xref: /linux/mm/swap_state.c (revision 00a6d7b6762c27d441e9ac8faff36384bc0fc180)
1 /*
2  *  linux/mm/swap_state.c
3  *
4  *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
5  *  Swap reorganised 29.12.95, Stephen Tweedie
6  *
7  *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
8  */
9 #include <linux/mm.h>
10 #include <linux/gfp.h>
11 #include <linux/kernel_stat.h>
12 #include <linux/swap.h>
13 #include <linux/swapops.h>
14 #include <linux/init.h>
15 #include <linux/pagemap.h>
16 #include <linux/backing-dev.h>
17 #include <linux/blkdev.h>
18 #include <linux/pagevec.h>
19 #include <linux/migrate.h>
20 #include <linux/page_cgroup.h>
21 
22 #include <asm/pgtable.h>
23 
24 /*
25  * swapper_space is a fiction, retained to simplify the path through
26  * vmscan's shrink_page_list.
27  */
28 static const struct address_space_operations swap_aops = {
29 	.writepage	= swap_writepage,
30 	.set_page_dirty	= swap_set_page_dirty,
31 	.migratepage	= migrate_page,
32 };
33 
34 static struct backing_dev_info swap_backing_dev_info = {
35 	.name		= "swap",
36 	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
37 };
38 
39 struct address_space swapper_spaces[MAX_SWAPFILES] = {
40 	[0 ... MAX_SWAPFILES - 1] = {
41 		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
42 		.a_ops		= &swap_aops,
43 		.backing_dev_info = &swap_backing_dev_info,
44 	}
45 };
46 
47 #define INC_CACHE_INFO(x)	do { swap_cache_info.x++; } while (0)
48 
49 static struct {
50 	unsigned long add_total;
51 	unsigned long del_total;
52 	unsigned long find_success;
53 	unsigned long find_total;
54 } swap_cache_info;
55 
56 unsigned long total_swapcache_pages(void)
57 {
58 	int i;
59 	unsigned long ret = 0;
60 
61 	for (i = 0; i < MAX_SWAPFILES; i++)
62 		ret += swapper_spaces[i].nrpages;
63 	return ret;
64 }
65 
66 static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);
67 
68 void show_swap_cache_info(void)
69 {
70 	printk("%lu pages in swap cache\n", total_swapcache_pages());
71 	printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
72 		swap_cache_info.add_total, swap_cache_info.del_total,
73 		swap_cache_info.find_success, swap_cache_info.find_total);
74 	printk("Free swap  = %ldkB\n",
75 		get_nr_swap_pages() << (PAGE_SHIFT - 10));
76 	printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
77 }
78 
79 /*
80  * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
81  * but sets SwapCache flag and private instead of mapping and index.
82  */
83 int __add_to_swap_cache(struct page *page, swp_entry_t entry)
84 {
85 	int error;
86 	struct address_space *address_space;
87 
88 	VM_BUG_ON_PAGE(!PageLocked(page), page);
89 	VM_BUG_ON_PAGE(PageSwapCache(page), page);
90 	VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
91 
92 	page_cache_get(page);
93 	SetPageSwapCache(page);
94 	set_page_private(page, entry.val);
95 
96 	address_space = swap_address_space(entry);
97 	spin_lock_irq(&address_space->tree_lock);
98 	error = radix_tree_insert(&address_space->page_tree,
99 					entry.val, page);
100 	if (likely(!error)) {
101 		address_space->nrpages++;
102 		__inc_zone_page_state(page, NR_FILE_PAGES);
103 		INC_CACHE_INFO(add_total);
104 	}
105 	spin_unlock_irq(&address_space->tree_lock);
106 
107 	if (unlikely(error)) {
108 		/*
109 		 * Only the context which have set SWAP_HAS_CACHE flag
110 		 * would call add_to_swap_cache().
111 		 * So add_to_swap_cache() doesn't returns -EEXIST.
