already in 122 use by an acquired volume or ENOMEM if an allocation failure occurred. It may 123 also return a NULL volume cookie if fscache is not enabled. It is safe to 124 pass a NULL cookie to any function that takes a volume cookie. This will 125 cause that function to do nothing. 126 127 128 When the network filesystem has finished with a volume, it should relinquish it 129 by calling:: 130 131 void fscache_relinquish_volume(struct fscache_volume *volume, 132 const void *coherency_data, 133 bool invalidate); 134 135 This will cause the volume to be committed or removed, and if sealed the 136 coherency data will be set to the value supplied. The amount of coherency data 137 must match the length specified when the volume was acquired. Note that all 138 data cookies obtained in this volume must be relinquished before the volume is 139 relinquished. 140 141 142 Data File Registration 143 ====================== 144 145 Once it has a volume cookie, a network filesystem can use it to acquire a 146 cookie for data storage:: 147 148 struct fscache_cookie * 149 fscache_acquire_cookie(struct fscache_volume *volume, 150 u8 advice, 151 const void *index_key, 152 size_t index_key_len, 153 const void *aux_data, 154 size_t aux_data_len, 155 loff_t object_size) 156 157 This creates the cookie in the volume using the specified index key. The index 158 key is a binary blob of the given length and must be unique for the volume. 159 This is saved into the cookie. There are no restrictions on the content, but 160 its length shouldn't exceed about three quarters of the maximum filename length 161 to allow for encoding. 162 163 The caller should also pass in a piece of coherency data in aux_data. A buffer 164 of size aux_data_len will be allocated and the coherency data copied in. It is 165 assumed that the size is invariant over time. The coherency data is used to 166 check the validity of data in the cache. Functions are provided by which the 167 coherency data can be updated. 168 169 The file size of the object being cached should also be provided. This may be 170 used to trim the data and will be stored with the coherency data. 171 172 This function never returns an error, though it may return a NULL cookie on 173 allocation failure or if fscache is not enabled. It is safe to pass in a NULL 174 volume cookie and pass the NULL cookie returned to any function that takes it. 175 This will cause that function to do nothing. 176 177 178 When the network filesystem has finished with a cookie, it should relinquish it 179 by calling:: 180 181 void fscache_relinquish_cookie(struct fscache_cookie *cookie, 182 bool retire); 183 184 This will cause fscache to either commit the storage backing the cookie or 185 delete it. 186 187 188 Marking A Cookie In-Use 189 ======================= 190 191 Once a cookie has been acquired by a network filesystem, the filesystem should 192 tell fscache when it intends to use the cookie (typically done on file open) 193 and should say when it has finished with it (typically on file close):: 194 195 void fscache_use_cookie(struct fscache_cookie *cookie, 196 bool will_modify); 197 void fscache_unuse_cookie(struct fscache_cookie *cookie, 198 const void *aux_data, 199 const loff_t *object_size); 200 201 The *use* function tells fscache that it will use the cookie and, additionally, 202 indicate if the user is intending to modify the contents locally. If not yet 203 done, this will trigger the cache backend to go and gather the resources it 204 needs to access/store data in the cache. This is done in the background, and 205 so may not be complete by the time the function returns. 206 207 The *unuse* function indicates that a filesystem has finished using a cookie. 208 It optionally updates the stored coherency data and object size and then 209 decreases the in-use counter. When the last user unuses the cookie, it is 210 scheduled for garbage collection. If not reused within a short time, the 211 resources will be released to reduce system resource consumption. 212 213 A cookie must be marked in-use before it can be accessed for read, write or 214 resize - and an in-use mark must be kept whilst there is dirty data in the 215 pagecache in order to avoid an oops due to trying to open a file during process 216 exit. 217 218 Note that in-use marks are cumulative. For each time a cookie is marked 219 in-use, it must be unused. 