mm/readahead.c

   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/fs.h>
  12 #include <linux/gfp.h>
  13 #include <linux/mm.h>
  14 #include <linux/export.h>
  15 #include <linux/blkdev.h>
  16 #include <linux/backing-dev.h>
  17 #include <linux/task_io_accounting_ops.h>
  18 #include <linux/pagevec.h>
  19 #include <linux/pagemap.h>
  20 #include <linux/syscalls.h>
  21 #include <linux/file.h>
  22
  23 /*
  24  * Initialise a struct file's readahead state.  Assumes that the caller has
  25  * memset *ra to zero.
  26  */
  27 void
  28 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
  29 {
  30         ra->ra_pages = mapping->backing_dev_info->ra_pages;
  31         ra->prev_pos = -1;
  32 }
  33 EXPORT_SYMBOL_GPL(file_ra_state_init);
  34
  35 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
  36
  37 /*
  38  * see if a page needs releasing upon read_cache_pages() failure
  39  * - the caller of read_cache_pages() may have set PG_private or PG_fscache
  40  *   before calling, such as the NFS fs marking pages that are cached locally
  41  *   on disk, thus we need to give the fs a chance to clean up in the event of
  42  *   an error
  43  */
  44 static void read_cache_pages_invalidate_page(struct address_space *mapping,
  45                                              struct page *page)
  46 {
  47         if (page_has_private(page)) {
  48                 if (!trylock_page(page))
  49                         BUG();
  50                 page->mapping = mapping;
  51                 do_invalidatepage(page, 0);
  52                 page->mapping = NULL;
  53                 unlock_page(page);
  54         }
  55         page_cache_release(page);
  56 }
  57
  58 /*
  59  * release a list of pages, invalidating them first if need be
  60  */
  61 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
  62                                               struct list_head *pages)
  63 {
  64         struct page *victim;
  65
  66         while (!list_empty(pages)) {
  67                 victim = list_to_page(pages);
  68                 list_del(&victim->lru);
  69                 read_cache_pages_invalidate_page(mapping, victim);
  70         }
  71 }
  72
  73 /**
  74  * read_cache_pages - populate an address space with some pages & start reads against them
  75  * @mapping: the address_space
  76  * @pages: The address of a list_head which contains the target pages.  These
  77  *   pages have their ->index populated and are otherwise uninitialised.
  78  * @filler: callback routine for filling a single page.
  79  * @data: private data for the callback routine.
  80  *
  81  * Hides the details of the LRU cache etc from the filesystems.
  82  */
  83 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
  84                         int (*filler)(void *, struct page *), void *data)
  85 {
  86         struct page *page;
  87         int ret = 0;
  88
  89         while (!list_empty(pages)) {
  90                 page = list_to_page(pages);
  91                 list_del(&page->lru);
  92                 if (add_to_page_cache_lru(page, mapping,
  93                                         page->index, GFP_KERNEL)) {
  94                         read_cache_pages_invalidate_page(mapping, page);
  95                         continue;
  96                 }
  97                 page_cache_release(page);
  98
  99                 ret = filler(data, page);
 100                 if (unlikely(ret)) {
 101                         read_cache_pages_invalidate_pages(mapping, pages);
 102                         break;
 103                 }
 104                 task_io_account_read(PAGE_CACHE_SIZE);
 105         }
 106         return ret;
 107 }
 108
 109 EXPORT_SYMBOL(read_cache_pages);
 110
 111 static int read_pages(struct address_space *mapping, struct file *filp,
 112                 struct list_head *pages, unsigned nr_pages)
 113 {
 114         struct blk_plug plug;
 115         unsigned page_idx;
 116         int ret;
 117
 118         blk_start_plug(&plug);
 119
 120         if (mapping->a_ops->readpages) {
 121                 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
 122                 /* Clean up the remaining pages */
 123                 put_pages_list(pages);
 124                 goto out;
 125         }
 126
 127         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 128                 struct page *page = list_to_page(pages);
 129                 list_del(&page->lru);
 130                 if (!add_to_page_cache_lru(page, mapping,
 131                                         page->index, GFP_KERNEL)) {
 132                         mapping->a_ops->readpage(filp, page);
 133                 }
 134                 page_cache_release(page);
 135         }
 136         ret = 0;
 137
 138 out:
 139         blk_finish_plug(&plug);
 140
 141         return ret;
 142 }
 143
 144 /*
 145  * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
 146  * the pages first, then submits them all for I/O. This avoids the very bad
 147  * behaviour which would occur if page allocations are causing VM writeback.
