Based on kernel version 4.8. Page generated on 2016-10-06 23:15 EST.
1 =============================================== 2 CacheFiles: CACHE ON ALREADY MOUNTED FILESYSTEM 3 =============================================== 4 5 Contents: 6 7 (*) Overview. 8 9 (*) Requirements. 10 11 (*) Configuration. 12 13 (*) Starting the cache. 14 15 (*) Things to avoid. 16 17 (*) Cache culling. 18 19 (*) Cache structure. 20 21 (*) Security model and SELinux. 22 23 (*) A note on security. 24 25 (*) Statistical information. 26 27 (*) Debugging. 28 29 30 ======== 31 OVERVIEW 32 ======== 33 34 CacheFiles is a caching backend that's meant to use as a cache a directory on 35 an already mounted filesystem of a local type (such as Ext3). 36 37 CacheFiles uses a userspace daemon to do some of the cache management - such as 38 reaping stale nodes and culling. This is called cachefilesd and lives in 39 /sbin. 40 41 The filesystem and data integrity of the cache are only as good as those of the 42 filesystem providing the backing services. Note that CacheFiles does not 43 attempt to journal anything since the journalling interfaces of the various 44 filesystems are very specific in nature. 45 46 CacheFiles creates a misc character device - "/dev/cachefiles" - that is used 47 to communication with the daemon. Only one thing may have this open at once, 48 and whilst it is open, a cache is at least partially in existence. The daemon 49 opens this and sends commands down it to control the cache. 50 51 CacheFiles is currently limited to a single cache. 52 53 CacheFiles attempts to maintain at least a certain percentage of free space on 54 the filesystem, shrinking the cache by culling the objects it contains to make 55 space if necessary - see the "Cache Culling" section. This means it can be 56 placed on the same medium as a live set of data, and will expand to make use of 57 spare space and automatically contract when the set of data requires more 58 space. 59 60 61 ============ 62 REQUIREMENTS 63 ============ 64 65 The use of CacheFiles and its daemon requires the following features to be 66 available in the system and in the cache filesystem: 67 68 - dnotify. 69 70 - extended attributes (xattrs). 71 72 - openat() and friends. 73 74 - bmap() support on files in the filesystem (FIBMAP ioctl). 75 76 - The use of bmap() to detect a partial page at the end of the file. 77 78 It is strongly recommended that the "dir_index" option is enabled on Ext3 79 filesystems being used as a cache. 80 81 82 ============= 83 CONFIGURATION 84 ============= 85 86 The cache is configured by a script in /etc/cachefilesd.conf. These commands 87 set up cache ready for use. The following script commands are available: 88 89 (*) brun <N>% 90 (*) bcull <N>% 91 (*) bstop <N>% 92 (*) frun <N>% 93 (*) fcull <N>% 94 (*) fstop <N>% 95 96 Configure the culling limits. Optional. See the section on culling 97 The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. 98 99 The commands beginning with a 'b' are file space (block) limits, those 100 beginning with an 'f' are file count limits. 101 102 (*) dir <path> 103 104 Specify the directory containing the root of the cache. Mandatory. 105 106 (*) tag <name> 107 108 Specify a tag to FS-Cache to use in distinguishing multiple caches. 109 Optional. The default is "CacheFiles". 110 111 (*) debug <mask> 112 113 Specify a numeric bitmask to control debugging in the kernel module. 114 Optional. The default is zero (all off). The following values can be 115 OR'd into the mask to collect various information: 116 117 1 Turn on trace of function entry (_enter() macros) 118 2 Turn on trace of function exit (_leave() macros) 119 4 Turn on trace of internal debug points (_debug()) 120 121 This mask can also be set through sysfs, eg: 122 123 echo 5 >/sys/modules/cachefiles/parameters/debug 124 125 126 ================== 127 STARTING THE CACHE 128 ================== 129 130 The cache is started by running the daemon. The daemon opens the cache device, 131 configures the cache and tells it to begin caching. At that point the cache 132 binds to fscache and the cache becomes live. 133 134 The daemon is run as follows: 135 136 /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] 137 138 The flags are: 139 140 (*) -d 141 142 Increase the debugging level. This can be specified multiple times and 143 is cumulative with itself. 144 145 (*) -s 146 147 Send messages to stderr instead of syslog. 148 149 (*) -n 150 151 Don't daemonise and go into background. 