220 221 222 Resizing A Data File (Truncation) 223 ================================= 224 225 If a network filesystem file is resized locally by truncation, the following 226 should be called to notify the cache:: 227 228 void fscache_resize_cookie(struct fscache_cookie *cookie, 229 loff_t new_size); 230 231 The caller must have first marked the cookie in-use. The cookie and the new 232 size are passed in and the cache is synchronously resized. This is expected to 233 be called from ``->setattr()`` inode operation under the inode lock. 234 235 236 Data I/O API 237 ============ 238 239 To do data I/O operations directly through a cookie, the following functions 240 are available:: 241 242 int fscache_begin_read_operation(struct netfs_cache_resources *cres, 243 struct fscache_cookie *cookie); 244 int fscache_read(struct netfs_cache_resources *cres, 245 loff_t start_pos, 246 struct iov_iter *iter, 247 enum netfs_read_from_hole read_hole, 248 netfs_io_terminated_t term_func, 249 void *term_func_priv); 250 int fscache_write(struct netfs_cache_resources *cres, 251 loff_t start_pos, 252 struct iov_iter *iter, 253 netfs_io_terminated_t term_func, 254 void *term_func_priv); 255 256 The *begin* function sets up an operation, attaching the resources required to 257 the cache resources block from the cookie. Assuming it doesn't return an error 258 (for instance, it will return -ENOBUFS if given a NULL cookie, but otherwise do 259 nothing), then one of the other two functions can be issued. 260 261 The *read* and *write* functions initiate a direct-IO operation. Both take the 262 previously set up cache resources block, an indication of the start file 263 position, and an I/O iterator that describes buffer and indicates the amount of 264 data. 265 266 The read function also takes a parameter to indicate how it should handle a 267 partially populated region (a hole) in the disk content. This may be to ignore 268 it, skip over an initial hole and place zeros in the buffer or give an error. 269 270 The read and write functions can be given an optional termination function that 271 will be run on completion:: 272 273 typedef 274 void (*netfs_io_terminated_t)(void *priv, ssize_t transferred_or_error, 275 bool was_async); 276 277 If a termination function is given, the operation will be run asynchronously 278 and the termination function will be called upon completion. If not given, the 279 operation will be run synchronously. Note that in the asynchronous case, it is 280 possible for the operation to complete before the function returns. 281 282 Both the read and write functions end the operation when they complete, 283 detaching any pinned resources. 284 285 The read operation will fail with ESTALE if invalidation occurred whilst the 286 operation was ongoing. 287 288 289 Data File Coherency 290 =================== 291 292 To request an update of the coherency data and file size on a cookie, the 293 following should be called:: 294 295 void fscache_update_cookie(struct fscache_cookie *cookie, 296 const void *aux_data, 297 const loff_t *object_size); 298 299 This will update the cookie's coherency data and/or file size. 300 301 302 Data File Invalidation 303 ====================== 304 305 Sometimes it will be necessary to invalidate an object that contains data. 306 Typically this will be necessary when the server informs the network filesystem 307 of a remote third-party change - at which point the filesystem has to throw 308 away the state and cached data that it had for an file and reload from the 309 server. 310 311 To indicate that a cache object should be invalidated, the following should be 312 called:: 313 314 void fscache_invalidate(struct fscache_cookie *cookie, 315 const void *aux_data, 316 loff_t size, 317 unsigned int flags); 318 319 This increases the invalidation counter in the cookie to cause outstanding 320 reads to fail with -ESTALE, sets the coherency data and file size from the 321 information supplied, blocks new I/O on the cookie and dispatches the cache to 322 go and get rid of the old data. 323 324 Invalidation runs asynchronously in a worker thread so that it doesn't block 325 too much. 326 327 328 Write-Back Resource Management 329 ============================== 330 331 To write data to the cache from network filesystem writeback, the cache 332 resources required need to be pinned at the point the modification is made (for 333 instance when the page is marked dirty) as it's not possible to open a file in 334 a thread that's exiting. 335 336 The following facilities are provided to manage this: 337 338 * An inode flag, ``I_PINNING_FSCACHE_WB``, is provided to indicate that an 339 in-use is held on the cookie for this inode. It can only be changed if the 340 the inode lock is held. 341 342 * A flag, ``unpinned_fscache_wb`` is placed in the ``writeback_control`` 343 struct that gets set if ``__writeback_single_inode()`` clears 344 ``I_PINNING_FSCACHE_WB`` because all the dirty pages were cleared. 