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/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                                 readahead_gfp_mask(mapping))) {
  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 int nr_pages, gfp_t gfp)
 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, gfp))
 130                         mapping->a_ops->readpage(filp, page);
 131                 put_page(page);
 132         }
 133         ret = 0;
 134
 135 out:
 136         blk_finish_plug(&plug);
 137
 138         return ret;
 139 }
 140
 141 /*
 142  * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
 143  * the pages first, then submits them all for I/O. This avoids the very bad
 144  * behaviour which would occur if page allocations are causing VM writeback.
 145  * We really don't want to intermingle reads and writes like that.
 146  *
 147  * Returns the number of pages requested, or the maximum amount of I/O allowed.
 148  */
 149 int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 150                         pgoff_t offset, unsigned long nr_to_read,
 151                         unsigned long lookahead_size)
 152 {
 153         struct inode *inode = mapping->host;
 154         struct page *page;
 155         unsigned long end_index;        /* The last page we want to read */
 156         LIST_HEAD(page_pool);
 157         int page_idx;
 158         int ret = 0;
 159         loff_t isize = i_size_read(inode);
 160         gfp_t gfp_mask = readahead_gfp_mask(mapping);
 161
 162         if (isize == 0)
 163                 goto out;
 164
 165         end_index = ((isize - 1) >> PAGE_SHIFT);
 166
 167         /*
 168          * Preallocate as many pages as we will need.
 169          */
 170         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
 171                 pgoff_t page_offset = offset + page_idx;
 172
 173                 if (page_offset > end_index)
 174                         break;
 175
 176                 rcu_read_lock();
 177                 page = radix_tree_lookup(&mapping->page_tree, page_offset);
 178                 rcu_read_unlock();
 179                 if (page && !radix_tree_exceptional_entry(page))
 180                         continue;
 181
 182                 page = __page_cache_alloc(gfp_mask);
 183                 if (!page)
 184                         break;
 185                 page->index = page_offset;
 186                 list_add(&page->lru, &page_pool);
 187                 if (page_idx == nr_to_read - lookahead_size)
 188                         SetPageReadahead(page);
 189                 ret++;
 190         }
 191
 192         /*
 193          * Now start the IO.  We ignore I/O errors - if the page is not
 194          * uptodate then the caller will launch readpage again, and
 195          * will then handle the error.
 196          */
 197         if (ret)
 198                 read_pages(mapping, filp, &page_pool, ret, gfp_mask);
 199         BUG_ON(!list_empty(&page_pool));
 200 out:
 201         return ret;
 202 }
 203
 204 /*
 205  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 206  * memory at once.
 207  */
 208 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 209                 pgoff_t offset, unsigned long nr_to_read)
 210 {
 211         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 212                 return -EINVAL;
 213
 214         nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
 215         while (nr_to_read) {
 216                 int err;
 217
 218                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 219
 220                 if (this_chunk > nr_to_read)
 221                         this_chunk = nr_to_read;
 222                 err = __do_page_cache_readahead(mapping, filp,
 223                                                 offset, this_chunk, 0);
 224                 if (err < 0)
 225                         return err;
 226
 227                 offset += this_chunk;
 228                 nr_to_read -= this_chunk;
 229         }
 230         return 0;
 231 }
 232
 233 /*
 234  * Set the initial window size, round to next power of 2 and square
 235  * for small size, x 4 for medium, and x 2 for large
 236  * for 128k (32 page) max ra
 237  * 1-8 page = 32k initial, > 8 page = 128k initial
 238  */
 239 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 240 {
 241         unsigned long newsize = roundup_pow_of_two(size);
 242
 243         if (newsize <= max / 32)
 244                 newsize = newsize * 4;
 245         else if (newsize <= max / 4)
 246                 newsize = newsize * 2;
 247         else
 248                 newsize = max;
 249
 250         return newsize;
 251 }
 252
 253 /*
 254  *  Get the previous window size, ramp it up, and
 255  *  return it as the new window size.
 256  */
 257 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 258                                                 unsigned long max)
 259 {
 260         unsigned long cur = ra->size;
 261         unsigned long newsize;
 262
 263         if (cur < max / 16)
 264                 newsize = 4 * cur;
 265         else
 266                 newsize = 2 * cur;
 267
 268         return min(newsize, max);
 269 }
 270
 271 /*
 272  * On-demand readahead design.
 273  *
 274  * The fields in struct file_ra_state represent the most-recently-executed
 275  * readahead attempt:
 276  *
 277  *                        |<----- async_size ---------|
 278  *     |------------------- size -------------------->|
 279  *     |==================#===========================|
 280  *     ^start             ^page marked with PG_readahead
 281  *
 282  * To overlap application thinking time and disk I/O time, we do
 283  * `readahead pipelining': Do not wait until the application consumed all
 284  * readahead pages and stalled on the missing page at readahead_index;
 285  * Instead, submit an asynchronous readahead I/O as soon as there are
 286  * only async_size pages left in the readahead window. Normally async_size
 287  * will be equal to size, for maximum pipelining.