152 153 (*) -f <configfile> 154 155 Use an alternative configuration file rather than the default one. 156 157 158 =============== 159 THINGS TO AVOID 160 =============== 161 162 Do not mount other things within the cache as this will cause problems. The 163 kernel module contains its own very cut-down path walking facility that ignores 164 mountpoints, but the daemon can't avoid them. 165 166 Do not create, rename or unlink files and directories in the cache whilst the 167 cache is active, as this may cause the state to become uncertain. 168 169 Renaming files in the cache might make objects appear to be other objects (the 170 filename is part of the lookup key). 171 172 Do not change or remove the extended attributes attached to cache files by the 173 cache as this will cause the cache state management to get confused. 174 175 Do not create files or directories in the cache, lest the cache get confused or 176 serve incorrect data. 177 178 Do not chmod files in the cache. The module creates things with minimal 179 permissions to prevent random users being able to access them directly. 180 181 182 ============= 183 CACHE CULLING 184 ============= 185 186 The cache may need culling occasionally to make space. This involves 187 discarding objects from the cache that have been used less recently than 188 anything else. Culling is based on the access time of data objects. Empty 189 directories are culled if not in use. 190 191 Cache culling is done on the basis of the percentage of blocks and the 192 percentage of files available in the underlying filesystem. There are six 193 "limits": 194 195 (*) brun 196 (*) frun 197 198 If the amount of free space and the number of available files in the cache 199 rises above both these limits, then culling is turned off. 200 201 (*) bcull 202 (*) fcull 203 204 If the amount of available space or the number of available files in the 205 cache falls below either of these limits, then culling is started. 206 207 (*) bstop 208 (*) fstop 209 210 If the amount of available space or the number of available files in the 211 cache falls below either of these limits, then no further allocation of 212 disk space or files is permitted until culling has raised things above 213 these limits again. 214 215 These must be configured thusly: 216 217 0 <= bstop < bcull < brun < 100 218 0 <= fstop < fcull < frun < 100 219 220 Note that these are percentages of available space and available files, and do 221 _not_ appear as 100 minus the percentage displayed by the "df" program. 222 223 The userspace daemon scans the cache to build up a table of cullable objects. 224 These are then culled in least recently used order. A new scan of the cache is 225 started as soon as space is made in the table. Objects will be skipped if 226 their atimes have changed or if the kernel module says it is still using them. 227 228 229 =============== 230 CACHE STRUCTURE 231 =============== 232 233 The CacheFiles module will create two directories in the directory it was 234 given: 235 236 (*) cache/ 237 238 (*) graveyard/ 239 240 The active cache objects all reside in the first directory. The CacheFiles 241 kernel module moves any retired or culled objects that it can't simply unlink 242 to the graveyard from which the daemon will actually delete them. 243 244 The daemon uses dnotify to monitor the graveyard directory, and will delete 245 anything that appears therein. 246 247 248 The module represents index objects as directories with the filename "I..." or 249 "J...". Note that the "cache/" directory is itself a special index. 250 251 Data objects are represented as files if they have no children, or directories 252 if they do. Their filenames all begin "D..." or "E...". If represented as a 253 directory, data objects will have a file in the directory called "data" that 254 actually holds the data. 255 256 Special objects are similar to data objects, except their filenames begin 257 "S..." or "T...". 258 259 260 If an object has children, then it will be represented as a directory. 261 Immediately in the representative directory are a collection of directories 262 named for hash values of the child object keys with an '@' prepended. Into 263 this directory, if possible, will be placed the representations of the child 264 objects: 265 266 INDEX INDEX INDEX DATA FILES 267 ========= ========== ================================= ================ 268 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 269 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry 270 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry 271 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry 272 273 274 If the key is so long that it exceeds NAME_MAX with the decorations added on to 275 it, then it will be cut into pieces, the first few of which will be used to 276 make a nest of directories, and the last one of which will be the objects 277 inside the last directory. The names of the intermediate directories will have 278 '+' prepended: 279 280 J1223/@23/+xy...z/+kl...m/Epqr 281 282 283 Note that keys are raw data, and not only may they exceed NAME_MAX in size, 284 they may also contain things like '/' and NUL characters, and so they may not 285 be suitable for turning directly into a filename. 