| 1 | /* |
| 2 | * mm/readahead.c - address_space-level file readahead. |
| 3 | * |
| 4 | * Copyright (C) 2002, Linus Torvalds |
| 5 | * |
| 6 | * 09Apr2002 Andrew Morton |
| 7 | * Initial version. |
| 8 | */ |
| 9 | |
| 10 | #include <linux/kernel.h> |
| 11 | #include <linux/gfp.h> |
| 12 | #include <linux/export.h> |
| 13 | #include <linux/blkdev.h> |
| 14 | #include <linux/backing-dev.h> |
| 15 | #include <linux/task_io_accounting_ops.h> |
| 16 | #include <linux/pagevec.h> |
| 17 | #include <linux/pagemap.h> |
| 18 | #include <linux/syscalls.h> |
| 19 | #include <linux/file.h> |
| 20 | #include <linux/mm_inline.h> |
| 21 | |
| 22 | #include "internal.h" |
| 23 | |
| 24 | /* |
| 25 | * Initialise a struct file's readahead state. Assumes that the caller has |
| 26 | * memset *ra to zero. |
| 27 | */ |
| 28 | void |
| 29 | file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping) |
| 30 | { |
| 31 | ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages; |
| 32 | ra->prev_pos = -1; |
| 33 | } |
| 34 | EXPORT_SYMBOL_GPL(file_ra_state_init); |
| 35 | |
| 36 | /* |
| 37 | * see if a page needs releasing upon read_cache_pages() failure |
| 38 | * - the caller of read_cache_pages() may have set PG_private or PG_fscache |
| 39 | * before calling, such as the NFS fs marking pages that are cached locally |
| 40 | * on disk, thus we need to give the fs a chance to clean up in the event of |
| 41 | * an error |
| 42 | */ |
| 43 | static void read_cache_pages_invalidate_page(struct address_space *mapping, |
| 44 | struct page *page) |
| 45 | { |
| 46 | if (page_has_private(page)) { |
| 47 | if (!trylock_page(page)) |
| 48 | BUG(); |
| 49 | page->mapping = mapping; |
| 50 | do_invalidatepage(page, 0, PAGE_SIZE); |
| 51 | page->mapping = NULL; |
| 52 | unlock_page(page); |
| 53 | } |
| 54 | put_page(page); |
| 55 | } |
| 56 | |
| 57 | /* |
| 58 | * release a list of pages, invalidating them first if need be |
| 59 | */ |
| 60 | static void read_cache_pages_invalidate_pages(struct address_space *mapping, |
| 61 | struct list_head *pages) |
| 62 | { |
| 63 | struct page *victim; |
| 64 | |
| 65 | while (!list_empty(pages)) { |
| 66 | victim = lru_to_page(pages); |
| 67 | list_del(&victim->lru); |
| 68 | read_cache_pages_invalidate_page(mapping, victim); |
| 69 | } |
| 70 | } |
| 71 | |
| 72 | /** |
| 73 | * read_cache_pages - populate an address space with some pages & start reads against them |
| 74 | * @mapping: the address_space |
| 75 | * @pages: The address of a list_head which contains the target pages. These |
| 76 | * pages have their ->index populated and are otherwise uninitialised. |
| 77 | * @filler: callback routine for filling a single page. |
| 78 | * @data: private data for the callback routine. |
| 79 | * |
| 80 | * Hides the details of the LRU cache etc from the filesystems. |
| 81 | */ |
| 82 | int read_cache_pages(struct address_space *mapping, struct list_head *pages, |
| 83 | int (*filler)(void *, struct page *), void *data) |
| 84 | { |
| 85 | struct page *page; |
| 86 | int ret = 0; |
| 87 | |
| 88 | while (!list_empty(pages)) { |
| 89 | page = lru_to_page(pages); |
| 90 | list_del(&page->lru); |
| 91 | if (add_to_page_cache_lru(page, mapping, page->index, |
| 92 | mapping_gfp_constraint(mapping, GFP_KERNEL))) { |
| 93 | read_cache_pages_invalidate_page(mapping, page); |
| 94 | continue; |
| 95 | } |
