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