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/dax.h>
  12 #include <linux/gfp.h>
  13 #include <linux/export.h>
  14 #include <linux/blkdev.h>
  15 #include <linux/backing-dev.h>
  16 #include <linux/task_io_accounting_ops.h>
  17 #include <linux/pagevec.h>
  18 #include <linux/pagemap.h>
  19 #include <linux/syscalls.h>
  20 #include <linux/file.h>
  21 #include <linux/mm_inline.h>
  22 #include <linux/blk-cgroup.h>
  23 #include <linux/fadvise.h>
  24
  25 #include "internal.h"
  26
  27 /*
  28  * Initialise a struct file's readahead state.  Assumes that the caller has
  29  * memset *ra to zero.
  30  */
  31 void
  32 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
  33 {
  34         ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
  35         ra->prev_pos = -1;
  36 }
  37 EXPORT_SYMBOL_GPL(file_ra_state_init);
  38
  39 /*
  40  * see if a page needs releasing upon read_cache_pages() failure
  41  * - the caller of read_cache_pages() may have set PG_private or PG_fscache
  42  *   before calling, such as the NFS fs marking pages that are cached locally
  43  *   on disk, thus we need to give the fs a chance to clean up in the event of
  44  *   an error
  45  */
  46 static void read_cache_pages_invalidate_page(struct address_space *mapping,
  47                                              struct page *page)
  48 {
  49         if (page_has_private(page)) {
  50                 if (!trylock_page(page))
  51                         BUG();
  52                 page->mapping = mapping;
  53                 do_invalidatepage(page, 0, PAGE_SIZE);
  54                 page->mapping = NULL;
  55                 unlock_page(page);
  56         }
  57         put_page(page);
  58 }
  59
  60 /*
  61  * release a list of pages, invalidating them first if need be
  62  */
  63 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
  64                                               struct list_head *pages)
  65 {
  66         struct page *victim;
  67
  68         while (!list_empty(pages)) {
  69                 victim = lru_to_page(pages);
  70                 list_del(&victim->lru);
  71                 read_cache_pages_invalidate_page(mapping, victim);
  72         }
  73 }
  74
  75 /**
  76  * read_cache_pages - populate an address space with some pages & start reads against them
  77  * @mapping: the address_space
  78  * @pages: The address of a list_head which contains the target pages.  These
  79  *   pages have their ->index populated and are otherwise uninitialised.
  80  * @filler: callback routine for filling a single page.
  81  * @data: private data for the callback routine.
  82  *
  83  * Hides the details of the LRU cache etc from the filesystems.
  84  */
  85 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
  86                         int (*filler)(void *, struct page *), void *data)
  87 {
  88         struct page *page;
  89         int ret = 0;
  90
  91         while (!list_empty(pages)) {
  92                 page = lru_to_page(pages);
  93                 list_del(&page->lru);
  94                 if (add_to_page_cache_lru(page, mapping, page->index,
  95                                 readahead_gfp_mask(mapping))) {
  96                         read_cache_pages_invalidate_page(mapping, page);
  97                         continue;
  98                 }
  99                 put_page(page);
 100
 101                 ret = filler(data, page);
 102                 if (unlikely(ret)) {
 103                         read_cache_pages_invalidate_pages(mapping, pages);
 104                         break;
 105                 }
 106                 task_io_account_read(PAGE_SIZE);
 107         }
 108         return ret;
 109 }
 110
 111 EXPORT_SYMBOL(read_cache_pages);
 112
 113 static int read_pages(struct address_space *mapping, struct file *filp,
 114                 struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
 115 {
 116         struct blk_plug plug;
 117         unsigned page_idx;
 118         int ret;
 119
 120         blk_start_plug(&plug);
 121
 122         if (mapping->a_ops->readpages) {
 123                 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
 124                 /* Clean up the remaining pages */
 125                 put_pages_list(pages);
 126                 goto out;
 127         }
 128
 129         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 130                 struct page *page = lru_to_page(pages);
 131                 list_del(&page->lru);
 132                 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
 133                         mapping->a_ops->readpage(filp, page);
 134                 put_page(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
 146  * the pages first, then submits them 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 unsigned int __do_page_cache_readahead(struct address_space *mapping,
 153                 struct file *filp, pgoff_t offset, unsigned long nr_to_read,
 154                 unsigned long lookahead_size)
 155 {
 156         struct inode *inode = mapping->host;
 157         struct page *page;
 158         unsigned long end_index;        /* The last page we want to read */
 159         LIST_HEAD(page_pool);
 160         int page_idx;
 161         unsigned int nr_pages = 0;
 162         loff_t isize = i_size_read(inode);
 163         gfp_t gfp_mask = readahead_gfp_mask(mapping);
 164
 165         if (isize == 0)
 166                 goto out;
 167
 168         end_index = ((isize - 1) >> PAGE_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                 page = xa_load(&mapping->i_pages, page_offset);
 180                 if (page && !xa_is_value(page)) {
 181                         /*
 182                          * Page already present?  Kick off the current batch of
 183                          * contiguous pages before continuing with the next
 184                          * batch.