| 96 | put_page(page); |
| 97 | |
| 98 | ret = filler(data, page); |
| 99 | if (unlikely(ret)) { |
| 100 | read_cache_pages_invalidate_pages(mapping, pages); |
| 101 | break; |
| 102 | } |
| 103 | task_io_account_read(PAGE_SIZE); |
| 104 | } |
| 105 | return ret; |
| 106 | } |
| 107 | |
| 108 | EXPORT_SYMBOL(read_cache_pages); |
| 109 | |
| 110 | static int read_pages(struct address_space *mapping, struct file *filp, |
| 111 | struct list_head *pages, unsigned nr_pages) |
| 112 | { |
| 113 | struct blk_plug plug; |
| 114 | unsigned page_idx; |
| 115 | int ret; |
| 116 | |
| 117 | blk_start_plug(&plug); |
| 118 | |
| 119 | if (mapping->a_ops->readpages) { |
| 120 | ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages); |
| 121 | /* Clean up the remaining pages */ |
| 122 | put_pages_list(pages); |
| 123 | goto out; |
| 124 | } |
| 125 | |
| 126 | for (page_idx = 0; page_idx < nr_pages; page_idx++) { |
| 127 | struct page *page = lru_to_page(pages); |
| 128 | list_del(&page->lru); |
| 129 | if (!add_to_page_cache_lru(page, mapping, page->index, |
| 130 | mapping_gfp_constraint(mapping, GFP_KERNEL))) { |
| 131 | mapping->a_ops->readpage(filp, page); |
| 132 | } |
| 133 | put_page(page); |
| 134 | } |
| 135 | ret = 0; |
| 136 | |
| 137 | out: |
| 138 | blk_finish_plug(&plug); |
| 139 | |
| 140 | return ret; |
| 141 | } |
| 142 | |
| 143 | /* |
| 144 | * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all |
| 145 | * the pages first, then submits them all for I/O. This avoids the very bad |
| 146 | * behaviour which would occur if page allocations are causing VM writeback. |
| 147 | * We really don't want to intermingle reads and writes like that. |
| 148 | * |
| 149 | * Returns the number of pages requested, or the maximum amount of I/O allowed. |
| 150 | */ |
| 151 | int __do_page_cache_readahead(struct address_space *mapping, struct file *filp, |
| 152 | pgoff_t offset, unsigned long nr_to_read, |
| 153 | unsigned long lookahead_size) |
| 154 | { |
| 155 | struct inode *inode = mapping->host; |
| 156 | struct page *page; |
| 157 | unsigned long end_index; /* The last page we want to read */ |
| 158 | LIST_HEAD(page_pool); |
| 159 | int page_idx; |
| 160 | int ret = 0; |
| 161 | loff_t isize = i_size_read(inode); |
| 162 | |
| 163 | if (isize == 0) |
| 164 | goto out; |
| 165 | |
| 166 | end_index = ((isize - 1) >> PAGE_SHIFT); |
| 167 | |
| 168 | /* |
| 169 | * Preallocate as many pages as we will need. |
| 170 | */ |
| 171 | for (page_idx = 0; page_idx < nr_to_read; page_idx++) { |
| 172 | pgoff_t page_offset = offset + page_idx; |
| 173 | |
| 174 | if (page_offset > end_index) |
| 175 | break; |
| 176 | |
| 177 | rcu_read_lock(); |
| 178 | page = radix_tree_lookup(&mapping->page_tree, page_offset); |
| 179 | rcu_read_unlock(); |
| 180 | if (page && !radix_tree_exceptional_entry(page)) |
| 181 | continue; |
| 182 | |
| 183 | page = page_cache_alloc_readahead(mapping); |
| 184 | if (!page) |
| 185 | break; |
| 186 | page->index = page_offset; |
| 187 | list_add(&page->lru, &page_pool); |
| 188 | if (page_idx == nr_to_read - lookahead_size) |
| 189 | SetPageReadahead(page); |
| 190 | ret++; |
| 191 | } |
| 192 | |
| 193 | /* |
| 194 | * Now start the IO. We ignore I/O errors - if the page is not |
| 195 | * uptodate then the caller will launch readpage again, and |
| 196 | * will then handle the error. |
| 197 | */ |