112 		 */
113 		VM_BUG_ON(error == -EEXIST);
114 		set_page_private(page, 0UL);
115 		ClearPageSwapCache(page);
116 		page_cache_release(page);
117 	}
118 
119 	return error;
120 }
121 
122 
123 int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
124 {
125 	int error;
126 
127 	error = radix_tree_maybe_preload(gfp_mask);
128 	if (!error) {
129 		error = __add_to_swap_cache(page, entry);
130 		radix_tree_preload_end();
131 	}
132 	return error;
133 }
134 
135 /*
136  * This must be called only on pages that have
137  * been verified to be in the swap cache.
138  */
139 void __delete_from_swap_cache(struct page *page)
140 {
141 	swp_entry_t entry;
142 	struct address_space *address_space;
143 
144 	VM_BUG_ON_PAGE(!PageLocked(page), page);
145 	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
146 	VM_BUG_ON_PAGE(PageWriteback(page), page);
147 
148 	entry.val = page_private(page);
149 	address_space = swap_address_space(entry);
150 	radix_tree_delete(&address_space->page_tree, page_private(page));
151 	set_page_private(page, 0);
152 	ClearPageSwapCache(page);
153 	address_space->nrpages--;
154 	__dec_zone_page_state(page, NR_FILE_PAGES);
155 	INC_CACHE_INFO(del_total);
156 }
157 
158 /**
159  * add_to_swap - allocate swap space for a page
160  * @page: page we want to move to swap
161  *
162  * Allocate swap space for the page and add the page to the
163  * swap cache.  Caller needs to hold the page lock.
164  */
165 int add_to_swap(struct page *page, struct list_head *list)
166 {
167 	swp_entry_t entry;
168 	int err;
169 
170 	VM_BUG_ON_PAGE(!PageLocked(page), page);
171 	VM_BUG_ON_PAGE(!PageUptodate(page), page);
172 
173 	entry = get_swap_page();
174 	if (!entry.val)
175 		return 0;
176 
177 	if (unlikely(PageTransHuge(page)))
178 		if (unlikely(split_huge_page_to_list(page, list))) {
179 			swapcache_free(entry, NULL);
180 			return 0;
181 		}
182 
183 	/*
184 	 * Radix-tree node allocations from PF_MEMALLOC contexts could
185 	 * completely exhaust the page allocator. __GFP_NOMEMALLOC
186 	 * stops emergency reserves from being allocated.
187 	 *
188 	 * TODO: this could cause a theoretical memory reclaim
189 	 * deadlock in the swap out path.
190 	 */
191 	/*
192 	 * Add it to the swap cache and mark it dirty
193 	 */
194 	err = add_to_swap_cache(page, entry,
195 			__GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
196 
197 	if (!err) {	/* Success */
198 		SetPageDirty(page);
199 		return 1;
200 	} else {	/* -ENOMEM radix-tree allocation failure */
201 		/*
202 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
203 		 * clear SWAP_HAS_CACHE flag.
204 		 */
205 		swapcache_free(entry, NULL);
206 		return 0;
207 	}
208 }
209 
210 /*
211  * This must be called only on pages that have
212  * been verified to be in the swap cache and locked.
213  * It will never put the page into the free list,
214  * the caller has a reference on the page.
215  */
216 void delete_from_swap_cache(struct page *page)
217 {
218 	swp_entry_t entry;
219 	struct address_space *address_space;
220 
221 	entry.val = page_private(page);
222 
223 	address_space = swap_address_space(entry);
224 	spin_lock_irq(&address_space->tree_lock);
225 	__delete_from_swap_cache(page);
226 	spin_unlock_irq(&address_space->tree_lock);
227 
228 	swapcache_free(entry, page);
229 	page_cache_release(page);
230 }
231 
232 /*
233  * If we are the only user, then try to free up the swap cache.
234  *
235  * Its ok to check for PageSwapCache without the page lock
236  * here because we are going to recheck again inside
237  * try_to_free_swap() _with_ the lock.
238  * 					- Marcelo
239  */
240 static inline void free_swap_cache(struct page *page)
241 {
242 	if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
243 		try_to_free_swap(page);
244 		unlock_page(page);
245 	}
246 }
247 
248 /*
249  * Perform a free_page(), also freeing any swap cache associated with
250  * this page if it is the last user of the page.