 185                          */
 186                         if (nr_pages)
 187                                 read_pages(mapping, filp, &page_pool, nr_pages,
 188                                                 gfp_mask);
 189                         nr_pages = 0;
 190                         continue;
 191                 }
 192
 193                 page = __page_cache_alloc(gfp_mask);
 194                 if (!page)
 195                         break;
 196                 page->index = page_offset;
 197                 list_add(&page->lru, &page_pool);
 198                 if (page_idx == nr_to_read - lookahead_size)
 199                         SetPageReadahead(page);
 200                 nr_pages++;
 201         }
 202
 203         /*
 204          * Now start the IO.  We ignore I/O errors - if the page is not
 205          * uptodate then the caller will launch readpage again, and
 206          * will then handle the error.
 207          */
 208         if (nr_pages)
 209                 read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
 210         BUG_ON(!list_empty(&page_pool));
 211 out:
 212         return nr_pages;
 213 }
 214
 215 /*
 216  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 217  * memory at once.
 218  */
 219 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 220                                pgoff_t offset, unsigned long nr_to_read)
 221 {
 222         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 223         struct file_ra_state *ra = &filp->f_ra;
 224         unsigned long max_pages;
 225
 226         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 227                 return -EINVAL;
 228
 229         /*
 230          * If the request exceeds the readahead window, allow the read to
 231          * be up to the optimal hardware IO size
 232          */
 233         max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
 234         nr_to_read = min(nr_to_read, max_pages);
 235         while (nr_to_read) {
 236                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 237
 238                 if (this_chunk > nr_to_read)
 239                         this_chunk = nr_to_read;
 240                 __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
 241
 242                 offset += this_chunk;
 243                 nr_to_read -= this_chunk;
 244         }
 245         return 0;
 246 }
 247
 248 /*
 249  * Set the initial window size, round to next power of 2 and square
 250  * for small size, x 4 for medium, and x 2 for large
 251  * for 128k (32 page) max ra
 252  * 1-8 page = 32k initial, > 8 page = 128k initial
 253  */
 254 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 255 {
 256         unsigned long newsize = roundup_pow_of_two(size);
 257
 258         if (newsize <= max / 32)
 259                 newsize = newsize * 4;
 260         else if (newsize <= max / 4)
 261                 newsize = newsize * 2;
 262         else
 263                 newsize = max;
 264
 265         return newsize;
 266 }
 267
 268 /*
 269  *  Get the previous window size, ramp it up, and
 270  *  return it as the new window size.
 271  */
 272 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 273                                                 unsigned long max)
 274 {
 275         unsigned long cur = ra->size;
 276         unsigned long newsize;
 277
 278         if (cur < max / 16)
 279                 newsize = 4 * cur;
 280         else
 281                 newsize = 2 * cur;
 282
 283         return min(newsize, max);
 284 }
 285
 286 /*
 287  * On-demand readahead design.
 288  *
 289  * The fields in struct file_ra_state represent the most-recently-executed
 290  * readahead attempt:
 291  *
 292  *                        |<----- async_size ---------|
 293  *     |------------------- size -------------------->|
 294  *     |==================#===========================|
 295  *     ^start             ^page marked with PG_readahead
 296  *
 297  * To overlap application thinking time and disk I/O time, we do
 298  * `readahead pipelining': Do not wait until the application consumed all
 299  * readahead pages and stalled on the missing page at readahead_index;
 300  * Instead, submit an asynchronous readahead I/O as soon as there are
 301  * only async_size pages left in the readahead window. Normally async_size
 302  * will be equal to size, for maximum pipelining.
 303  *
 304  * In interleaved sequential reads, concurrent streams on the same fd can
 305  * be invalidating each other's readahead state. So we flag the new readahead
 306  * page at (start+size-async_size) with PG_readahead, and use it as readahead
 307  * indicator. The flag won't be set on already cached pages, to avoid the
 308  * readahead-for-nothing fuss, saving pointless page cache lookups.
 309  *
 310  * prev_pos tracks the last visited byte in the _previous_ read request.
 311  * It should be maintained by the caller, and will be used for detecting
 312  * small random reads. Note that the readahead algorithm checks loosely
 313  * for sequential patterns. Hence interleaved reads might be served as
 314  * sequential ones.