| 198 | if (ret) |
| 199 | read_pages(mapping, filp, &page_pool, ret); |
| 200 | BUG_ON(!list_empty(&page_pool)); |
| 201 | out: |
| 202 | return ret; |
| 203 | } |
| 204 | |
| 205 | /* |
| 206 | * Chunk the readahead into 2 megabyte units, so that we don't pin too much |
| 207 | * memory at once. |
| 208 | */ |
| 209 | int force_page_cache_readahead(struct address_space *mapping, struct file *filp, |
| 210 | pgoff_t offset, unsigned long nr_to_read) |
| 211 | { |
| 212 | if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) |
| 213 | return -EINVAL; |
| 214 | |
| 215 | nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages); |
| 216 | while (nr_to_read) { |
| 217 | int err; |
| 218 | |
| 219 | unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE; |
| 220 | |
| 221 | if (this_chunk > nr_to_read) |
| 222 | this_chunk = nr_to_read; |
| 223 | err = __do_page_cache_readahead(mapping, filp, |
| 224 | offset, this_chunk, 0); |
| 225 | if (err < 0) |
| 226 | return err; |
| 227 | |
| 228 | offset += this_chunk; |
| 229 | nr_to_read -= this_chunk; |
| 230 | } |
| 231 | return 0; |
| 232 | } |
| 233 | |
| 234 | /* |
| 235 | * Set the initial window size, round to next power of 2 and square |
| 236 | * for small size, x 4 for medium, and x 2 for large |
| 237 | * for 128k (32 page) max ra |
| 238 | * 1-8 page = 32k initial, > 8 page = 128k initial |
| 239 | */ |
| 240 | static unsigned long get_init_ra_size(unsigned long size, unsigned long max) |
| 241 | { |
| 242 | unsigned long newsize = roundup_pow_of_two(size); |
| 243 | |
| 244 | if (newsize <= max / 32) |
| 245 | newsize = newsize * 4; |
| 246 | else if (newsize <= max / 4) |
| 247 | newsize = newsize * 2; |
| 248 | else |
| 249 | newsize = max; |
| 250 | |
| 251 | return newsize; |
| 252 | } |
| 253 | |
| 254 | /* |
| 255 | * Get the previous window size, ramp it up, and |
| 256 | * return it as the new window size. |
| 257 | */ |
| 258 | static unsigned long get_next_ra_size(struct file_ra_state *ra, |
| 259 | unsigned long max) |
| 260 | { |
| 261 | unsigned long cur = ra->size; |
| 262 | unsigned long newsize; |
| 263 | |
| 264 | if (cur < max / 16) |
| 265 | newsize = 4 * cur; |
| 266 | else |
| 267 | newsize = 2 * cur; |
| 268 | |
| 269 | return min(newsize, max); |
| 270 | } |
| 271 | |
| 272 | /* |
| 273 | * On-demand readahead design. |
| 274 | * |
| 275 | * The fields in struct file_ra_state represent the most-recently-executed |
| 276 | * readahead attempt: |
| 277 | * |
| 278 | * |<----- async_size ---------| |
| 279 | * |------------------- size -------------------->| |
| 280 | * |==================#===========================| |
| 281 | * ^start ^page marked with PG_readahead |
| 282 | * |
| 283 | * To overlap application thinking time and disk I/O time, we do |
| 284 | * `readahead pipelining': Do not wait until the application consumed all |
| 285 | * readahead pages and stalled on the missing page at readahead_index; |
| 286 | * Instead, submit an asynchronous readahead I/O as soon as there are |
| 287 | * only async_size pages left in the readahead window. Normally async_size |
| 288 | * will be equal to size, for maximum pipelining. |
| 289 | * |
| 290 | * In interleaved sequential reads, concurrent streams on the same fd can |
| 291 | * be invalidating each other's readahead state. So we flag the new readahead |
