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
  23 #include "internal.h"
  24
  25 /*
  26  * Initialise a struct file's readahead state.  Assumes that the caller has
  27  * memset *ra to zero.
  28  */
  29 void
  30 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
  31 {
  32         ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
  33         ra->prev_pos = -1;
  34 }
  35 EXPORT_SYMBOL_GPL(file_ra_state_init);
  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, PAGE_SIZE);
  52                 page->mapping = NULL;
  53                 unlock_page(page);
  54         }
  55         put_page(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 = lru_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 = lru_to_page(pages);
  91                 list_del(&page->lru);
  92                 if (add_to_page_cache_lru(page, mapping, page->index,
  93                                 readahead_gfp_mask(mapping))) {
  94                         read_cache_pages_invalidate_page(mapping, page);
  95                         continue;
  96                 }
  97                 put_page(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_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 int nr_pages, gfp_t gfp)
 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 = lru_to_page(pages);
 129                 list_del(&page->lru);
 130                 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
 131                         mapping->a_ops->readpage(filp, page);
 132                 put_page(page);
 133         }
 134         ret = 0;
 135
 136 out:
 137         blk_finish_plug(&plug);
 138
 139         return ret;
 140 }
 141
 142 /*
 143  * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
 144  * the pages first, then submits them all for I/O. This avoids the very bad
 145  * behaviour which would occur if page allocations are causing VM writeback.
 146  * We really don't want to intermingle reads and writes like that.
 147  *
 148  * Returns the number of pages requested, or the maximum amount of I/O allowed.
 149  */
 150 int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 151                         pgoff_t offset, unsigned long nr_to_read,
 152                         unsigned long lookahead_size)
 153 {
 154         struct inode *inode = mapping->host;
 155         struct page *page;
 156         unsigned long end_index;        /* The last page we want to read */
 157         LIST_HEAD(page_pool);
 158         int page_idx;
 159         int ret = 0;
 160         loff_t isize = i_size_read(inode);
 161         gfp_t gfp_mask = readahead_gfp_mask(mapping);
 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(gfp_mask);
 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, gfp_mask);
 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         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 213         struct file_ra_state *ra = &filp->f_ra;
 214         unsigned long max_pages;
 215
 216         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 217                 return -EINVAL;
 218
 219         /*
 220          * If the request exceeds the readahead window, allow the read to
 221          * be up to the optimal hardware IO size
 222          */
 223         max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
 224         nr_to_read = min(nr_to_read, max_pages);
 225         while (nr_to_read) {
 226                 int err;
 227
 228                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 229
 230                 if (this_chunk > nr_to_read)
 231                         this_chunk = nr_to_read;
 232                 err = __do_page_cache_readahead(mapping, filp,
 233                                                 offset, this_chunk, 0);
 234                 if (err < 0)
 235                         return err;
 236
 237                 offset += this_chunk;
 238                 nr_to_read -= this_chunk;
 239         }
 240         return 0;
 241 }
 242
 243 /*
 244  * Set the initial window size, round to next power of 2 and square
 245  * for small size, x 4 for medium, and x 2 for large
 246  * for 128k (32 page) max ra
 247  * 1-8 page = 32k initial, > 8 page = 128k initial
 248  */
 249 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 250 {
 251         unsigned long newsize = roundup_pow_of_two(size);
 252
 253         if (newsize <= max / 32)
 254                 newsize = newsize * 4;
 255         else if (newsize <= max / 4)
 256                 newsize = newsize * 2;
 257         else
 258                 newsize = max;
 259
 260         return newsize;
 261 }
 262
 263 /*
 264  *  Get the previous window size, ramp it up, and
 265  *  return it as the new window size.
 266  */
 267 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 268                                                 unsigned long max)
 269 {
 270         unsigned long cur = ra->size;
 271         unsigned long newsize;
 272
 273         if (cur < max / 16)
 274                 newsize = 4 * cur;
 275         else
 276                 newsize = 2 * cur;
 277
 278         return min(newsize, max);
 279 }
 280
 281 /*
 282  * On-demand readahead design.
 283  *
 284  * The fields in struct file_ra_state represent the most-recently-executed
 285  * readahead attempt:
 286  *
 287  *                        |<----- async_size ---------|
 288  *     |------------------- size -------------------->|
 289  *     |==================#===========================|
 290  *     ^start             ^page marked with PG_readahead
 291  *
 292  * To overlap application thinking time and disk I/O time, we do
 293  * `readahead pipelining': Do not wait until the application consumed all
 294  * readahead pages and stalled on the missing page at readahead_index;
 295  * Instead, submit an asynchronous readahead I/O as soon as there are
 296  * only async_size pages left in the readahead window. Normally async_size
 297  * will be equal to size, for maximum pipelining.
 298  *
 299  * In interleaved sequential reads, concurrent streams on the same fd can
 300  * be invalidating each other's readahead state. So we flag the new readahead
 301  * page at (start+size-async_size) with PG_readahead, and use it as readahead
 302  * indicator. The flag won't be set on already cached pages, to avoid the
 303  * readahead-for-nothing fuss, saving pointless page cache lookups.
