readahead.c 14.9 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551
/*
 * mm/readahead.c - address_space-level file readahead.
 *
 * Copyright (C) 2002, Linus Torvalds
 *
 * 09Apr2002	Andrew Morton
 *		Initial version.
 */

#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/pagevec.h>
#include <linux/pagemap.h>

/*
 * Initialise a struct file's readahead state.  Assumes that the caller has
 * memset *ra to zero.
 */
void
file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
{
	ra->ra_pages = mapping->backing_dev_info->ra_pages;
	ra->prev_pos = -1;
}
EXPORT_SYMBOL_GPL(file_ra_state_init);

#define list_to_page(head) (list_entry((head)->prev, struct page, lru))

/*
 * see if a page needs releasing upon read_cache_pages() failure
 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
 *   before calling, such as the NFS fs marking pages that are cached locally
 *   on disk, thus we need to give the fs a chance to clean up in the event of
 *   an error
 */
static void read_cache_pages_invalidate_page(struct address_space *mapping,
					     struct page *page)
{
	if (page_has_private(page)) {
		if (!trylock_page(page))
			BUG();
		page->mapping = mapping;
		do_invalidatepage(page, 0);
		page->mapping = NULL;
		unlock_page(page);
	}
	page_cache_release(page);
}

/*
 * release a list of pages, invalidating them first if need be
 */
static void read_cache_pages_invalidate_pages(struct address_space *mapping,
					      struct list_head *pages)
{
	struct page *victim;

	while (!list_empty(pages)) {
		victim = list_to_page(pages);
		list_del(&victim->lru);
		read_cache_pages_invalidate_page(mapping, victim);
	}
}

/**
 * read_cache_pages - populate an address space with some pages & start reads against them
 * @mapping: the address_space
 * @pages: The address of a list_head which contains the target pages.  These
 *   pages have their ->index populated and are otherwise uninitialised.
 * @filler: callback routine for filling a single page.
 * @data: private data for the callback routine.
 *
 * Hides the details of the LRU cache etc from the filesystems.
 */
int read_cache_pages(struct address_space *mapping, struct list_head *pages,
			int (*filler)(void *, struct page *), void *data)
{
	struct page *page;
	int ret = 0;

	while (!list_empty(pages)) {
		page = list_to_page(pages);
		list_del(&page->lru);
		if (add_to_page_cache_lru(page, mapping,
					page->index, GFP_KERNEL)) {
			read_cache_pages_invalidate_page(mapping, page);
			continue;
		}
		page_cache_release(page);

		ret = filler(data, page);
		if (unlikely(ret)) {
			read_cache_pages_invalidate_pages(mapping, pages);
			break;
		}
		task_io_account_read(PAGE_CACHE_SIZE);
	}
	return ret;
}

EXPORT_SYMBOL(read_cache_pages);

static int read_pages(struct address_space *mapping, struct file *filp,
		struct list_head *pages, unsigned nr_pages)
{
	unsigned page_idx;
	int ret;

	if (mapping->a_ops->readpages) {
		ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
		/* Clean up the remaining pages */
		put_pages_list(pages);
		goto out;
	}

	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
		struct page *page = list_to_page(pages);
		list_del(&page->lru);
		if (!add_to_page_cache_lru(page, mapping,
					page->index, GFP_KERNEL)) {
			mapping->a_ops->readpage(filp, page);
		}
		page_cache_release(page);
	}
	ret = 0;
out:
	return ret;
}

/*
 * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
 * the pages first, then submits them all for I/O. This avoids the very bad
 * behaviour which would occur if page allocations are causing VM writeback.
 * We really don't want to intermingle reads and writes like that.
 *
 * Returns the number of pages requested, or the maximum amount of I/O allowed.
 */
static int
__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
			pgoff_t offset, unsigned long nr_to_read,
			unsigned long lookahead_size)
{
	struct inode *inode = mapping->host;
	struct page *page;
	unsigned long end_index;	/* The last page we want to read */
	LIST_HEAD(page_pool);
	int page_idx;
	int ret = 0;
	loff_t isize = i_size_read(inode);

	if (isize == 0)
		goto out;

	end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);

	/*
	 * Preallocate as many pages as we will need.
	 */
	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
		pgoff_t page_offset = offset + page_idx;

		if (page_offset > end_index)
			break;

		rcu_read_lock();
		page = radix_tree_lookup(&mapping->page_tree, page_offset);
		rcu_read_unlock();
		if (page)
			continue;

		page = page_cache_alloc_cold(mapping);
		if (!page)
			break;
		page->index = page_offset;
		list_add(&page->lru, &page_pool);
		if (page_idx == nr_to_read - lookahead_size)
			SetPageReadahead(page);
		ret++;
	}

	/*
	 * Now start the IO.  We ignore I/O errors - if the page is not
	 * uptodate then the caller will launch readpage again, and
	 * will then handle the error.
	 */
	if (ret)
		read_pages(mapping, filp, &page_pool, ret);
	BUG_ON(!list_empty(&page_pool));
out:
	return ret;
}