| 292 | * page at (start+size-async_size) with PG_readahead, and use it as readahead |
| 293 | * indicator. The flag won't be set on already cached pages, to avoid the |
| 294 | * readahead-for-nothing fuss, saving pointless page cache lookups. |
| 295 | * |
| 296 | * prev_pos tracks the last visited byte in the _previous_ read request. |
| 297 | * It should be maintained by the caller, and will be used for detecting |
| 298 | * small random reads. Note that the readahead algorithm checks loosely |
| 299 | * for sequential patterns. Hence interleaved reads might be served as |
| 300 | * sequential ones. |
| 301 | * |
| 302 | * There is a special-case: if the first page which the application tries to |
| 303 | * read happens to be the first page of the file, it is assumed that a linear |
| 304 | * read is about to happen and the window is immediately set to the initial size |
| 305 | * based on I/O request size and the max_readahead. |
| 306 | * |
| 307 | * The code ramps up the readahead size aggressively at first, but slow down as |
| 308 | * it approaches max_readhead. |
| 309 | */ |
| 310 | |
| 311 | /* |
| 312 | * Count contiguously cached pages from @offset-1 to @offset-@max, |
| 313 | * this count is a conservative estimation of |
| 314 | * - length of the sequential read sequence, or |
| 315 | * - thrashing threshold in memory tight systems |
| 316 | */ |
| 317 | static pgoff_t count_history_pages(struct address_space *mapping, |
| 318 | pgoff_t offset, unsigned long max) |
| 319 | { |
| 320 | pgoff_t head; |
| 321 | |
| 322 | rcu_read_lock(); |
| 323 | head = page_cache_prev_hole(mapping, offset - 1, max); |
| 324 | rcu_read_unlock(); |
| 325 | |
| 326 | return offset - 1 - head; |
| 327 | } |
| 328 | |
| 329 | /* |
| 330 | * page cache context based read-ahead |
| 331 | */ |
| 332 | static int try_context_readahead(struct address_space *mapping, |
| 333 | struct file_ra_state *ra, |
| 334 | pgoff_t offset, |
| 335 | unsigned long req_size, |
| 336 | unsigned long max) |
| 337 | { |
| 338 | pgoff_t size; |
| 339 | |
| 340 | size = count_history_pages(mapping, offset, max); |
| 341 | |
| 342 | /* |
| 343 | * not enough history pages: |
| 344 | * it could be a random read |
| 345 | */ |
| 346 | if (size <= req_size) |
| 347 | return 0; |
| 348 | |
| 349 | /* |
| 350 | * starts from beginning of file: |
| 351 | * it is a strong indication of long-run stream (or whole-file-read) |
| 352 | */ |
| 353 | if (size >= offset) |
| 354 | size *= 2; |
| 355 | |
| 356 | ra->start = offset; |
| 357 | ra->size = min(size + req_size, max); |
| 358 | ra->async_size = 1; |
| 359 | |
| 360 | return 1; |
| 361 | } |
| 362 | |
| 363 | /* |
| 364 | * A minimal readahead algorithm for trivial sequential/random reads. |
| 365 | */ |
| 366 | static unsigned long |
| 367 | ondemand_readahead(struct address_space *mapping, |
| 368 | struct file_ra_state *ra, struct file *filp, |
| 369 | bool hit_readahead_marker, pgoff_t offset, |
| 370 | unsigned long req_size) |
| 371 | { |
| 372 | unsigned long max = ra->ra_pages; |
| 373 | pgoff_t prev_offset; |
| 374 | |
| 375 | /* |
| 376 | * start of file |
| 377 | */ |
| 378 | if (!offset) |
| 379 | goto initial_readahead; |
| 380 | |
| 381 | /* |
| 382 | * It's the expected callback offset, assume sequential access. |
| 383 | * Ramp up sizes, and push forward the readahead window. |
| 384 | */ |
| 385 | if ((offset == (ra->start + ra->size - ra->async_size) || |