 304  *
 305  * prev_pos tracks the last visited byte in the _previous_ read request.
 306  * It should be maintained by the caller, and will be used for detecting
 307  * small random reads. Note that the readahead algorithm checks loosely
 308  * for sequential patterns. Hence interleaved reads might be served as
 309  * sequential ones.
 310  *
 311  * There is a special-case: if the first page which the application tries to
 312  * read happens to be the first page of the file, it is assumed that a linear
 313  * read is about to happen and the window is immediately set to the initial size
 314  * based on I/O request size and the max_readahead.
 315  *
 316  * The code ramps up the readahead size aggressively at first, but slow down as
 317  * it approaches max_readhead.
 318  */
 319
 320 /*
 321  * Count contiguously cached pages from @offset-1 to @offset-@max,
 322  * this count is a conservative estimation of
 323  *      - length of the sequential read sequence, or
 324  *      - thrashing threshold in memory tight systems
 325  */
 326 static pgoff_t count_history_pages(struct address_space *mapping,
 327                                    pgoff_t offset, unsigned long max)
 328 {
 329         pgoff_t head;
 330
 331         rcu_read_lock();
 332         head = page_cache_prev_hole(mapping, offset - 1, max);
 333         rcu_read_unlock();
 334
 335         return offset - 1 - head;
 336 }
 337
 338 /*
 339  * page cache context based read-ahead
 340  */
 341 static int try_context_readahead(struct address_space *mapping,
 342                                  struct file_ra_state *ra,
 343                                  pgoff_t offset,
 344                                  unsigned long req_size,
 345                                  unsigned long max)
 346 {
 347         pgoff_t size;
 348
 349         size = count_history_pages(mapping, offset, max);
 350
 351         /*
 352          * not enough history pages:
 353          * it could be a random read
 354          */
 355         if (size <= req_size)
 356                 return 0;
 357
 358         /*
 359          * starts from beginning of file:
 360          * it is a strong indication of long-run stream (or whole-file-read)
 361          */
 362         if (size >= offset)
 363                 size *= 2;
 364
 365         ra->start = offset;
 366         ra->size = min(size + req_size, max);
 367         ra->async_size = 1;
 368
 369         return 1;
 370 }
 371
 372 /*
 373  * A minimal readahead algorithm for trivial sequential/random reads.
 374  */
 375 static unsigned long
 376 ondemand_readahead(struct address_space *mapping,
 377                    struct file_ra_state *ra, struct file *filp,
 378                    bool hit_readahead_marker, pgoff_t offset,
 379                    unsigned long req_size)
 380 {
 381         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 382         unsigned long max_pages = ra->ra_pages;
 383         unsigned long add_pages;
 384         pgoff_t prev_offset;
 385
 386         /*
 387          * If the request exceeds the readahead window, allow the read to
 388          * be up to the optimal hardware IO size
 389          */
 390         if (req_size > max_pages && bdi->io_pages > max_pages)
 391                 max_pages = min(req_size, bdi->io_pages);
 392
 393         /*
 394          * start of file
 395          */
 396         if (!offset)
 397                 goto initial_readahead;
 398
 399         /*
 400          * It's the expected callback offset, assume sequential access.
 401          * Ramp up sizes, and push forward the readahead window.
 402          */
 403         if ((offset == (ra->start + ra->size - ra->async_size) ||
 404              offset == (ra->start + ra->size))) {
 405                 ra->start += ra->size;
 406                 ra->size = get_next_ra_size(ra, max_pages);
 407                 ra->async_size = ra->size;
 408                 goto readit;
 409         }
 410
 411         /*
 412          * Hit a marked page without valid readahead state.
 413          * E.g. interleaved reads.
 414          * Query the pagecache for async_size, which normally equals to
 415          * readahead size. Ramp it up and use it as the new readahead size.
 416          */
 417         if (hit_readahead_marker) {
 418                 pgoff_t start;
 419
 420                 rcu_read_lock();
 421                 start = page_cache_next_hole(mapping, offset + 1, max_pages);
 422                 rcu_read_unlock();
 423
 424                 if (!start || start - offset > max_pages)
 425                         return 0;
 426
 427                 ra->start = start;
 428                 ra->size = start - offset;      /* old async_size */
 429                 ra->size += req_size;
 430                 ra->size = get_next_ra_size(ra, max_pages);
 431                 ra->async_size = ra->size;
 432                 goto readit;
 433         }
 434
 435         /*
 436          * oversize read
 437          */
 438         if (req_size > max_pages)
 439                 goto initial_readahead;
 440
 441         /*
 442          * sequential cache miss
 443          * trivial case: (offset - prev_offset) == 1
 444          * unaligned reads: (offset - prev_offset) == 0
 445          */
 446         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
 447         if (offset - prev_offset <= 1UL)
 448                 goto initial_readahead;
 449
 450         /*
 451          * Query the page cache and look for the traces(cached history pages)
 452          * that a sequential stream would leave behind.