251  */
252 void free_page_and_swap_cache(struct page *page)
253 {
254 	free_swap_cache(page);
255 	page_cache_release(page);
256 }
257 
258 /*
259  * Passed an array of pages, drop them all from swapcache and then release
260  * them.  They are removed from the LRU and freed if this is their last use.
261  */
262 void free_pages_and_swap_cache(struct page **pages, int nr)
263 {
264 	struct page **pagep = pages;
265 
266 	lru_add_drain();
267 	while (nr) {
268 		int todo = min(nr, PAGEVEC_SIZE);
269 		int i;
270 
271 		for (i = 0; i < todo; i++)
272 			free_swap_cache(pagep[i]);
273 		release_pages(pagep, todo, 0);
274 		pagep += todo;
275 		nr -= todo;
276 	}
277 }
278 
279 /*
280  * Lookup a swap entry in the swap cache. A found page will be returned
281  * unlocked and with its refcount incremented - we rely on the kernel
282  * lock getting page table operations atomic even if we drop the page
283  * lock before returning.
284  */
285 struct page * lookup_swap_cache(swp_entry_t entry)
286 {
287 	struct page *page;
288 
289 	page = find_get_page(swap_address_space(entry), entry.val);
290 
291 	if (page) {
292 		INC_CACHE_INFO(find_success);
293 		if (TestClearPageReadahead(page))
294 			atomic_inc(&swapin_readahead_hits);
295 	}
296 
297 	INC_CACHE_INFO(find_total);
298 	return page;
299 }
300 
301 /*
302  * Locate a page of swap in physical memory, reserving swap cache space
303  * and reading the disk if it is not already cached.
304  * A failure return means that either the page allocation failed or that
305  * the swap entry is no longer in use.
306  */
307 struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
308 			struct vm_area_struct *vma, unsigned long addr)
309 {
310 	struct page *found_page, *new_page = NULL;
311 	int err;
312 
313 	do {
314 		/*
315 		 * First check the swap cache.  Since this is normally
316 		 * called after lookup_swap_cache() failed, re-calling
317 		 * that would confuse statistics.
318 		 */
319 		found_page = find_get_page(swap_address_space(entry),
320 					entry.val);
321 		if (found_page)
322 			break;
323 
324 		/*
325 		 * Get a new page to read into from swap.
326 		 */
327 		if (!new_page) {
328 			new_page = alloc_page_vma(gfp_mask, vma, addr);
329 			if (!new_page)
330 				break;		/* Out of memory */
331 		}
332 
333 		/*
334 		 * call radix_tree_preload() while we can wait.
335 		 */
336 		err = radix_tree_maybe_preload(gfp_mask & GFP_KERNEL);
337 		if (err)
338 			break;
339 
340 		/*
341 		 * Swap entry may have been freed since our caller observed it.
342 		 */
343 		err = swapcache_prepare(entry);
344 		if (err == -EEXIST) {
345 			radix_tree_preload_end();
346 			/*
347 			 * We might race against get_swap_page() and stumble
348 			 * across a SWAP_HAS_CACHE swap_map entry whose page
349 			 * has not been brought into the swapcache yet, while
350 			 * the other end is scheduled away waiting on discard
351 			 * I/O completion at scan_swap_map().
352 			 *
353 			 * In order to avoid turning this transitory state
354 			 * into a permanent loop around this -EEXIST case
355 			 * if !CONFIG_PREEMPT and the I/O completion happens
356 			 * to be waiting on the CPU waitqueue where we are now
357 			 * busy looping, we just conditionally invoke the
358 			 * scheduler here, if there are some more important
359 			 * tasks to run.
360 			 */
361 			cond_resched();
362 			continue;
363 		}
364 		if (err) {		/* swp entry is obsolete ? */
365 			radix_tree_preload_end();
366 			break;
367 		}
368 
369 		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
370 		__set_page_locked(new_page);
371 		SetPageSwapBacked(new_page);
372 		err = __add_to_swap_cache(new_page, entry);
373 		if (likely(!err)) {
374 			radix_tree_preload_end();
375 			/*
376 			 * Initiate read into locked page and return.