286 287 To handle this, CacheFiles will use a suitably printable filename directly and 288 "base-64" encode ones that aren't directly suitable. The two versions of 289 object filenames indicate the encoding: 290 291 OBJECT TYPE PRINTABLE ENCODED 292 =============== =============== =============== 293 Index "I..." "J..." 294 Data "D..." "E..." 295 Special "S..." "T..." 296 297 Intermediate directories are always "@" or "+" as appropriate. 298 299 300 Each object in the cache has an extended attribute label that holds the object 301 type ID (required to distinguish special objects) and the auxiliary data from 302 the netfs. The latter is used to detect stale objects in the cache and update 303 or retire them. 304 305 306 Note that CacheFiles will erase from the cache any file it doesn't recognise or 307 any file of an incorrect type (such as a FIFO file or a device file). 308 309 310 ========================== 311 SECURITY MODEL AND SELINUX 312 ========================== 313 314 CacheFiles is implemented to deal properly with the LSM security features of 315 the Linux kernel and the SELinux facility. 316 317 One of the problems that CacheFiles faces is that it is generally acting on 318 behalf of a process, and running in that process's context, and that includes a 319 security context that is not appropriate for accessing the cache - either 320 because the files in the cache are inaccessible to that process, or because if 321 the process creates a file in the cache, that file may be inaccessible to other 322 processes. 323 324 The way CacheFiles works is to temporarily change the security context (fsuid, 325 fsgid and actor security label) that the process acts as - without changing the 326 security context of the process when it the target of an operation performed by 327 some other process (so signalling and suchlike still work correctly). 328 329 330 When the CacheFiles module is asked to bind to its cache, it: 331 332 (1) Finds the security label attached to the root cache directory and uses 333 that as the security label with which it will create files. By default, 334 this is: 335 336 cachefiles_var_t 337 338 (2) Finds the security label of the process which issued the bind request 339 (presumed to be the cachefilesd daemon), which by default will be: 340 341 cachefilesd_t 342 343 and asks LSM to supply a security ID as which it should act given the 344 daemon's label. By default, this will be: 345 346 cachefiles_kernel_t 347 348 SELinux transitions the daemon's security ID to the module's security ID 349 based on a rule of this form in the policy. 350 351 type_transition <daemon's-ID> kernel_t : process <module's-ID>; 352 353 For instance: 354 355 type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; 356 357 358 The module's security ID gives it permission to create, move and remove files 359 and directories in the cache, to find and access directories and files in the 360 cache, to set and access extended attributes on cache objects, and to read and 361 write files in the cache. 362 363 The daemon's security ID gives it only a very restricted set of permissions: it 364 may scan directories, stat files and erase files and directories. It may 365 not read or write files in the cache, and so it is precluded from accessing the 366 data cached therein; nor is it permitted to create new files in the cache. 367 368 369 There are policy source files available in: 370 371 http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 372 373 and later versions. In that tarball, see the files: 374 375 cachefilesd.te 376 cachefilesd.fc 377 cachefilesd.if 378 379 They are built and installed directly by the RPM. 380 381 If a non-RPM based system is being used, then copy the above files to their own 382 directory and run: 383 384 make -f /usr/share/selinux/devel/Makefile 385 semodule -i cachefilesd.pp 386 387 You will need checkpolicy and selinux-policy-devel installed prior to the 388 build. 