| 386 | offset == (ra->start + ra->size))) { |
| 387 | ra->start += ra->size; |
| 388 | ra->size = get_next_ra_size(ra, max); |
| 389 | ra->async_size = ra->size; |
| 390 | goto readit; |
| 391 | } |
| 392 | |
| 393 | /* |
| 394 | * Hit a marked page without valid readahead state. |
| 395 | * E.g. interleaved reads. |
| 396 | * Query the pagecache for async_size, which normally equals to |
| 397 | * readahead size. Ramp it up and use it as the new readahead size. |
| 398 | */ |
| 399 | if (hit_readahead_marker) { |
| 400 | pgoff_t start; |
| 401 | |
| 402 | rcu_read_lock(); |
| 403 | start = page_cache_next_hole(mapping, offset + 1, max); |
| 404 | rcu_read_unlock(); |
| 405 | |
| 406 | if (!start || start - offset > max) |
| 407 | return 0; |
| 408 | |
| 409 | ra->start = start; |
| 410 | ra->size = start - offset; /* old async_size */ |
| 411 | ra->size += req_size; |
| 412 | ra->size = get_next_ra_size(ra, max); |
| 413 | ra->async_size = ra->size; |
| 414 | goto readit; |
| 415 | } |
| 416 | |
| 417 | /* |
| 418 | * oversize read |
| 419 | */ |
| 420 | if (req_size > max) |
| 421 | goto initial_readahead; |
| 422 | |
| 423 | /* |
| 424 | * sequential cache miss |
| 425 | * trivial case: (offset - prev_offset) == 1 |
| 426 | * unaligned reads: (offset - prev_offset) == 0 |
| 427 | */ |
| 428 | prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT; |
| 429 | if (offset - prev_offset <= 1UL) |
| 430 | goto initial_readahead; |
| 431 | |
| 432 | /* |
| 433 | * Query the page cache and look for the traces(cached history pages) |
| 434 | * that a sequential stream would leave behind. |
| 435 | */ |
| 436 | if (try_context_readahead(mapping, ra, offset, req_size, max)) |
| 437 | goto readit; |
| 438 | |
| 439 | /* |
| 440 | * standalone, small random read |
| 441 | * Read as is, and do not pollute the readahead state. |
| 442 | */ |
| 443 | return __do_page_cache_readahead(mapping, filp, offset, req_size, 0); |
| 444 | |
| 445 | initial_readahead: |
| 446 | ra->start = offset; |
| 447 | ra->size = get_init_ra_size(req_size, max); |
| 448 | ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size; |
| 449 | |
| 450 | readit: |
| 451 | /* |
| 452 | * Will this read hit the readahead marker made by itself? |
| 453 | * If so, trigger the readahead marker hit now, and merge |
| 454 | * the resulted next readahead window into the current one. |
| 455 | */ |
| 456 | if (offset == ra->start && ra->size == ra->async_size) { |
| 457 | ra->async_size = get_next_ra_size(ra, max); |
| 458 | ra->size += ra->async_size; |
| 459 | } |
| 460 | |
| 461 | return ra_submit(ra, mapping, filp); |
| 462 | } |
| 463 | |
| 464 | /** |
| 465 | * page_cache_sync_readahead - generic file readahead |
| 466 | * @mapping: address_space which holds the pagecache and I/O vectors |
| 467 | * @ra: file_ra_state which holds the readahead state |
| 468 | * @filp: passed on to ->readpage() and ->readpages() |
| 469 | * @offset: start offset into @mapping, in pagecache page-sized units |
| 470 | * @req_size: hint: total size of the read which the caller is performing in |
| 471 | * pagecache pages |
| 472 | * |
| 473 | * page_cache_sync_readahead() should be called when a cache miss happened: |
| 474 | * it will submit the read. The readahead logic may decide to piggyback more |
| 475 | * pages onto the read request if access patterns suggest it will improve |
