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
  24 #include "internal.h"
  25
  26 /*
  27  * Initialise a struct file's readahead state.  Assumes that the caller has
  28  * memset *ra to zero.
  29  */
  30 void
  31 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
  32 {
  33         ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
  34         ra->prev_pos = -1;
  35 }
  36 EXPORT_SYMBOL_GPL(file_ra_state_init);
  37
  38 /*
  39  * see if a page needs releasing upon read_cache_pages() failure
  40  * - the caller of read_cache_pages() may have set PG_private or PG_fscache
  41  *   before calling, such as the NFS fs marking pages that are cached locally
  42  *   on disk, thus we need to give the fs a chance to clean up in the event of
  43  *   an error
  44  */
  45 static void read_cache_pages_invalidate_page(struct address_space *mapping,
  46                                              struct page *page)
  47 {
  48         if (page_has_private(page)) {
  49                 if (!trylock_page(page))
  50                         BUG();
  51                 page->mapping = mapping;
  52                 do_invalidatepage(page, 0, PAGE_SIZE);
  53                 page->mapping = NULL;
  54                 unlock_page(page);
  55         }
  56         put_page(page);
  57 }
  58
  59 /*
  60  * release a list of pages, invalidating them first if need be
  61  */
  62 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
  63                                               struct list_head *pages)
  64 {
  65         struct page *victim;
  66
  67         while (!list_empty(pages)) {
  68                 victim = lru_to_page(pages);
  69                 list_del(&victim->lru);
  70                 read_cache_pages_invalidate_page(mapping, victim);
  71         }
  72 }
  73
  74 /**
  75  * read_cache_pages - populate an address space with some pages & start reads against them
  76  * @mapping: the address_space
  77  * @pages: The address of a list_head which contains the target pages.  These
  78  *   pages have their ->index populated and are otherwise uninitialised.
  79  * @filler: callback routine for filling a single page.
  80  * @data: private data for the callback routine.
  81  *
  82  * Hides the details of the LRU cache etc from the filesystems.
  83  */
  84 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
  85                         int (*filler)(void *, struct page *), void *data)
  86 {
  87         struct page *page;
  88         int ret = 0;
  89
  90         while (!list_empty(pages)) {
  91                 page = lru_to_page(pages);
  92                 list_del(&page->lru);
  93                 if (add_to_page_cache_lru(page, mapping, page->index,
  94                                 readahead_gfp_mask(mapping))) {
  95                         read_cache_pages_invalidate_page(mapping, page);
  96                         continue;
  97                 }
  98                 put_page(page);
  99
 100                 ret = filler(data, page);
 101                 if (unlikely(ret)) {
 102                         read_cache_pages_invalidate_pages(mapping, pages);
 103                         break;
 104                 }
 105                 task_io_account_read(PAGE_SIZE);
 106         }
 107         return ret;
 108 }
 109
 110 EXPORT_SYMBOL(read_cache_pages);
 111
 112 static int read_pages(struct address_space *mapping, struct file *filp,
 113                 struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
 114 {
 115         struct blk_plug plug;
 116         unsigned page_idx;
 117         int ret;
 118
 119         blk_start_plug(&plug);
 120
 121         if (mapping->a_ops->readpages) {
 122                 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
 123                 /* Clean up the remaining pages */
 124                 put_pages_list(pages);
 125                 goto out;
 126         }
 127
 128         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 129                 struct page *page = lru_to_page(pages);
 130                 list_del(&page->lru);
 131                 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
 132                         mapping->a_ops->readpage(filp, page);
 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
 145  * the pages first, then submits them 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 unsigned int __do_page_cache_readahead(struct address_space *mapping,
 152                 struct file *filp, 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         unsigned int nr_pages = 0;
 161         loff_t isize = i_size_read(inode);
 162         gfp_t gfp_mask = readahead_gfp_mask(mapping);
 163
 164         if (isize == 0)
 165                 goto out;
 166
 167         end_index = ((isize - 1) >> PAGE_SHIFT);
 168
 169         /*
 170          * Preallocate as many pages as we will need.
 171          */
 172         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
 173                 pgoff_t page_offset = offset + page_idx;
 174
 175                 if (page_offset > end_index)
 176                         break;
 177
 178                 rcu_read_lock();
 179                 page = radix_tree_lookup(&mapping->i_pages, page_offset);
 180                 rcu_read_unlock();
 181                 if (page && !radix_tree_exceptional_entry(page)) {
 182                         /*
 183                          * Page already present?  Kick off the current batch of
 184                          * contiguous pages before continuing with the next
 185                          * batch.