345 346 To support this, the following functions are provided:: 347 348 bool fscache_dirty_folio(struct address_space *mapping, 349 struct folio *folio, 350 struct fscache_cookie *cookie); 351 void fscache_unpin_writeback(struct writeback_control *wbc, 352 struct fscache_cookie *cookie); 353 void fscache_clear_inode_writeback(struct fscache_cookie *cookie, 354 struct inode *inode, 355 const void *aux); 356 357 The *set* function is intended to be called from the filesystem's 358 ``dirty_folio`` address space operation. If ``I_PINNING_FSCACHE_WB`` is not 359 set, it sets that flag and increments the use count on the cookie (the caller 360 must already have called ``fscache_use_cookie()``). 361 362 The *unpin* function is intended to be called from the filesystem's 363 ``write_inode`` superblock operation. It cleans up after writing by unusing 364 the cookie if unpinned_fscache_wb is set in the writeback_control struct. 365 366 The *clear* function is intended to be called from the netfs's ``evict_inode`` 367 superblock operation. It must be called *after* 368 ``truncate_inode_pages_final()``, but *before* ``clear_inode()``. This cleans 369 up any hanging ``I_PINNING_FSCACHE_WB``. It also allows the coherency data to 370 be updated. 371 372 373 Caching of Local Modifications 374 ============================== 375 376 If a network filesystem has locally modified data that it wants to write to the 377 cache, it needs to mark the pages to indicate that a write is in progress, and 378 if the mark is already present, it needs to wait for it to be removed first 379 (presumably due to an already in-progress operation). This prevents multiple 380 competing DIO writes to the same storage in the cache. 381 382 Firstly, the netfs should determine if caching is available by doing something 383 like:: 384 385 bool caching = fscache_cookie_enabled(cookie); 386 387 If caching is to be attempted, pages should be waited for and then marked using 388 the following functions provided by the netfs helper library:: 389 390 void set_page_fscache(struct page *page); 391 void wait_on_page_fscache(struct page *page); 392 int wait_on_page_fscache_killable(struct page *page); 393 394 Once all the pages in the span are marked, the netfs can ask fscache to 395 schedule a write of that region:: 396 397 void fscache_write_to_cache(struct fscache_cookie *cookie, 398 struct address_space *mapping, 399 loff_t start, size_t len, loff_t i_size, 400 netfs_io_terminated_t term_func, 401 void *term_func_priv, 402 bool caching) 403 404 And if an error occurs before that point is reached, the marks can be removed 405 by calling:: 406 407 void fscache_clear_page_bits(struct address_space *mapping, 408 loff_t start, size_t len, 409 bool caching) 410 411 In these functions, a pointer to the mapping to which the source pages are 412 attached is passed in and start and len indicate the size of the region that's 413 going to be written (it doesn't have to align to page boundaries necessarily, 414 but it does have to align to DIO boundaries on the backing filesystem). The 415 caching parameter indicates if caching should be skipped, and if false, the 416 functions do nothing. 417 418 The write function takes some additional parameters: the cookie representing 419 the cache object to be written to, i_size indicates the size of the netfs file 420 and term_func indicates an optional completion function, to which 421 term_func_priv will be passed, along with the error or amount written. 422 423 Note that the write function will always run asynchronously and will unmark all 424 the pages upon completion before calling term_func. 425 426 427 Page Release and Invalidation 428 ============================= 429 430 Fscache keeps track of whether we have any data in the cache yet for a cache 431 object we've just created. It knows it doesn't have to do any reading until it 432 has done a write and then the page it wrote from has been released by the VM, 433 after which it *has* to look in the cache. 434 435 To inform fscache that a page might now be in the cache, the following function 436 should be called from the ``release_folio`` address space op:: 437 438 void fscache_note_page_release(struct fscache_cookie *cookie); 439 440 if the page has been released (ie. release_folio returned true). 441 442 Page release and page invalidation should also wait for any mark left on the 443 page to say that a DIO write is underway from that page:: 444 445 void wait_on_page_fscache(struct page *page); 446 int wait_on_page_fscache_killable(struct page *page); 447 448 449 API Function Reference 450 ====================== 451 452 .. kernel-doc:: include/linux/fscache.h
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