/*
 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
 * memory at once.
 */
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
		pgoff_t offset, unsigned long nr_to_read)
{
	int ret = 0;

	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
		return -EINVAL;

	nr_to_read = max_sane_readahead(nr_to_read);
	while (nr_to_read) {
		int err;

		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;

		if (this_chunk > nr_to_read)
			this_chunk = nr_to_read;
		err = __do_page_cache_readahead(mapping, filp,
						offset, this_chunk, 0);
		if (err < 0) {
			ret = err;
			break;
		}
		ret += err;
		offset += this_chunk;
		nr_to_read -= this_chunk;
	}
	return ret;
}

/*
 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
 * sensible upper limit.
 */
unsigned long max_sane_readahead(unsigned long nr)
{
	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
}

/*
 * Submit IO for the read-ahead request in file_ra_state.
 */
unsigned long ra_submit(struct file_ra_state *ra,
		       struct address_space *mapping, struct file *filp)
{
	int actual;

	actual = __do_page_cache_readahead(mapping, filp,
					ra->start, ra->size, ra->async_size);

	return actual;
}

/*
 * Set the initial window size, round to next power of 2 and square
 * for small size, x 4 for medium, and x 2 for large
 * for 128k (32 page) max ra
 * 1-8 page = 32k initial, > 8 page = 128k initial
 */
static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
{
	unsigned long newsize = roundup_pow_of_two(size);

	if (newsize <= max / 32)
		newsize = newsize * 4;
	else if (newsize <= max / 4)
		newsize = newsize * 2;
	else
		newsize = max;

	return newsize;
}

/*
 *  Get the previous window size, ramp it up, and
 *  return it as the new window size.
 */
static unsigned long get_next_ra_size(struct file_ra_state *ra,
						unsigned long max)
{
	unsigned long cur = ra->size;
	unsigned long newsize;

	if (cur < max / 16)
		newsize = 4 * cur;
	else
		newsize = 2 * cur;

	return min(newsize, max);
}

/*
 * On-demand readahead design.
 *
 * The fields in struct file_ra_state represent the most-recently-executed
 * readahead attempt:
 *
 *                        |<----- async_size ---------|
 *     |------------------- size -------------------->|
 *     |==================#===========================|
 *     ^start             ^page marked with PG_readahead
 *
 * To overlap application thinking time and disk I/O time, we do
 * `readahead pipelining': Do not wait until the application consumed all
 * readahead pages and stalled on the missing page at readahead_index;
 * Instead, submit an asynchronous readahead I/O as soon as there are
 * only async_size pages left in the readahead window. Normally async_size
 * will be equal to size, for maximum pipelining.
 *
 * In interleaved sequential reads, concurrent streams on the same fd can
 * be invalidating each other's readahead state. So we flag the new readahead
 * page at (start+size-async_size) with PG_readahead, and use it as readahead
 * indicator. The flag won't be set on already cached pages, to avoid the
 * readahead-for-nothing fuss, saving pointless page cache lookups.
 *
 * prev_pos tracks the last visited byte in the _previous_ read request.
 * It should be maintained by the caller, and will be used for detecting
 * small random reads. Note that the readahead algorithm checks loosely
 * for sequential patterns. Hence interleaved reads might be served as
 * sequential ones.
 *
 * There is a special-case: if the first page which the application tries to
 * read happens to be the first page of the file, it is assumed that a linear
 * read is about to happen and the window is immediately set to the initial size
 * based on I/O request size and the max_readahead.
 *
 * The code ramps up the readahead size aggressively at first, but slow down as
 * it approaches max_readhead.
 */

/*
 * Count contiguously cached pages from @offset-1 to @offset-@max,
 * this count is a conservative estimation of
 * 	- length of the sequential read sequence, or
 * 	- thrashing threshold in memory tight systems
 */
static pgoff_t count_history_pages(struct address_space *mapping,
				   struct file_ra_state *ra,
				   pgoff_t offset, unsigned long max)
{
	pgoff_t head;

	rcu_read_lock();
	head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
	rcu_read_unlock();

	return offset - 1 - head;
}

/*
 * page cache context based read-ahead
 */
static int try_context_readahead(struct address_space *mapping,
				 struct file_ra_state *ra,
				 pgoff_t offset,
				 unsigned long req_size,
				 unsigned long max)
{
	pgoff_t size;

	size = count_history_pages(mapping, ra, offset, max);

	/*
	 * no history pages:
	 * it could be a random read
	 */
	if (!size)
		return 0;