 186                          */
 187                         if (nr_pages)
 188                                 read_pages(mapping, filp, &page_pool, nr_pages,
 189                                                 gfp_mask);
 190                         nr_pages = 0;
 191                         continue;
 192                 }
 193
 194                 page = __page_cache_alloc(gfp_mask);
 195                 if (!page)
 196                         break;
 197                 page->index = page_offset;
 198                 list_add(&page->lru, &page_pool);
 199                 if (page_idx == nr_to_read - lookahead_size)
 200                         SetPageReadahead(page);
 201                 nr_pages++;
 202         }
 203
 204         /*
 205          * Now start the IO.  We ignore I/O errors - if the page is not
 206          * uptodate then the caller will launch readpage again, and
 207          * will then handle the error.
 208          */
 209         if (nr_pages)
 210                 read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
 211         BUG_ON(!list_empty(&page_pool));
 212 out:
 213         return nr_pages;
 214 }
 215
 216 /*
 217  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 218  * memory at once.
 219  */
 220 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 221                                pgoff_t offset, unsigned long nr_to_read)
 222 {
 223         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 224         struct file_ra_state *ra = &filp->f_ra;
 225         unsigned long max_pages;
 226
 227         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 228                 return -EINVAL;
 229
 230         /*
 231          * If the request exceeds the readahead window, allow the read to
 232          * be up to the optimal hardware IO size
 233          */
 234         max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
 235         nr_to_read = min(nr_to_read, max_pages);
 236         while (nr_to_read) {
 237                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 238
 239                 if (this_chunk > nr_to_read)
 240                         this_chunk = nr_to_read;
 241                 __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
 242
 243                 offset += this_chunk;
 244                 nr_to_read -= this_chunk;
 245         }
 246         return 0;
 247 }
 248
 249 /*
 250  * Set the initial window size, round to next power of 2 and square
 251  * for small size, x 4 for medium, and x 2 for large
 252  * for 128k (32 page) max ra
 253  * 1-8 page = 32k initial, > 8 page = 128k initial
 254  */
 255 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 256 {
 257         unsigned long newsize = roundup_pow_of_two(size);
 258
 259         if (newsize <= max / 32)
 260                 newsize = newsize * 4;
 261         else if (newsize <= max / 4)
 262                 newsize = newsize * 2;
 263         else
 264                 newsize = max;
 265
 266         return newsize;
 267 }
 268
 269 /*
 270  *  Get the previous window size, ramp it up, and
 271  *  return it as the new window size.
 272  */
 273 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 274                                                 unsigned long max)
 275 {
 276         unsigned long cur = ra->size;
 277         unsigned long newsize;
 278
 279         if (cur < max / 16)
 280                 newsize = 4 * cur;
 281         else
 282                 newsize = 2 * cur;
 283
 284         return min(newsize, max);
 285 }
 286
 287 /*
 288  * On-demand readahead design.
 289  *
 290  * The fields in struct file_ra_state represent the most-recently-executed
 291  * readahead attempt:
 292  *
 293  *                        |<----- async_size ---------|
 294  *     |------------------- size -------------------->|
 295  *     |==================#===========================|
 296  *     ^start             ^page marked with PG_readahead
 297  *
 298  * To overlap application thinking time and disk I/O time, we do
 299  * `readahead pipelining': Do not wait until the application consumed all
 300  * readahead pages and stalled on the missing page at readahead_index;
 301  * Instead, submit an asynchronous readahead I/O as soon as there are
 302  * only async_size pages left in the readahead window. Normally async_size
 303  * will be equal to size, for maximum pipelining.
 304  *
 305  * In interleaved sequential reads, concurrent streams on the same fd can
 306  * be invalidating each other's readahead state. So we flag the new readahead
 307  * page at (start+size-async_size) with PG_readahead, and use it as readahead
 308  * indicator. The flag won't be set on already cached pages, to avoid the
 309  * readahead-for-nothing fuss, saving pointless page cache lookups.
 310  *
 311  * prev_pos tracks the last visited byte in the _previous_ read request.
 312  * It should be maintained by the caller, and will be used for detecting
 313  * small random reads. Note that the readahead algorithm checks loosely
 314  * for sequential patterns. Hence interleaved reads might be served as
 315  * sequential ones.
 316  *
 317  * There is a special-case: if the first page which the application tries to
 318  * read happens to be the first page of the file, it is assumed that a linear
 319  * read is about to happen and the window is immediately set to the initial size
 320  * based on I/O request size and the max_readahead.
 321  *
 322  * The code ramps up the readahead size aggressively at first, but slow down as
 323  * it approaches max_readhead.