 453          */
 454         if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
 455                 goto readit;
 456
 457         /*
 458          * standalone, small random read
 459          * Read as is, and do not pollute the readahead state.
 460          */
 461         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
 462
 463 initial_readahead:
 464         ra->start = offset;
 465         ra->size = get_init_ra_size(req_size, max_pages);
 466         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 467
 468 readit:
 469         /*
 470          * Will this read hit the readahead marker made by itself?
 471          * If so, trigger the readahead marker hit now, and merge
 472          * the resulted next readahead window into the current one.
 473          * Take care of maximum IO pages as above.
 474          */
 475         if (offset == ra->start && ra->size == ra->async_size) {
 476                 add_pages = get_next_ra_size(ra, max_pages);
 477                 if (ra->size + add_pages <= max_pages) {
 478                         ra->async_size = add_pages;
 479                         ra->size += add_pages;
 480                 } else {
 481                         ra->size = max_pages;
 482                         ra->async_size = max_pages >> 1;
 483                 }
 484         }
 485
 486         return ra_submit(ra, mapping, filp);
 487 }
 488
 489 /**
 490  * page_cache_sync_readahead - generic file readahead
 491  * @mapping: address_space which holds the pagecache and I/O vectors
 492  * @ra: file_ra_state which holds the readahead state
 493  * @filp: passed on to ->readpage() and ->readpages()
 494  * @offset: start offset into @mapping, in pagecache page-sized units
 495  * @req_size: hint: total size of the read which the caller is performing in
 496  *            pagecache pages
 497  *
 498  * page_cache_sync_readahead() should be called when a cache miss happened:
 499  * it will submit the read.  The readahead logic may decide to piggyback more
 500  * pages onto the read request if access patterns suggest it will improve
 501  * performance.
 502  */
 503 void page_cache_sync_readahead(struct address_space *mapping,
 504                                struct file_ra_state *ra, struct file *filp,
 505                                pgoff_t offset, unsigned long req_size)
 506 {
 507         /* no read-ahead */
 508         if (!ra->ra_pages)
 509                 return;
 510
 511         /* be dumb */
 512         if (filp && (filp->f_mode & FMODE_RANDOM)) {
 513                 force_page_cache_readahead(mapping, filp, offset, req_size);
 514                 return;
 515         }
 516
 517         /* do read-ahead */
 518         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
 519 }
 520 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 521
 522 /**
 523  * page_cache_async_readahead - file readahead for marked pages
 524  * @mapping: address_space which holds the pagecache and I/O vectors
 525  * @ra: file_ra_state which holds the readahead state
 526  * @filp: passed on to ->readpage() and ->readpages()
 527  * @page: the page at @offset which has the PG_readahead flag set
 528  * @offset: start offset into @mapping, in pagecache page-sized units
 529  * @req_size: hint: total size of the read which the caller is performing in
 530  *            pagecache pages
 531  *
 532  * page_cache_async_readahead() should be called when a page is used which
 533  * has the PG_readahead flag; this is a marker to suggest that the application
 534  * has used up enough of the readahead window that we should start pulling in
 535  * more pages.
 536  */
 537 void
 538 page_cache_async_readahead(struct address_space *mapping,
 539                            struct file_ra_state *ra, struct file *filp,
 540                            struct page *page, pgoff_t offset,
 541                            unsigned long req_size)
 542 {
 543         /* no read-ahead */
 544         if (!ra->ra_pages)
 545                 return;
 546
 547         /*
 548          * Same bit is used for PG_readahead and PG_reclaim.
 549          */
 550         if (PageWriteback(page))
 551                 return;
 552
 553         ClearPageReadahead(page);
 554
 555         /*
 556          * Defer asynchronous read-ahead on IO congestion.
 557          */
 558         if (inode_read_congested(mapping->host))
 559                 return;
 560
 561         /* do read-ahead */
 562         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 563 }
 564 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 565
 566 static ssize_t
 567 do_readahead(struct address_space *mapping, struct file *filp,
 568              pgoff_t index, unsigned long nr)
 569 {
 570         if (!mapping || !mapping->a_ops)
 571                 return -EINVAL;
 572
 573         /*
 574          * Readahead doesn't make sense for DAX inodes, but we don't want it
 575          * to report a failure either.  Instead, we just return success and
 576          * don't do any work.
 577          */
 578         if (dax_mapping(mapping))
 579                 return 0;
 580
 581         return force_page_cache_readahead(mapping, filp, index, nr);
 582 }
 583
 584 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 585 {
 586         ssize_t ret;
 587         struct fd f;
 588
 589         ret = -EBADF;
 590         f = fdget(fd);
 591         if (f.file) {
 592                 if (f.file->f_mode & FMODE_READ) {
 593                         struct address_space *mapping = f.file->f_mapping;
 594                         pgoff_t start = offset >> PAGE_SHIFT;
 595                         pgoff_t end = (offset + count - 1) >> PAGE_SHIFT;
 596                         unsigned long len = end - start + 1;
 597                         ret = do_readahead(mapping, f.file, start, len);
 598                 }
 599                 fdput(f);
 600         }
 601         return ret;
 602 }