 288  *
 289  * In interleaved sequential reads, concurrent streams on the same fd can
 290  * be invalidating each other's readahead state. So we flag the new readahead
 291  * page at (start+size-async_size) with PG_readahead, and use it as readahead
 292  * indicator. The flag won't be set on already cached pages, to avoid the
 293  * readahead-for-nothing fuss, saving pointless page cache lookups.
 294  *
 295  * prev_pos tracks the last visited byte in the _previous_ read request.
 296  * It should be maintained by the caller, and will be used for detecting
 297  * small random reads. Note that the readahead algorithm checks loosely
 298  * for sequential patterns. Hence interleaved reads might be served as
 299  * sequential ones.
 300  *
 301  * There is a special-case: if the first page which the application tries to
 302  * read happens to be the first page of the file, it is assumed that a linear
 303  * read is about to happen and the window is immediately set to the initial size
 304  * based on I/O request size and the max_readahead.
 305  *
 306  * The code ramps up the readahead size aggressively at first, but slow down as
 307  * it approaches max_readhead.
 308  */
 309
 310 /*
 311  * Count contiguously cached pages from @offset-1 to @offset-@max,
 312  * this count is a conservative estimation of
 313  *      - length of the sequential read sequence, or
 314  *      - thrashing threshold in memory tight systems
 315  */
 316 static pgoff_t count_history_pages(struct address_space *mapping,
 317                                    pgoff_t offset, unsigned long max)
 318 {
 319         pgoff_t head;
 320
 321         rcu_read_lock();
 322         head = page_cache_prev_hole(mapping, offset - 1, max);
 323         rcu_read_unlock();
 324
 325         return offset - 1 - head;
 326 }
 327
 328 /*
 329  * page cache context based read-ahead
 330  */
 331 static int try_context_readahead(struct address_space *mapping,
 332                                  struct file_ra_state *ra,
 333                                  pgoff_t offset,
 334                                  unsigned long req_size,
 335                                  unsigned long max)
 336 {
 337         pgoff_t size;
 338
 339         size = count_history_pages(mapping, offset, max);
 340
 341         /*
 342          * not enough history pages:
 343          * it could be a random read
 344          */
 345         if (size <= req_size)
 346                 return 0;
 347
 348         /*
 349          * starts from beginning of file:
 350          * it is a strong indication of long-run stream (or whole-file-read)
 351          */
 352         if (size >= offset)
 353                 size *= 2;
 354
 355         ra->start = offset;
 356         ra->size = min(size + req_size, max);
 357         ra->async_size = 1;
 358
 359         return 1;
 360 }
 361
 362 /*
 363  * A minimal readahead algorithm for trivial sequential/random reads.
 364  */
 365 static unsigned long
 366 ondemand_readahead(struct address_space *mapping,
 367                    struct file_ra_state *ra, struct file *filp,
 368                    bool hit_readahead_marker, pgoff_t offset,
 369                    unsigned long req_size)
 370 {
 371         unsigned long max = ra->ra_pages;
 372         pgoff_t prev_offset;
 373
 374         /*
 375          * start of file
 376          */
 377         if (!offset)
 378                 goto initial_readahead;
 379
 380         /*
 381          * It's the expected callback offset, assume sequential access.
 382          * Ramp up sizes, and push forward the readahead window.
 383          */
 384         if ((offset == (ra->start + ra->size - ra->async_size) ||
 385              offset == (ra->start + ra->size))) {
 386                 ra->start += ra->size;
 387                 ra->size = get_next_ra_size(ra, max);
 388                 ra->async_size = ra->size;
 389                 goto readit;
 390         }
 391
 392         /*
 393          * Hit a marked page without valid readahead state.
 394          * E.g. interleaved reads.
 395          * Query the pagecache for async_size, which normally equals to
 396          * readahead size. Ramp it up and use it as the new readahead size.
 397          */
 398         if (hit_readahead_marker) {
 399                 pgoff_t start;
 400
 401                 rcu_read_lock();
 402                 start = page_cache_next_hole(mapping, offset + 1, max);
 403                 rcu_read_unlock();
 404
 405                 if (!start || start - offset > max)
 406                         return 0;
 407
 408                 ra->start = start;
 409                 ra->size = start - offset;      /* old async_size */
 410                 ra->size += req_size;
 411                 ra->size = get_next_ra_size(ra, max);
 412                 ra->async_size = ra->size;
 413                 goto readit;
 414         }
 415
 416         /*
 417          * oversize read
 418          */
 419         if (req_size > max)
 420                 goto initial_readahead;
 421
 422         /*
 423          * sequential cache miss
 424          * trivial case: (offset - prev_offset) == 1
 425          * unaligned reads: (offset - prev_offset) == 0
 426          */
 427         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
 428         if (offset - prev_offset <= 1UL)
 429                 goto initial_readahead;
 430
 431         /*
 432          * Query the page cache and look for the traces(cached history pages)
 433          * that a sequential stream would leave behind.