 324  */
 325
 326 /*
 327  * Count contiguously cached pages from @offset-1 to @offset-@max,
 328  * this count is a conservative estimation of
 329  *      - length of the sequential read sequence, or
 330  *      - thrashing threshold in memory tight systems
 331  */
 332 static pgoff_t count_history_pages(struct address_space *mapping,
 333                                    pgoff_t offset, unsigned long max)
 334 {
 335         pgoff_t head;
 336
 337         rcu_read_lock();
 338         head = page_cache_prev_hole(mapping, offset - 1, max);
 339         rcu_read_unlock();
 340
 341         return offset - 1 - head;
 342 }
 343
 344 /*
 345  * page cache context based read-ahead
 346  */
 347 static int try_context_readahead(struct address_space *mapping,
 348                                  struct file_ra_state *ra,
 349                                  pgoff_t offset,
 350                                  unsigned long req_size,
 351                                  unsigned long max)
 352 {
 353         pgoff_t size;
 354
 355         size = count_history_pages(mapping, offset, max);
 356
 357         /*
 358          * not enough history pages:
 359          * it could be a random read
 360          */
 361         if (size <= req_size)
 362                 return 0;
 363
 364         /*
 365          * starts from beginning of file:
 366          * it is a strong indication of long-run stream (or whole-file-read)
 367          */
 368         if (size >= offset)
 369                 size *= 2;
 370
 371         ra->start = offset;
 372         ra->size = min(size + req_size, max);
 373         ra->async_size = 1;
 374
 375         return 1;
 376 }
 377
 378 /*
 379  * A minimal readahead algorithm for trivial sequential/random reads.
 380  */
 381 static unsigned long
 382 ondemand_readahead(struct address_space *mapping,
 383                    struct file_ra_state *ra, struct file *filp,
 384                    bool hit_readahead_marker, pgoff_t offset,
 385                    unsigned long req_size)
 386 {
 387         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 388         unsigned long max_pages = ra->ra_pages;
 389         unsigned long add_pages;
 390         pgoff_t prev_offset;
 391
 392         /*
 393          * If the request exceeds the readahead window, allow the read to
 394          * be up to the optimal hardware IO size
 395          */
 396         if (req_size > max_pages && bdi->io_pages > max_pages)
 397                 max_pages = min(req_size, bdi->io_pages);
 398
 399         /*
 400          * start of file
 401          */
 402         if (!offset)
 403                 goto initial_readahead;
 404
 405         /*
 406          * It's the expected callback offset, assume sequential access.
 407          * Ramp up sizes, and push forward the readahead window.
 408          */
 409         if ((offset == (ra->start + ra->size - ra->async_size) ||
 410              offset == (ra->start + ra->size))) {
 411                 ra->start += ra->size;
 412                 ra->size = get_next_ra_size(ra, max_pages);
 413                 ra->async_size = ra->size;
 414                 goto readit;
 415         }
 416
 417         /*
 418          * Hit a marked page without valid readahead state.
 419          * E.g. interleaved reads.
 420          * Query the pagecache for async_size, which normally equals to
 421          * readahead size. Ramp it up and use it as the new readahead size.
 422          */
 423         if (hit_readahead_marker) {
 424                 pgoff_t start;
 425
 426                 rcu_read_lock();
 427                 start = page_cache_next_hole(mapping, offset + 1, max_pages);
 428                 rcu_read_unlock();
 429
 430                 if (!start || start - offset > max_pages)
 431                         return 0;
 432
 433                 ra->start = start;
 434                 ra->size = start - offset;      /* old async_size */
 435                 ra->size += req_size;
 436                 ra->size = get_next_ra_size(ra, max_pages);
 437                 ra->async_size = ra->size;
 438                 goto readit;
 439         }
 440
 441         /*
 442          * oversize read
 443          */
 444         if (req_size > max_pages)
 445                 goto initial_readahead;
 446
 447         /*
 448          * sequential cache miss
 449          * trivial case: (offset - prev_offset) == 1
 450          * unaligned reads: (offset - prev_offset) == 0
 451          */
 452         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
 453         if (offset - prev_offset <= 1UL)
 454                 goto initial_readahead;
 455
 456         /*
 457          * Query the page cache and look for the traces(cached history pages)
 458          * that a sequential stream would leave behind.
 459          */
 460         if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
 461                 goto readit;
 462
 463         /*
 464          * standalone, small random read
 465          * Read as is, and do not pollute the readahead state.
 466          */
 467         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
 468
 469 initial_readahead:
 470         ra->start = offset;
 471         ra->size = get_init_ra_size(req_size, max_pages);
 472         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 473
 474 readit:
 475         /*
 476          * Will this read hit the readahead marker made by itself?
 477          * If so, trigger the readahead marker hit now, and merge
 478          * the resulted next readahead window into the current one.
 479          * Take care of maximum IO pages as above.