 148  * We really don't want to intermingle reads and writes like that.
 149  *
 150  * Returns the number of pages requested, or the maximum amount of I/O allowed.
 151  */
 152 static int
 153 __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 154                         pgoff_t offset, unsigned long nr_to_read,
 155                         unsigned long lookahead_size)
 156 {
 157         struct inode *inode = mapping->host;
 158         struct page *page;
 159         unsigned long end_index;        /* The last page we want to read */
 160         LIST_HEAD(page_pool);
 161         int page_idx;
 162         int ret = 0;
 163         loff_t isize = i_size_read(inode);
 164
 165         if (isize == 0)
 166                 goto out;
 167
 168         end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
 169
 170         /*
 171          * Preallocate as many pages as we will need.
 172          */
 173         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
 174                 pgoff_t page_offset = offset + page_idx;
 175
 176                 if (page_offset > end_index)
 177                         break;
 178
 179                 rcu_read_lock();
 180                 page = radix_tree_lookup(&mapping->page_tree, page_offset);
 181                 rcu_read_unlock();
 182                 if (page)
 183                         continue;
 184
 185                 page = page_cache_alloc_readahead(mapping);
 186                 if (!page)
 187                         break;
 188                 page->index = page_offset;
 189                 list_add(&page->lru, &page_pool);
 190                 if (page_idx == nr_to_read - lookahead_size)
 191                         SetPageReadahead(page);
 192                 ret++;
 193         }
 194
 195         /*
 196          * Now start the IO.  We ignore I/O errors - if the page is not
 197          * uptodate then the caller will launch readpage again, and
 198          * will then handle the error.
 199          */
 200         if (ret)
 201                 read_pages(mapping, filp, &page_pool, ret);
 202         BUG_ON(!list_empty(&page_pool));
 203 out:
 204         return ret;
 205 }
 206
 207 /*
 208  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 209  * memory at once.
 210  */
 211 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 212                 pgoff_t offset, unsigned long nr_to_read)
 213 {
 214         int ret = 0;
 215
 216         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 217                 return -EINVAL;
 218
 219         nr_to_read = max_sane_readahead(nr_to_read);
 220         while (nr_to_read) {
 221                 int err;
 222
 223                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
 224
 225                 if (this_chunk > nr_to_read)
 226                         this_chunk = nr_to_read;
 227                 err = __do_page_cache_readahead(mapping, filp,
 228                                                 offset, this_chunk, 0);
 229                 if (err < 0) {
 230                         ret = err;
 231                         break;
 232                 }
 233                 ret += err;
 234                 offset += this_chunk;
 235                 nr_to_read -= this_chunk;
 236         }
 237         return ret;
 238 }
 239
 240 /*
 241  * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
 242  * sensible upper limit.
 243  */
 244 unsigned long max_sane_readahead(unsigned long nr)
 245 {
 246         return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
 247                 + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
 248 }
 249
 250 /*
 251  * Submit IO for the read-ahead request in file_ra_state.