377 			 */
378 			lru_cache_add_anon(new_page);
379 			swap_readpage(new_page);
380 			return new_page;
381 		}
382 		radix_tree_preload_end();
383 		ClearPageSwapBacked(new_page);
384 		__clear_page_locked(new_page);
385 		/*
386 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
387 		 * clear SWAP_HAS_CACHE flag.
388 		 */
389 		swapcache_free(entry, NULL);
390 	} while (err != -ENOMEM);
391 
392 	if (new_page)
393 		page_cache_release(new_page);
394 	return found_page;
395 }
396 
397 static unsigned long swapin_nr_pages(unsigned long offset)
398 {
399 	static unsigned long prev_offset;
400 	unsigned int pages, max_pages, last_ra;
401 	static atomic_t last_readahead_pages;
402 
403 	max_pages = 1 << ACCESS_ONCE(page_cluster);
404 	if (max_pages <= 1)
405 		return 1;
406 
407 	/*
408 	 * This heuristic has been found to work well on both sequential and
409 	 * random loads, swapping to hard disk or to SSD: please don't ask
410 	 * what the "+ 2" means, it just happens to work well, that's all.
411 	 */
412 	pages = atomic_xchg(&swapin_readahead_hits, 0) + 2;
413 	if (pages == 2) {
414 		/*
415 		 * We can have no readahead hits to judge by: but must not get
416 		 * stuck here forever, so check for an adjacent offset instead
417 		 * (and don't even bother to check whether swap type is same).
418 		 */
419 		if (offset != prev_offset + 1 && offset != prev_offset - 1)
420 			pages = 1;
421 		prev_offset = offset;
422 	} else {
423 		unsigned int roundup = 4;
424 		while (roundup < pages)
425 			roundup <<= 1;
426 		pages = roundup;
427 	}
428 
429 	if (pages > max_pages)
430 		pages = max_pages;
431 
432 	/* Don't shrink readahead too fast */
433 	last_ra = atomic_read(&last_readahead_pages) / 2;
434 	if (pages < last_ra)
435 		pages = last_ra;
436 	atomic_set(&last_readahead_pages, pages);
437 
438 	return pages;
439 }
440 
441 /**
442  * swapin_readahead - swap in pages in hope we need them soon
443  * @entry: swap entry of this memory
444  * @gfp_mask: memory allocation flags
445  * @vma: user vma this address belongs to
446  * @addr: target address for mempolicy
447  *
448  * Returns the struct page for entry and addr, after queueing swapin.
449  *
450  * Primitive swap readahead code. We simply read an aligned block of
451  * (1 << page_cluster) entries in the swap area. This method is chosen
452  * because it doesn't cost us any seek time.  We also make sure to queue
453  * the 'original' request together with the readahead ones...
454  *
455  * This has been extended to use the NUMA policies from the mm triggering
456  * the readahead.
457  *
458  * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
459  */
460 struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
461 			struct vm_area_struct *vma, unsigned long addr)
462 {
463 	struct page *page;
464 	unsigned long entry_offset = swp_offset(entry);
465 	unsigned long offset = entry_offset;
466 	unsigned long start_offset, end_offset;
467 	unsigned long mask;
468 	struct blk_plug plug;
469 
470 	mask = swapin_nr_pages(offset) - 1;
471 	if (!mask)
472 		goto skip;
473 
474 	/* Read a page_cluster sized and aligned cluster around offset. */
475 	start_offset = offset & ~mask;
476 	end_offset = offset | mask;
477 	if (!start_offset)	/* First page is swap header. */
478 		start_offset++;
479 
480 	blk_start_plug(&plug);
481 	for (offset = start_offset; offset <= end_offset ; offset++) {
482 		/* Ok, do the async read-ahead now */
483 		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
484 						gfp_mask, vma, addr);
485 		if (!page)
486 			continue;
487 		if (offset != entry_offset)
488 			SetPageReadahead(page);
489 		page_cache_release(page);
490 	}
491 	blk_finish_plug(&plug);
492 
493 	lru_add_drain();	/* Push any new pages onto the LRU now */
494 skip:
495 	return read_swap_cache_async(entry, gfp_mask, vma, addr);
496 }
497