 315  *
 316  * There is a special-case: if the first page which the application tries to
 317  * read happens to be the first page of the file, it is assumed that a linear
 318  * read is about to happen and the window is immediately set to the initial size
 319  * based on I/O request size and the max_readahead.
 320  *
 321  * The code ramps up the readahead size aggressively at first, but slow down as
 322  * it approaches max_readhead.
 323  */
 324
 325 /*
 326  * Count contiguously cached pages from @offset-1 to @offset-@max,
 327  * this count is a conservative estimation of
 328  *      - length of the sequential read sequence, or
 329  *      - thrashing threshold in memory tight systems
 330  */
 331 static pgoff_t count_history_pages(struct address_space *mapping,
 332                                    pgoff_t offset, unsigned long max)
 333 {
 334         pgoff_t head;
 335
 336         rcu_read_lock();
 337         head = page_cache_prev_miss(mapping, offset - 1, max);
 338         rcu_read_unlock();
 339
 340         return offset - 1 - head;
 341 }
 342
 343 /*
 344  * page cache context based read-ahead
 345  */
 346 static int try_context_readahead(struct address_space *mapping,
 347                                  struct file_ra_state *ra,
 348                                  pgoff_t offset,
 349                                  unsigned long req_size,
 350                                  unsigned long max)
 351 {
 352         pgoff_t size;
 353
 354         size = count_history_pages(mapping, offset, max);
 355
 356         /*
 357          * not enough history pages:
 358          * it could be a random read
 359          */
 360         if (size <= req_size)
 361                 return 0;
 362
 363         /*
 364          * starts from beginning of file:
 365          * it is a strong indication of long-run stream (or whole-file-read)
 366          */
 367         if (size >= offset)
 368                 size *= 2;
 369
 370         ra->start = offset;
 371         ra->size = min(size + req_size, max);
 372         ra->async_size = 1;
 373
 374         return 1;
 375 }
 376
 377 /*
 378  * A minimal readahead algorithm for trivial sequential/random reads.
 379  */
 380 static unsigned long
 381 ondemand_readahead(struct address_space *mapping,
 382                    struct file_ra_state *ra, struct file *filp,
 383                    bool hit_readahead_marker, pgoff_t offset,
 384                    unsigned long req_size)
 385 {
 386         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 387         unsigned long max_pages = ra->ra_pages;
 388         unsigned long add_pages;
 389         pgoff_t prev_offset;
 390
 391         /*
 392          * If the request exceeds the readahead window, allow the read to
 393          * be up to the optimal hardware IO size
 394          */
 395         if (req_size > max_pages && bdi->io_pages > max_pages)
 396                 max_pages = min(req_size, bdi->io_pages);
 397
 398         /*
 399          * start of file
 400          */
 401         if (!offset)
 402                 goto initial_readahead;
 403
 404         /*
 405          * It's the expected callback offset, assume sequential access.
 406          * Ramp up sizes, and push forward the readahead window.
 407          */
 408         if ((offset == (ra->start + ra->size - ra->async_size) ||
 409              offset == (ra->start + ra->size))) {
 410                 ra->start += ra->size;
 411                 ra->size = get_next_ra_size(ra, max_pages);
 412                 ra->async_size = ra->size;
 413                 goto readit;
 414         }
 415
 416         /*
 417          * Hit a marked page without valid readahead state.
 418          * E.g. interleaved reads.
 419          * Query the pagecache for async_size, which normally equals to
 420          * readahead size. Ramp it up and use it as the new readahead size.
 421          */
 422         if (hit_readahead_marker) {
 423                 pgoff_t start;
 424
 425                 rcu_read_lock();
 426                 start = page_cache_next_miss(mapping, offset + 1, max_pages);
 427                 rcu_read_unlock();
 428
 429                 if (!start || start - offset > max_pages)
 430                         return 0;
 431
 432                 ra->start = start;
 433                 ra->size = start - offset;      /* old async_size */
 434                 ra->size += req_size;
 435                 ra->size = get_next_ra_size(ra, max_pages);
 436                 ra->async_size = ra->size;
 437                 goto readit;
 438         }
 439
 440         /*
 441          * oversize read
 442          */
 443         if (req_size > max_pages)
 444                 goto initial_readahead;
 445
 446         /*
 447          * sequential cache miss
 448          * trivial case: (offset - prev_offset) == 1
 449          * unaligned reads: (offset - prev_offset) == 0
 450          */
 451         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
 452         if (offset - prev_offset <= 1UL)
 453                 goto initial_readahead;
 454
 455         /*
 456          * Query the page cache and look for the traces(cached history pages)
 457          * that a sequential stream would leave behind.
 458          */
 459         if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
 460                 goto readit;
 461
 462         /*
 463          * standalone, small random read
 464          * Read as is, and do not pollute the readahead state.