 252  */
 253 unsigned long ra_submit(struct file_ra_state *ra,
 254                        struct address_space *mapping, struct file *filp)
 255 {
 256         int actual;
 257
 258         actual = __do_page_cache_readahead(mapping, filp,
 259                                         ra->start, ra->size, ra->async_size);
 260
 261         return actual;
 262 }
 263
 264 /*
 265  * Set the initial window size, round to next power of 2 and square
 266  * for small size, x 4 for medium, and x 2 for large
 267  * for 128k (32 page) max ra
 268  * 1-8 page = 32k initial, > 8 page = 128k initial
 269  */
 270 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 271 {
 272         unsigned long newsize = roundup_pow_of_two(size);
 273
 274         if (newsize <= max / 32)
 275                 newsize = newsize * 4;
 276         else if (newsize <= max / 4)
 277                 newsize = newsize * 2;
 278         else
 279                 newsize = max;
 280
 281         return newsize;
 282 }
 283
 284 /*
 285  *  Get the previous window size, ramp it up, and
 286  *  return it as the new window size.
 287  */
 288 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 289                                                 unsigned long max)
 290 {
 291         unsigned long cur = ra->size;
 292         unsigned long newsize;
 293
 294         if (cur < max / 16)
 295                 newsize = 4 * cur;
 296         else
 297                 newsize = 2 * cur;
 298
 299         return min(newsize, max);
 300 }
 301
 302 /*
 303  * On-demand readahead design.
 304  *
 305  * The fields in struct file_ra_state represent the most-recently-executed
 306  * readahead attempt:
 307  *
 308  *                        |<----- async_size ---------|
 309  *     |------------------- size -------------------->|
 310  *     |==================#===========================|
 311  *     ^start             ^page marked with PG_readahead
 312  *
 313  * To overlap application thinking time and disk I/O time, we do
 314  * `readahead pipelining': Do not wait until the application consumed all
 315  * readahead pages and stalled on the missing page at readahead_index;
 316  * Instead, submit an asynchronous readahead I/O as soon as there are
 317  * only async_size pages left in the readahead window. Normally async_size
 318  * will be equal to size, for maximum pipelining.
 319  *
 320  * In interleaved sequential reads, concurrent streams on the same fd can
 321  * be invalidating each other's readahead state. So we flag the new readahead
 322  * page at (start+size-async_size) with PG_readahead, and use it as readahead
 323  * indicator. The flag won't be set on already cached pages, to avoid the
 324  * readahead-for-nothing fuss, saving pointless page cache lookups.
 325  *
 326  * prev_pos tracks the last visited byte in the _previous_ read request.
 327  * It should be maintained by the caller, and will be used for detecting
 328  * small random reads. Note that the readahead algorithm checks loosely
 329  * for sequential patterns. Hence interleaved reads might be served as
 330  * sequential ones.
 331  *
 332  * There is a special-case: if the first page which the application tries to
 333  * read happens to be the first page of the file, it is assumed that a linear
 334  * read is about to happen and the window is immediately set to the initial size
 335  * based on I/O request size and the max_readahead.
 336  *
 337  * The code ramps up the readahead size aggressively at first, but slow down as
 338  * it approaches max_readhead.
 339  */
 340
 341 /*
 342  * Count contiguously cached pages from @offset-1 to @offset-@max,
 343  * this count is a conservative estimation of
 344  *      - length of the sequential read sequence, or
 345  *      - thrashing threshold in memory tight systems
 346  */
 347 static pgoff_t count_history_pages(struct address_space *mapping,
 348                                    struct file_ra_state *ra,
 349                                    pgoff_t offset, unsigned long max)
 350 {
 351         pgoff_t head;
 352
 353         rcu_read_lock();
 354         head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
 355         rcu_read_unlock();
 356
 357         return offset - 1 - head;
 358 }
 359
 360 /*
 361  * page cache context based read-ahead
 362  */
 363 static int try_context_readahead(struct address_space *mapping,
 364                                  struct file_ra_state *ra,
 365                                  pgoff_t offset,
 366                                  unsigned long req_size,
 367                                  unsigned long max)
 368 {
 369         pgoff_t size;
 370
 371         size = count_history_pages(mapping, ra, offset, max);
 372
 373         /*
 374          * no history pages:
 375          * it could be a random read
 376          */
 377         if (!size)
 378                 return 0;
 379
 380         /*
 381          * starts from beginning of file:
 382          * it is a strong indication of long-run stream (or whole-file-read)
 383          */
 384         if (size >= offset)
 385                 size *= 2;
 386
 387         ra->start = offset;
 388         ra->size = get_init_ra_size(size + req_size, max);
 389         ra->async_size = ra->size;
 390
 391         return 1;
 392 }
 393
 394 /*
 395  * A minimal readahead algorithm for trivial sequential/random reads.
