readahead.c revision d30a11004e3411909f2448546f036a011978062e
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/fs.h> 12#include <linux/mm.h> 13#include <linux/module.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 20/* 21 * Initialise a struct file's readahead state. Assumes that the caller has 22 * memset *ra to zero. 23 */ 24void 25file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping) 26{ 27 ra->ra_pages = mapping->backing_dev_info->ra_pages; 28 ra->prev_pos = -1; 29} 30EXPORT_SYMBOL_GPL(file_ra_state_init); 31 32#define list_to_page(head) (list_entry((head)->prev, struct page, lru)) 33 34/* 35 * see if a page needs releasing upon read_cache_pages() failure 36 * - the caller of read_cache_pages() may have set PG_private or PG_fscache 37 * before calling, such as the NFS fs marking pages that are cached locally 38 * on disk, thus we need to give the fs a chance to clean up in the event of 39 * an error 40 */ 41static void read_cache_pages_invalidate_page(struct address_space *mapping, 42 struct page *page) 43{ 44 if (page_has_private(page)) { 45 if (!trylock_page(page)) 46 BUG(); 47 page->mapping = mapping; 48 do_invalidatepage(page, 0); 49 page->mapping = NULL; 50 unlock_page(page); 51 } 52 page_cache_release(page); 53} 54 55/* 56 * release a list of pages, invalidating them first if need be 57 */ 58static void read_cache_pages_invalidate_pages(struct address_space *mapping, 59 struct list_head *pages) 60{ 61 struct page *victim; 62 63 while (!list_empty(pages)) { 64 victim = list_to_page(pages); 65 list_del(&victim->lru); 66 read_cache_pages_invalidate_page(mapping, victim); 67 } 68} 69 70/** 71 * read_cache_pages - populate an address space with some pages & start reads against them 72 * @mapping: the address_space 73 * @pages: The address of a list_head which contains the target pages. These 74 * pages have their ->index populated and are otherwise uninitialised. 75 * @filler: callback routine for filling a single page. 76 * @data: private data for the callback routine. 77 * 78 * Hides the details of the LRU cache etc from the filesystems. 79 */ 80int read_cache_pages(struct address_space *mapping, struct list_head *pages, 81 int (*filler)(void *, struct page *), void *data) 82{ 83 struct page *page; 84 int ret = 0; 85 86 while (!list_empty(pages)) { 87 page = list_to_page(pages); 88 list_del(&page->lru); 89 if (add_to_page_cache_lru(page, mapping, 90 page->index, GFP_KERNEL)) { 91 read_cache_pages_invalidate_page(mapping, page); 92 continue; 93 } 94 page_cache_release(page); 95 96 ret = filler(data, page); 97 if (unlikely(ret)) { 98 read_cache_pages_invalidate_pages(mapping, pages); 99 break; 100 } 101 task_io_account_read(PAGE_CACHE_SIZE); 102 } 103 return ret; 104} 105 106EXPORT_SYMBOL(read_cache_pages); 107 108static int read_pages(struct address_space *mapping, struct file *filp, 109 struct list_head *pages, unsigned nr_pages) 110{ 111 unsigned page_idx; 112 int ret; 113 114 if (mapping->a_ops->readpages) { 115 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages); 116 /* Clean up the remaining pages */ 117 put_pages_list(pages); 118 goto out; 119 } 120 121 for (page_idx = 0; page_idx < nr_pages; page_idx++) { 122 struct page *page = list_to_page(pages); 123 list_del(&page->lru); 124 if (!add_to_page_cache_lru(page, mapping, 125 page->index, GFP_KERNEL)) { 126 mapping->a_ops->readpage(filp, page); 127 } 128 page_cache_release(page); 129 } 130 ret = 0; 131out: 132 return ret; 133} 134 135/* 136 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all 137 * the pages first, then submits them all for I/O. This avoids the very bad 138 * behaviour which would occur if page allocations are causing VM writeback. 139 * We really don't want to intermingle reads and writes like that. 140 * 141 * Returns the number of pages requested, or the maximum amount of I/O allowed. 142 */ 143static int 144__do_page_cache_readahead(struct address_space *mapping, struct file *filp, 145 pgoff_t offset, unsigned long nr_to_read, 146 unsigned long lookahead_size) 147{ 148 struct inode *inode = mapping->host; 149 struct page *page; 150 unsigned long end_index; /* The last page we want to read */ 151 LIST_HEAD(page_pool); 152 int page_idx; 153 int ret = 0; 154 loff_t isize = i_size_read(inode); 155 156 if (isize == 0) 157 goto out; 158 159 end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); 160 161 /* 162 * Preallocate as many pages as we will need. 163 */ 164 for (page_idx = 0; page_idx < nr_to_read; page_idx++) { 165 pgoff_t page_offset = offset + page_idx; 166 167 if (page_offset > end_index) 168 break; 169 170 rcu_read_lock(); 171 page = radix_tree_lookup(&mapping->page_tree, page_offset); 172 rcu_read_unlock(); 173 if (page) 174 continue; 175 176 page = page_cache_alloc_cold(mapping); 177 if (!page) 178 break; 179 page->index = page_offset; 180 list_add(&page->lru, &page_pool); 181 if (page_idx == nr_to_read - lookahead_size) 182 SetPageReadahead(page); 183 ret++; 184 } 185 186 /* 187 * Now start the IO. We ignore I/O errors - if the page is not 188 * uptodate then the caller will launch readpage again, and 189 * will then handle the error. 