 434          */
 435         if (try_context_readahead(mapping, ra, offset, req_size, max))
 436                 goto readit;
 437
 438         /*
 439          * standalone, small random read
 440          * Read as is, and do not pollute the readahead state.
 441          */
 442         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
 443
 444 initial_readahead:
 445         ra->start = offset;
 446         ra->size = get_init_ra_size(req_size, max);
 447         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 448
 449 readit:
 450         /*
 451          * Will this read hit the readahead marker made by itself?
 452          * If so, trigger the readahead marker hit now, and merge
 453          * the resulted next readahead window into the current one.
 454          */
 455         if (offset == ra->start && ra->size == ra->async_size) {
 456                 ra->async_size = get_next_ra_size(ra, max);
 457                 ra->size += ra->async_size;
 458         }
 459
 460         return ra_submit(ra, mapping, filp);
 461 }
 462
 463 /**
 464  * page_cache_sync_readahead - generic file readahead
 465  * @mapping: address_space which holds the pagecache and I/O vectors
 466  * @ra: file_ra_state which holds the readahead state
 467  * @filp: passed on to ->readpage() and ->readpages()
 468  * @offset: start offset into @mapping, in pagecache page-sized units
 469  * @req_size: hint: total size of the read which the caller is performing in
 470  *            pagecache pages
 471  *
 472  * page_cache_sync_readahead() should be called when a cache miss happened:
 473  * it will submit the read.  The readahead logic may decide to piggyback more
 474  * pages onto the read request if access patterns suggest it will improve
 475  * performance.
 476  */
 477 void page_cache_sync_readahead(struct address_space *mapping,
 478                                struct file_ra_state *ra, struct file *filp,
 479                                pgoff_t offset, unsigned long req_size)
 480 {
 481         /* no read-ahead */
 482         if (!ra->ra_pages)
 483                 return;
 484
 485         /* be dumb */
 486         if (filp && (filp->f_mode & FMODE_RANDOM)) {
 487                 force_page_cache_readahead(mapping, filp, offset, req_size);
 488                 return;
 489         }
 490
 491         /* do read-ahead */
 492         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
 493 }
 494 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 495
 496 /**
 497  * page_cache_async_readahead - file readahead for marked pages
 498  * @mapping: address_space which holds the pagecache and I/O vectors
 499  * @ra: file_ra_state which holds the readahead state
 500  * @filp: passed on to ->readpage() and ->readpages()
 501  * @page: the page at @offset which has the PG_readahead flag set
 502  * @offset: start offset into @mapping, in pagecache page-sized units
 503  * @req_size: hint: total size of the read which the caller is performing in
 504  *            pagecache pages
 505  *
 506  * page_cache_async_readahead() should be called when a page is used which
 507  * has the PG_readahead flag; this is a marker to suggest that the application
 508  * has used up enough of the readahead window that we should start pulling in
 509  * more pages.
 510  */
 511 void
 512 page_cache_async_readahead(struct address_space *mapping,
 513                            struct file_ra_state *ra, struct file *filp,
 514                            struct page *page, pgoff_t offset,
 515                            unsigned long req_size)
 516 {
 517         /* no read-ahead */
 518         if (!ra->ra_pages)
 519                 return;
 520
 521         /*
 522          * Same bit is used for PG_readahead and PG_reclaim.
 523          */
 524         if (PageWriteback(page))
 525                 return;
 526
 527         ClearPageReadahead(page);
 528
 529         /*
 530          * Defer asynchronous read-ahead on IO congestion.
 531          */
 532         if (inode_read_congested(mapping->host))
 533                 return;
 534
 535         /* do read-ahead */
 536         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 537 }
 538 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 539
 540 static ssize_t
 541 do_readahead(struct address_space *mapping, struct file *filp,
 542              pgoff_t index, unsigned long nr)
 543 {
 544         if (!mapping || !mapping->a_ops)
 545                 return -EINVAL;
 546
 547         return force_page_cache_readahead(mapping, filp, index, nr);
 548 }
 549
 550 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 551 {
 552         ssize_t ret;
 553         struct fd f;
 554
 555         ret = -EBADF;
 556         f = fdget(fd);
 557         if (f.file) {
 558                 if (f.file->f_mode & FMODE_READ) {
 559                         struct address_space *mapping = f.file->f_mapping;
 560                         pgoff_t start = offset >> PAGE_SHIFT;
 561                         pgoff_t end = (offset + count - 1) >> PAGE_SHIFT;
 562                         unsigned long len = end - start + 1;
 563                         ret = do_readahead(mapping, f.file, start, len);
 564                 }
 565                 fdput(f);
 566         }
 567         return ret;
 568 }