	/*
	 * starts from beginning of file:
	 * it is a strong indication of long-run stream (or whole-file-read)
	 */
	if (size >= offset)
		size *= 2;

	ra->start = offset;
	ra->size = get_init_ra_size(size + req_size, max);
	ra->async_size = ra->size;

	return 1;
}

/*
 * A minimal readahead algorithm for trivial sequential/random reads.
 */
static unsigned long
ondemand_readahead(struct address_space *mapping,
		   struct file_ra_state *ra, struct file *filp,
		   bool hit_readahead_marker, pgoff_t offset,
		   unsigned long req_size)
{
	unsigned long max = max_sane_readahead(ra->ra_pages);

	/*
	 * start of file
	 */
	if (!offset)
		goto initial_readahead;

	/*
	 * It's the expected callback offset, assume sequential access.
	 * Ramp up sizes, and push forward the readahead window.
	 */
	if ((offset == (ra->start + ra->size - ra->async_size) ||
	     offset == (ra->start + ra->size))) {
		ra->start += ra->size;
		ra->size = get_next_ra_size(ra, max);
		ra->async_size = ra->size;
		goto readit;
	}

	/*
	 * Hit a marked page without valid readahead state.
	 * E.g. interleaved reads.
	 * Query the pagecache for async_size, which normally equals to
	 * readahead size. Ramp it up and use it as the new readahead size.
	 */
	if (hit_readahead_marker) {
		pgoff_t start;

		rcu_read_lock();
		start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
		rcu_read_unlock();

		if (!start || start - offset > max)
			return 0;

		ra->start = start;
		ra->size = start - offset;	/* old async_size */
		ra->size += req_size;
		ra->size = get_next_ra_size(ra, max);
		ra->async_size = ra->size;
		goto readit;
	}

	/*
	 * oversize read
	 */
	if (req_size > max)
		goto initial_readahead;

	/*
	 * sequential cache miss
	 */
	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
		goto initial_readahead;

	/*
	 * Query the page cache and look for the traces(cached history pages)
	 * that a sequential stream would leave behind.
	 */
	if (try_context_readahead(mapping, ra, offset, req_size, max))
		goto readit;

	/*
	 * standalone, small random read
	 * Read as is, and do not pollute the readahead state.
	 */
	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);

initial_readahead:
	ra->start = offset;
	ra->size = get_init_ra_size(req_size, max);
	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;

readit:
	/*
	 * Will this read hit the readahead marker made by itself?
	 * If so, trigger the readahead marker hit now, and merge
	 * the resulted next readahead window into the current one.
	 */
	if (offset == ra->start && ra->size == ra->async_size) {
		ra->async_size = get_next_ra_size(ra, max);
		ra->size += ra->async_size;
	}

	return ra_submit(ra, mapping, filp);
}

/**
 * page_cache_sync_readahead - generic file readahead
 * @mapping: address_space which holds the pagecache and I/O vectors
 * @ra: file_ra_state which holds the readahead state
 * @filp: passed on to ->readpage() and ->readpages()
 * @offset: start offset into @mapping, in pagecache page-sized units
 * @req_size: hint: total size of the read which the caller is performing in
 *            pagecache pages
 *
 * page_cache_sync_readahead() should be called when a cache miss happened:
 * it will submit the read.  The readahead logic may decide to piggyback more
 * pages onto the read request if access patterns suggest it will improve
 * performance.
 */
void page_cache_sync_readahead(struct address_space *mapping,
			       struct file_ra_state *ra, struct file *filp,
			       pgoff_t offset, unsigned long req_size)
{
	/* no read-ahead */
	if (!ra->ra_pages)
		return;

	/* do read-ahead */
	ondemand_readahead(mapping, ra, filp, false, offset, req_size);
}
EXPORT_SYMBOL_GPL(page_cache_sync_readahead);

/**
 * page_cache_async_readahead - file readahead for marked pages
 * @mapping: address_space which holds the pagecache and I/O vectors
 * @ra: file_ra_state which holds the readahead state
 * @filp: passed on to ->readpage() and ->readpages()
 * @page: the page at @offset which has the PG_readahead flag set
 * @offset: start offset into @mapping, in pagecache page-sized units
 * @req_size: hint: total size of the read which the caller is performing in
 *            pagecache pages
 *
 * page_cache_async_ondemand() should be called when a page is used which
 * has the PG_readahead flag; this is a marker to suggest that the application
 * has used up enough of the readahead window that we should start pulling in
 * more pages.
 */
void
page_cache_async_readahead(struct address_space *mapping,
			   struct file_ra_state *ra, struct file *filp,
			   struct page *page, pgoff_t offset,
			   unsigned long req_size)
{
	/* no read-ahead */
	if (!ra->ra_pages)
		return;

	/*
	 * Same bit is used for PG_readahead and PG_reclaim.
	 */
	if (PageWriteback(page))
		return;

	ClearPageReadahead(page);

	/*
	 * Defer asynchronous read-ahead on IO congestion.
	 */
	if (bdi_read_congested(mapping->backing_dev_info))
		return;

	/* do read-ahead */
	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
}
EXPORT_SYMBOL_GPL(page_cache_async_readahead);