190 */ 191 if (ret) 192 read_pages(mapping, filp, &page_pool, ret); 193 BUG_ON(!list_empty(&page_pool)); 194out: 195 return ret; 196} 197 198/* 199 * Chunk the readahead into 2 megabyte units, so that we don't pin too much 200 * memory at once. 201 */ 202int force_page_cache_readahead(struct address_space *mapping, struct file *filp, 203 pgoff_t offset, unsigned long nr_to_read) 204{ 205 int ret = 0; 206 207 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) 208 return -EINVAL; 209 210 nr_to_read = max_sane_readahead(nr_to_read); 211 while (nr_to_read) { 212 int err; 213 214 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE; 215 216 if (this_chunk > nr_to_read) 217 this_chunk = nr_to_read; 218 err = __do_page_cache_readahead(mapping, filp, 219 offset, this_chunk, 0); 220 if (err < 0) { 221 ret = err; 222 break; 223 } 224 ret += err; 225 offset += this_chunk; 226 nr_to_read -= this_chunk; 227 } 228 return ret; 229} 230 231/* 232 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a 233 * sensible upper limit. 234 */ 235unsigned long max_sane_readahead(unsigned long nr) 236{ 237 return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE) 238 + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2); 239} 240 241/* 242 * Submit IO for the read-ahead request in file_ra_state. 243 */ 244unsigned long ra_submit(struct file_ra_state *ra, 245 struct address_space *mapping, struct file *filp) 246{ 247 int actual; 248 249 actual = __do_page_cache_readahead(mapping, filp, 250 ra->start, ra->size, ra->async_size); 251 252 return actual; 253} 254 255/* 256 * Set the initial window size, round to next power of 2 and square 257 * for small size, x 4 for medium, and x 2 for large 258 * for 128k (32 page) max ra 259 * 1-8 page = 32k initial, > 8 page = 128k initial 260 */ 261static unsigned long get_init_ra_size(unsigned long size, unsigned long max) 262{ 263 unsigned long newsize = roundup_pow_of_two(size); 264 265 if (newsize <= max / 32) 266 newsize = newsize * 4; 267 else if (newsize <= max / 4) 268 newsize = newsize * 2; 269 else 270 newsize = max; 271 272 return newsize; 273} 274 275/* 276 * Get the previous window size, ramp it up, and 277 * return it as the new window size. 278 */ 279static unsigned long get_next_ra_size(struct file_ra_state *ra, 280 unsigned long max) 281{ 282 unsigned long cur = ra->size; 283 unsigned long newsize; 284 285 if (cur < max / 16) 286 newsize = 4 * cur; 287 else 288 newsize = 2 * cur; 289 290 return min(newsize, max); 291} 292 293/* 294 * On-demand readahead design. 295 * 296 * The fields in struct file_ra_state represent the most-recently-executed 297 * readahead attempt: 298 * 299 * |<----- async_size ---------| 300 * |------------------- size -------------------->| 301 * |==================#===========================| 302 * ^start ^page marked with PG_readahead 303 * 304 * To overlap application thinking time and disk I/O time, we do 305 * `readahead pipelining': Do not wait until the application consumed all 306 * readahead pages and stalled on the missing page at readahead_index; 307 * Instead, submit an asynchronous readahead I/O as soon as there are 308 * only async_size pages left in the readahead window. Normally async_size 309 * will be equal to size, for maximum pipelining. 310 * 311 * In interleaved sequential reads, concurrent streams on the same fd can 312 * be invalidating each other's readahead state. So we flag the new readahead 313 * page at (start+size-async_size) with PG_readahead, and use it as readahead 314 * indicator. The flag won't be set on already cached pages, to avoid the 315 * readahead-for-nothing fuss, saving pointless page cache lookups. 316 * 317 * prev_pos tracks the last visited byte in the _previous_ read request. 318 * It should be maintained by the caller, and will be used for detecting 319 * small random reads. Note that the readahead algorithm checks loosely 320 * for sequential patterns. Hence interleaved reads might be served as 321 * sequential ones. 322 * 323 * There is a special-case: if the first page which the application tries to 324 * read happens to be the first page of the file, it is assumed that a linear 325 * read is about to happen and the window is immediately set to the initial size 326 * based on I/O request size and the max_readahead. 327 * 328 * The code ramps up the readahead size aggressively at first, but slow down as 329 * it approaches max_readhead. 330 */ 331 332/* 333 * A minimal readahead algorithm for trivial sequential/random reads. 