 480          */
 481         if (offset == ra->start && ra->size == ra->async_size) {
 482                 add_pages = get_next_ra_size(ra, max_pages);
 483                 if (ra->size + add_pages <= max_pages) {
 484                         ra->async_size = add_pages;
 485                         ra->size += add_pages;
 486                 } else {
 487                         ra->size = max_pages;
 488                         ra->async_size = max_pages >> 1;
 489                 }
 490         }
 491
 492         return ra_submit(ra, mapping, filp);
 493 }
 494
 495 /**
 496  * page_cache_sync_readahead - generic file readahead
 497  * @mapping: address_space which holds the pagecache and I/O vectors
 498  * @ra: file_ra_state which holds the readahead state
 499  * @filp: passed on to ->readpage() and ->readpages()
 500  * @offset: start offset into @mapping, in pagecache page-sized units
 501  * @req_size: hint: total size of the read which the caller is performing in
 502  *            pagecache pages
 503  *
 504  * page_cache_sync_readahead() should be called when a cache miss happened:
 505  * it will submit the read.  The readahead logic may decide to piggyback more
 506  * pages onto the read request if access patterns suggest it will improve
 507  * performance.
 508  */
 509 void page_cache_sync_readahead(struct address_space *mapping,
 510                                struct file_ra_state *ra, struct file *filp,
 511                                pgoff_t offset, unsigned long req_size)
 512 {
 513         /* no read-ahead */
 514         if (!ra->ra_pages)
 515                 return;
 516
 517         if (blk_cgroup_congested())
 518                 return;
 519
 520         /* be dumb */
 521         if (filp && (filp->f_mode & FMODE_RANDOM)) {
 522                 force_page_cache_readahead(mapping, filp, offset, req_size);
 523                 return;
 524         }
 525
 526         /* do read-ahead */
 527         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
 528 }
 529 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 530
 531 /**
 532  * page_cache_async_readahead - file readahead for marked pages
 533  * @mapping: address_space which holds the pagecache and I/O vectors
 534  * @ra: file_ra_state which holds the readahead state
 535  * @filp: passed on to ->readpage() and ->readpages()
 536  * @page: the page at @offset which has the PG_readahead flag set
 537  * @offset: start offset into @mapping, in pagecache page-sized units
 538  * @req_size: hint: total size of the read which the caller is performing in
 539  *            pagecache pages
 540  *
 541  * page_cache_async_readahead() should be called when a page is used which
 542  * has the PG_readahead flag; this is a marker to suggest that the application
 543  * has used up enough of the readahead window that we should start pulling in
 544  * more pages.
 545  */
 546 void
 547 page_cache_async_readahead(struct address_space *mapping,
 548                            struct file_ra_state *ra, struct file *filp,
 549                            struct page *page, pgoff_t offset,
 550                            unsigned long req_size)
 551 {
 552         /* no read-ahead */
 553         if (!ra->ra_pages)
 554                 return;
 555
 556         /*
 557          * Same bit is used for PG_readahead and PG_reclaim.
 558          */
 559         if (PageWriteback(page))
 560                 return;
 561
 562         ClearPageReadahead(page);
 563
 564         /*
 565          * Defer asynchronous read-ahead on IO congestion.
 566          */
 567         if (inode_read_congested(mapping->host))
 568                 return;
 569
 570         if (blk_cgroup_congested())
 571                 return;
 572
 573         /* do read-ahead */
 574         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 575 }
 576 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 577
 578 static ssize_t
 579 do_readahead(struct address_space *mapping, struct file *filp,
 580              pgoff_t index, unsigned long nr)
 581 {
 582         if (!mapping || !mapping->a_ops)
 583                 return -EINVAL;
 584
 585         /*
 586          * Readahead doesn't make sense for DAX inodes, but we don't want it
 587          * to report a failure either.  Instead, we just return success and
 588          * don't do any work.
 589          */
 590         if (dax_mapping(mapping))
 591                 return 0;
 592
 593         return force_page_cache_readahead(mapping, filp, index, nr);
 594 }
 595
 596 ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
 597 {
 598         ssize_t ret;
 599         struct fd f;
 600
 601         ret = -EBADF;
 602         f = fdget(fd);
 603         if (f.file) {
 604                 if (f.file->f_mode & FMODE_READ) {
 605                         struct address_space *mapping = f.file->f_mapping;
 606                         pgoff_t start = offset >> PAGE_SHIFT;
 607                         pgoff_t end = (offset + count - 1) >> PAGE_SHIFT;
 608                         unsigned long len = end - start + 1;
 609                         ret = do_readahead(mapping, f.file, start, len);
 610                 }
 611                 fdput(f);
 612         }
 613         return ret;
 614 }
 615
 616 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 617 {
 618         return ksys_readahead(fd, offset, count);
 619 }