 396  */
 397 static unsigned long
 398 ondemand_readahead(struct address_space *mapping,
 399                    struct file_ra_state *ra, struct file *filp,
 400                    bool hit_readahead_marker, pgoff_t offset,
 401                    unsigned long req_size)
 402 {
 403         unsigned long max = max_sane_readahead(ra->ra_pages);
 404
 405         /*
 406          * start of file
 407          */
 408         if (!offset)
 409                 goto initial_readahead;
 410
 411         /*
 412          * It's the expected callback offset, assume sequential access.
 413          * Ramp up sizes, and push forward the readahead window.
 414          */
 415         if ((offset == (ra->start + ra->size - ra->async_size) ||
 416              offset == (ra->start + ra->size))) {
 417                 ra->start += ra->size;
 418                 ra->size = get_next_ra_size(ra, max);
 419                 ra->async_size = ra->size;
 420                 goto readit;
 421         }
 422
 423         /*
 424          * Hit a marked page without valid readahead state.
 425          * E.g. interleaved reads.
 426          * Query the pagecache for async_size, which normally equals to
 427          * readahead size. Ramp it up and use it as the new readahead size.
 428          */
 429         if (hit_readahead_marker) {
 430                 pgoff_t start;
 431
 432                 rcu_read_lock();
 433                 start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
 434                 rcu_read_unlock();
 435
 436                 if (!start || start - offset > max)
 437                         return 0;
 438
 439                 ra->start = start;
 440                 ra->size = start - offset;      /* old async_size */
 441                 ra->size += req_size;
 442                 ra->size = get_next_ra_size(ra, max);
 443                 ra->async_size = ra->size;
 444                 goto readit;
 445         }
 446
 447         /*
 448          * oversize read
 449          */
 450         if (req_size > max)
 451                 goto initial_readahead;
 452
 453         /*
 454          * sequential cache miss
 455          */
 456         if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
 457                 goto initial_readahead;
 458
 459         /*
 460          * Query the page cache and look for the traces(cached history pages)
 461          * that a sequential stream would leave behind.
 462          */
 463         if (try_context_readahead(mapping, ra, offset, req_size, max))
 464                 goto readit;
 465
 466         /*
 467          * standalone, small random read
 468          * Read as is, and do not pollute the readahead state.
 469          */
 470         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
 471
 472 initial_readahead:
 473         ra->start = offset;
 474         ra->size = get_init_ra_size(req_size, max);
 475         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 476
 477 readit:
 478         /*
 479          * Will this read hit the readahead marker made by itself?
 480          * If so, trigger the readahead marker hit now, and merge
 481          * the resulted next readahead window into the current one.
 482          */
 483         if (offset == ra->start && ra->size == ra->async_size) {
 484                 ra->async_size = get_next_ra_size(ra, max);
 485                 ra->size += ra->async_size;
 486         }
 487
 488         return ra_submit(ra, mapping, filp);
 489 }
 490
 491 /**
 492  * page_cache_sync_readahead - generic file readahead
 493  * @mapping: address_space which holds the pagecache and I/O vectors
 494  * @ra: file_ra_state which holds the readahead state
 495  * @filp: passed on to ->readpage() and ->readpages()
 496  * @offset: start offset into @mapping, in pagecache page-sized units
 497  * @req_size: hint: total size of the read which the caller is performing in
 498  *            pagecache pages
 499  *
 500  * page_cache_sync_readahead() should be called when a cache miss happened:
 501  * it will submit the read.  The readahead logic may decide to piggyback more
 502  * pages onto the read request if access patterns suggest it will improve
 503  * performance.