 465          */
 466         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
 467
 468 initial_readahead:
 469         ra->start = offset;
 470         ra->size = get_init_ra_size(req_size, max_pages);
 471         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 472
 473 readit:
 474         /*
 475          * Will this read hit the readahead marker made by itself?
 476          * If so, trigger the readahead marker hit now, and merge
 477          * the resulted next readahead window into the current one.
 478          * Take care of maximum IO pages as above.
 479          */
 480         if (offset == ra->start && ra->size == ra->async_size) {
 481                 add_pages = get_next_ra_size(ra, max_pages);
 482                 if (ra->size + add_pages <= max_pages) {
 483                         ra->async_size = add_pages;
 484                         ra->size += add_pages;
 485                 } else {
 486                         ra->size = max_pages;
 487                         ra->async_size = max_pages >> 1;
 488                 }
 489         }
 490
 491         return ra_submit(ra, mapping, filp);
 492 }
 493
 494 /**
 495  * page_cache_sync_readahead - generic file readahead
 496  * @mapping: address_space which holds the pagecache and I/O vectors
 497  * @ra: file_ra_state which holds the readahead state
 498  * @filp: passed on to ->readpage() and ->readpages()
 499  * @offset: start offset into @mapping, in pagecache page-sized units
 500  * @req_size: hint: total size of the read which the caller is performing in
 501  *            pagecache pages
 502  *
 503  * page_cache_sync_readahead() should be called when a cache miss happened:
 504  * it will submit the read.  The readahead logic may decide to piggyback more
 505  * pages onto the read request if access patterns suggest it will improve
 506  * performance.
 507  */
 508 void page_cache_sync_readahead(struct address_space *mapping,
 509                                struct file_ra_state *ra, struct file *filp,
 510                                pgoff_t offset, unsigned long req_size)
 511 {
 512         /* no read-ahead */
 513         if (!ra->ra_pages)
 514                 return;
 515
 516         if (blk_cgroup_congested())
 517                 return;
 518
 519         /* be dumb */
 520         if (filp && (filp->f_mode & FMODE_RANDOM)) {
 521                 force_page_cache_readahead(mapping, filp, offset, req_size);
 522                 return;
 523         }
 524
 525         /* do read-ahead */
 526         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
 527 }
 528 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 529
 530 /**
 531  * page_cache_async_readahead - file readahead for marked pages
 532  * @mapping: address_space which holds the pagecache and I/O vectors
 533  * @ra: file_ra_state which holds the readahead state
 534  * @filp: passed on to ->readpage() and ->readpages()
 535  * @page: the page at @offset which has the PG_readahead flag set
 536  * @offset: start offset into @mapping, in pagecache page-sized units
 537  * @req_size: hint: total size of the read which the caller is performing in
 538  *            pagecache pages
 539  *
 540  * page_cache_async_readahead() should be called when a page is used which
 541  * has the PG_readahead flag; this is a marker to suggest that the application
 542  * has used up enough of the readahead window that we should start pulling in
 543  * more pages.
 544  */
 545 void
 546 page_cache_async_readahead(struct address_space *mapping,
 547                            struct file_ra_state *ra, struct file *filp,
 548                            struct page *page, pgoff_t offset,
 549                            unsigned long req_size)
 550 {
 551         /* no read-ahead */
 552         if (!ra->ra_pages)
 553                 return;
 554
 555         /*
 556          * Same bit is used for PG_readahead and PG_reclaim.
 557          */
 558         if (PageWriteback(page))
 559                 return;
 560
 561         ClearPageReadahead(page);
 562
 563         /*
 564          * Defer asynchronous read-ahead on IO congestion.
 565          */
 566         if (inode_read_congested(mapping->host))
 567                 return;
 568
 569         if (blk_cgroup_congested())
 570                 return;
 571
 572         /* do read-ahead */
 573         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 574 }
 575 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 576
 577 ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
 578 {
 579         ssize_t ret;
 580         struct fd f;
 581
 582         ret = -EBADF;
 583         f = fdget(fd);
 584         if (!f.file || !(f.file->f_mode & FMODE_READ))
 585                 goto out;
 586
 587         /*
 588          * The readahead() syscall is intended to run only on files
 589          * that can execute readahead. If readahead is not possible
 590          * on this file, then we must return -EINVAL.
 591          */
 592         ret = -EINVAL;
 593         if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
 594             !S_ISREG(file_inode(f.file)->i_mode))
 595                 goto out;
 596
 597         ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
 598 out:
 599         fdput(f);
 600         return ret;
 601 }
 602
 603 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 604 {
 605         return ksys_readahead(fd, offset, count);
 606 }