389 390 391 By default, the cache is located in /var/fscache, but if it is desirable that 392 it should be elsewhere, than either the above policy files must be altered, or 393 an auxiliary policy must be installed to label the alternate location of the 394 cache. 395 396 For instructions on how to add an auxiliary policy to enable the cache to be 397 located elsewhere when SELinux is in enforcing mode, please see: 398 399 /usr/share/doc/cachefilesd-*/move-cache.txt 400 401 When the cachefilesd rpm is installed; alternatively, the document can be found 402 in the sources. 403 404 405 ================== 406 A NOTE ON SECURITY 407 ================== 408 409 CacheFiles makes use of the split security in the task_struct. It allocates 410 its own task_security structure, and redirects current->cred to point to it 411 when it acts on behalf of another process, in that process's context. 412 413 The reason it does this is that it calls vfs_mkdir() and suchlike rather than 414 bypassing security and calling inode ops directly. Therefore the VFS and LSM 415 may deny the CacheFiles access to the cache data because under some 416 circumstances the caching code is running in the security context of whatever 417 process issued the original syscall on the netfs. 418 419 Furthermore, should CacheFiles create a file or directory, the security 420 parameters with that object is created (UID, GID, security label) would be 421 derived from that process that issued the system call, thus potentially 422 preventing other processes from accessing the cache - including CacheFiles's 423 cache management daemon (cachefilesd). 424 425 What is required is to temporarily override the security of the process that 426 issued the system call. We can't, however, just do an in-place change of the 427 security data as that affects the process as an object, not just as a subject. 428 This means it may lose signals or ptrace events for example, and affects what 429 the process looks like in /proc. 430 431 So CacheFiles makes use of a logical split in the security between the 432 objective security (task->real_cred) and the subjective security (task->cred). 433 The objective security holds the intrinsic security properties of a process and 434 is never overridden. This is what appears in /proc, and is what is used when a 435 process is the target of an operation by some other process (SIGKILL for 436 example). 437 438 The subjective security holds the active security properties of a process, and 439 may be overridden. This is not seen externally, and is used whan a process 440 acts upon another object, for example SIGKILLing another process or opening a 441 file. 442 443 LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request 444 for CacheFiles to run in a context of a specific security label, or to create 445 files and directories with another security label. 446 447 448 ======================= 449 STATISTICAL INFORMATION 450 ======================= 451 452 If FS-Cache is compiled with the following option enabled: 453 454 CONFIG_CACHEFILES_HISTOGRAM=y 455 456 then it will gather certain statistics and display them through a proc file. 457 458 (*) /proc/fs/cachefiles/histogram 459 460 cat /proc/fs/cachefiles/histogram 461 JIFS SECS LOOKUPS MKDIRS CREATES 462 ===== ===== ========= ========= ========= 463 464 This shows the breakdown of the number of times each amount of time 465 between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The 466 columns are as follows: 467 468 COLUMN TIME MEASUREMENT 469 ======= ======================================================= 470 LOOKUPS Length of time to perform a lookup on the backing fs 471 MKDIRS Length of time to perform a mkdir on the backing fs 472 CREATES Length of time to perform a create on the backing fs 473 474 Each row shows the number of events that took a particular range of times. 475 Each step is 1 jiffy in size. The JIFS column indicates the particular 476 jiffy range covered, and the SECS field the equivalent number of seconds. 477 478 479 ========= 480 DEBUGGING 481 ========= 482 483 If CONFIG_CACHEFILES_DEBUG is enabled, the CacheFiles facility can have runtime 484 debugging enabled by adjusting the value in: 485 486 /sys/module/cachefiles/parameters/debug 487 488 This is a bitmask of debugging streams to enable: 489 490 BIT VALUE STREAM POINT 491 ======= ======= =============================== ======================= 492 0 1 General Function entry trace 493 1 2 Function exit trace 494 2 4 General 495 496 The appropriate set of values should be OR'd together and the result written to 497 the control file. For example: 498 499 echo $((1|4|8)) >/sys/module/cachefiles/parameters/debug 500 501 will turn on all function entry debugging.