 504  */
 505 void page_cache_sync_readahead(struct address_space *mapping,
 506                                struct file_ra_state *ra, struct file *filp,
 507                                pgoff_t offset, unsigned long req_size)
 508 {
 509         /* no read-ahead */
 510         if (!ra->ra_pages)
 511                 return;
 512
 513         /* be dumb */
 514         if (filp && (filp->f_mode & FMODE_RANDOM)) {
 515                 force_page_cache_readahead(mapping, filp, offset, req_size);
 516                 return;
 517         }
 518
 519         /* do read-ahead */
 520         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
 521 }
 522 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 523
 524 /**
 525  * page_cache_async_readahead - file readahead for marked pages
 526  * @mapping: address_space which holds the pagecache and I/O vectors
 527  * @ra: file_ra_state which holds the readahead state
 528  * @filp: passed on to ->readpage() and ->readpages()
 529  * @page: the page at @offset which has the PG_readahead flag set
 530  * @offset: start offset into @mapping, in pagecache page-sized units
 531  * @req_size: hint: total size of the read which the caller is performing in
 532  *            pagecache pages
 533  *
 534  * page_cache_async_readahead() should be called when a page is used which
 535  * has the PG_readahead flag; this is a marker to suggest that the application
 536  * has used up enough of the readahead window that we should start pulling in
 537  * more pages.
 538  */
 539 void
 540 page_cache_async_readahead(struct address_space *mapping,
 541                            struct file_ra_state *ra, struct file *filp,
 542                            struct page *page, pgoff_t offset,
 543                            unsigned long req_size)
 544 {
 545         /* no read-ahead */
 546         if (!ra->ra_pages)
 547                 return;
 548
 549         /*
 550          * Same bit is used for PG_readahead and PG_reclaim.
 551          */
 552         if (PageWriteback(page))
 553                 return;
 554
 555         ClearPageReadahead(page);
 556
 557         /*
 558          * Defer asynchronous read-ahead on IO congestion.
 559          */
 560         if (bdi_read_congested(mapping->backing_dev_info))
 561                 return;
 562
 563         /* do read-ahead */
 564         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 565 }
 566 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 567
 568 static ssize_t
 569 do_readahead(struct address_space *mapping, struct file *filp,
 570              pgoff_t index, unsigned long nr)
 571 {
 572         if (!mapping || !mapping->a_ops || !mapping->a_ops->readpage)
 573                 return -EINVAL;
 574
 575         force_page_cache_readahead(mapping, filp, index, nr);
 576         return 0;
 577 }
 578
 579 SYSCALL_DEFINE(readahead)(int fd, loff_t offset, size_t count)
 580 {
 581         ssize_t ret;
 582         struct file *file;
 583
 584         ret = -EBADF;
 585         file = fget(fd);
 586         if (file) {
 587                 if (file->f_mode & FMODE_READ) {
 588                         struct address_space *mapping = file->f_mapping;
 589                         pgoff_t start = offset >> PAGE_CACHE_SHIFT;
 590                         pgoff_t end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
 591                         unsigned long len = end - start + 1;
 592                         ret = do_readahead(mapping, file, start, len);
 593                 }
 594                 fput(file);
 595         }
 596         return ret;
 597 }
 598 #ifdef CONFIG_HAVE_SYSCALL_WRAPPERS
 599 asmlinkage long SyS_readahead(long fd, loff_t offset, long count)
 600 {
 601         return SYSC_readahead((int) fd, offset, (size_t) count);
 602 }
 603 SYSCALL_ALIAS(sys_readahead, SyS_readahead);
 604 #endif