| 476 | * performance. |
| 477 | */ |
| 478 | void page_cache_sync_readahead(struct address_space *mapping, |
| 479 | struct file_ra_state *ra, struct file *filp, |
| 480 | pgoff_t offset, unsigned long req_size) |
| 481 | { |
| 482 | /* no read-ahead */ |
| 483 | if (!ra->ra_pages) |
| 484 | return; |
| 485 | |
| 486 | /* be dumb */ |
| 487 | if (filp && (filp->f_mode & FMODE_RANDOM)) { |
| 488 | force_page_cache_readahead(mapping, filp, offset, req_size); |
| 489 | return; |
| 490 | } |
| 491 | |
| 492 | /* do read-ahead */ |
| 493 | ondemand_readahead(mapping, ra, filp, false, offset, req_size); |
| 494 | } |
| 495 | EXPORT_SYMBOL_GPL(page_cache_sync_readahead); |
| 496 | |
| 497 | /** |
| 498 | * page_cache_async_readahead - file readahead for marked pages |
| 499 | * @mapping: address_space which holds the pagecache and I/O vectors |
| 500 | * @ra: file_ra_state which holds the readahead state |
| 501 | * @filp: passed on to ->readpage() and ->readpages() |
| 502 | * @page: the page at @offset which has the PG_readahead flag set |
| 503 | * @offset: start offset into @mapping, in pagecache page-sized units |
| 504 | * @req_size: hint: total size of the read which the caller is performing in |
| 505 | * pagecache pages |
| 506 | * |
| 507 | * page_cache_async_readahead() should be called when a page is used which |
| 508 | * has the PG_readahead flag; this is a marker to suggest that the application |
| 509 | * has used up enough of the readahead window that we should start pulling in |
| 510 | * more pages. |
| 511 | */ |
| 512 | void |
| 513 | page_cache_async_readahead(struct address_space *mapping, |
| 514 | struct file_ra_state *ra, struct file *filp, |
| 515 | struct page *page, pgoff_t offset, |
| 516 | unsigned long req_size) |
| 517 | { |
| 518 | /* no read-ahead */ |
| 519 | if (!ra->ra_pages) |
| 520 | return; |
| 521 | |
| 522 | /* |
| 523 | * Same bit is used for PG_readahead and PG_reclaim. |
| 524 | */ |
| 525 | if (PageWriteback(page)) |
| 526 | return; |
| 527 | |
| 528 | ClearPageReadahead(page); |
| 529 | |
| 530 | /* |
| 531 | * Defer asynchronous read-ahead on IO congestion. |
| 532 | */ |
| 533 | if (inode_read_congested(mapping->host)) |
| 534 | return; |
| 535 | |
| 536 | /* do read-ahead */ |
| 537 | ondemand_readahead(mapping, ra, filp, true, offset, req_size); |
| 538 | } |
| 539 | EXPORT_SYMBOL_GPL(page_cache_async_readahead); |
| 540 | |
| 541 | static ssize_t |
| 542 | do_readahead(struct address_space *mapping, struct file *filp, |
| 543 | pgoff_t index, unsigned long nr) |
| 544 | { |
| 545 | if (!mapping || !mapping->a_ops) |
| 546 | return -EINVAL; |
| 547 | |
| 548 | return force_page_cache_readahead(mapping, filp, index, nr); |
| 549 | } |
| 550 | |
| 551 | SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count) |
| 552 | { |
| 553 | ssize_t ret; |
| 554 | struct fd f; |
| 555 | |
| 556 | ret = -EBADF; |
| 557 | f = fdget(fd); |
| 558 | if (f.file) { |
| 559 | if (f.file->f_mode & FMODE_READ) { |
| 560 | struct address_space *mapping = f.file->f_mapping; |
| 561 | pgoff_t start = offset >> PAGE_SHIFT; |
| 562 | pgoff_t end = (offset + count - 1) >> PAGE_SHIFT; |
| 563 | unsigned long len = end - start + 1; |
| 564 | ret = do_readahead(mapping, f.file, start, len); |
| 565 | } |
| 566 | fdput(f); |
| 567 | } |
| 568 | return ret; |
| 569 | } |