334 */ 335static unsigned long 336ondemand_readahead(struct address_space *mapping, 337 struct file_ra_state *ra, struct file *filp, 338 bool hit_readahead_marker, pgoff_t offset, 339 unsigned long req_size) 340{ 341 unsigned long max = max_sane_readahead(ra->ra_pages); 342 pgoff_t prev_offset; 343 int sequential; 344 345 /* 346 * It's the expected callback offset, assume sequential access. 347 * Ramp up sizes, and push forward the readahead window. 348 */ 349 if (offset && (offset == (ra->start + ra->size - ra->async_size) || 350 offset == (ra->start + ra->size))) { 351 ra->start += ra->size; 352 ra->size = get_next_ra_size(ra, max); 353 ra->async_size = ra->size; 354 goto readit; 355 } 356 357 prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT; 358 sequential = offset - prev_offset <= 1UL || req_size > max; 359 360 /* 361 * Standalone, small read. 362 * Read as is, and do not pollute the readahead state. 363 */ 364 if (!hit_readahead_marker && !sequential) { 365 return __do_page_cache_readahead(mapping, filp, 366 offset, req_size, 0); 367 } 368 369 /* 370 * Hit a marked page without valid readahead state. 371 * E.g. interleaved reads. 372 * Query the pagecache for async_size, which normally equals to 373 * readahead size. Ramp it up and use it as the new readahead size. 374 */ 375 if (hit_readahead_marker) { 376 pgoff_t start; 377 378 rcu_read_lock(); 379 start = radix_tree_next_hole(&mapping->page_tree, offset+1,max); 380 rcu_read_unlock(); 381 382 if (!start || start - offset > max) 383 return 0; 384 385 ra->start = start; 386 ra->size = start - offset; /* old async_size */ 387 ra->size += req_size; 388 ra->size = get_next_ra_size(ra, max); 389 ra->async_size = ra->size; 390 goto readit; 391 } 392 393 /* 394 * It may be one of 395 * - first read on start of file 396 * - sequential cache miss 397 * - oversize random read 398 * Start readahead for it. 399 */ 400 ra->start = offset; 401 ra->size = get_init_ra_size(req_size, max); 402 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size; 403 404readit: 405 /* 406 * Will this read hit the readahead marker made by itself? 407 * If so, trigger the readahead marker hit now, and merge 408 * the resulted next readahead window into the current one. 409 */ 410 if (offset == ra->start && ra->size == ra->async_size) { 411 ra->async_size = get_next_ra_size(ra, max); 412 ra->size += ra->async_size; 413 } 414 415 return ra_submit(ra, mapping, filp); 416} 417 418/** 419 * page_cache_sync_readahead - generic file readahead 420 * @mapping: address_space which holds the pagecache and I/O vectors 421 * @ra: file_ra_state which holds the readahead state 422 * @filp: passed on to ->readpage() and ->readpages() 423 * @offset: start offset into @mapping, in pagecache page-sized units 424 * @req_size: hint: total size of the read which the caller is performing in 425 * pagecache pages 426 * 427 * page_cache_sync_readahead() should be called when a cache miss happened: 428 * it will submit the read. The readahead logic may decide to piggyback more 429 * pages onto the read request if access patterns suggest it will improve 430 * performance. 431 */ 432void page_cache_sync_readahead(struct address_space *mapping, 433 struct file_ra_state *ra, struct file *filp, 434 pgoff_t offset, unsigned long req_size) 435{ 436 /* no read-ahead */ 437 if (!ra->ra_pages) 438 return; 439 440 /* do read-ahead */ 441 ondemand_readahead(mapping, ra, filp, false, offset, req_size); 442} 443EXPORT_SYMBOL_GPL(page_cache_sync_readahead); 444 445/** 446 * page_cache_async_readahead - file readahead for marked pages 447 * @mapping: address_space which holds the pagecache and I/O vectors 448 * @ra: file_ra_state which holds the readahead state 449 * @filp: passed on to ->readpage() and ->readpages() 450 * @page: the page at @offset which has the PG_readahead flag set 451 * @offset: start offset into @mapping, in pagecache page-sized units 452 * @req_size: hint: total size of the read which the caller is performing in 453 * pagecache pages 454 * 455 * page_cache_async_ondemand() should be called when a page is used which 456 * has the PG_readahead flag; this is a marker to suggest that the application 457 * has used up enough of the readahead window that we should start pulling in 458 * more pages. 459 */ 460void 461page_cache_async_readahead(struct address_space *mapping, 462 struct file_ra_state *ra, struct file *filp, 463 struct page *page, pgoff_t offset, 464 unsigned long req_size) 465{ 466 /* no read-ahead */ 467 if (!ra->ra_pages) 468 return; 469 470 /* 471 * Same bit is used for PG_readahead and PG_reclaim. 472 */ 473 if (PageWriteback(page)) 474 return; 475 476 ClearPageReadahead(page); 477 478 /* 479 * Defer asynchronous read-ahead on IO congestion. 480 */ 481 if (bdi_read_congested(mapping->backing_dev_info)) 482 return; 483 484 /* do read-ahead */ 485 ondemand_readahead(mapping, ra, filp, true, offset, req_size); 486} 487EXPORT_SYMBOL_GPL(page_cache_async_readahead); 488