1/*
2 * Frontswap frontend
3 *
4 * This code provides the generic "frontend" layer to call a matching
5 * "backend" driver implementation of frontswap. See
6 * Documentation/vm/frontswap.rst for more information.
7 *
8 * Copyright (C) 2009-2012 Oracle Corp. All rights reserved.
9 * Author: Dan Magenheimer
10 *
11 * This work is licensed under the terms of the GNU GPL, version 2.
12 */
13
14#include <linux/mman.h>
15#include <linux/swap.h>
16#include <linux/swapops.h>
17#include <linux/security.h>
18#include <linux/module.h>
19#include <linux/debugfs.h>
20#include <linux/frontswap.h>
21#include <linux/swapfile.h>
22
23DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);
24
25/*
26 * frontswap_ops are added by frontswap_register_ops, and provide the
27 * frontswap "backend" implementation functions. Multiple implementations
28 * may be registered, but implementations can never deregister. This
29 * is a simple singly-linked list of all registered implementations.
30 */
31static struct frontswap_ops *frontswap_ops __read_mostly;
32
33#define for_each_frontswap_ops(ops) \
34 for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
35
36/*
37 * If enabled, frontswap_store will return failure even on success. As
38 * a result, the swap subsystem will always write the page to swap, in
39 * effect converting frontswap into a writethrough cache. In this mode,
40 * there is no direct reduction in swap writes, but a frontswap backend
41 * can unilaterally "reclaim" any pages in use with no data loss, thus
42 * providing increases control over maximum memory usage due to frontswap.
43 */
44static bool frontswap_writethrough_enabled __read_mostly;
45
46/*
47 * If enabled, the underlying tmem implementation is capable of doing
48 * exclusive gets, so frontswap_load, on a successful tmem_get must
49 * mark the page as no longer in frontswap AND mark it dirty.
50 */
51static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
52
53#ifdef CONFIG_DEBUG_FS
54/*
55 * Counters available via /sys/kernel/debug/frontswap (if debugfs is
56 * properly configured). These are for information only so are not protected
57 * against increment races.
58 */
59static u64 frontswap_loads;
60static u64 frontswap_succ_stores;
61static u64 frontswap_failed_stores;
62static u64 frontswap_invalidates;
63
64static inline void inc_frontswap_loads(void) {
65 frontswap_loads++;
66}
67static inline void inc_frontswap_succ_stores(void) {
68 frontswap_succ_stores++;
69}
70static inline void inc_frontswap_failed_stores(void) {
71 frontswap_failed_stores++;
72}
73static inline void inc_frontswap_invalidates(void) {
74 frontswap_invalidates++;
75}
76#else
77static inline void inc_frontswap_loads(void) { }
78static inline void inc_frontswap_succ_stores(void) { }
79static inline void inc_frontswap_failed_stores(void) { }
80static inline void inc_frontswap_invalidates(void) { }
81#endif
82
83/*
84 * Due to the asynchronous nature of the backends loading potentially
85 * _after_ the swap system has been activated, we have chokepoints
86 * on all frontswap functions to not call the backend until the backend
87 * has registered.
88 *
89 * This would not guards us against the user deciding to call swapoff right as
90 * we are calling the backend to initialize (so swapon is in action).
91 * Fortunatly for us, the swapon_mutex has been taked by the callee so we are
92 * OK. The other scenario where calls to frontswap_store (called via
93 * swap_writepage) is racing with frontswap_invalidate_area (called via
94 * swapoff) is again guarded by the swap subsystem.
95 *
96 * While no backend is registered all calls to frontswap_[store|load|
97 * invalidate_area|invalidate_page] are ignored or fail.
98 *
99 * The time between the backend being registered and the swap file system
100 * calling the backend (via the frontswap_* functions) is indeterminate as
101 * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
102 * That is OK as we are comfortable missing some of these calls to the newly
103 * registered backend.
104 *
105 * Obviously the opposite (unloading the backend) must be done after all
106 * the frontswap_[store|load|invalidate_area|invalidate_page] start
107 * ignoring or failing the requests. However, there is currently no way
108 * to unload a backend once it is registered.
109 */
110
111/*
112 * Register operations for frontswap
113 */
114void frontswap_register_ops(struct frontswap_ops *ops)
115{
116 DECLARE_BITMAP(a, MAX_SWAPFILES);
117 DECLARE_BITMAP(b, MAX_SWAPFILES);
118 struct swap_info_struct *si;
119 unsigned int i;
120
121 bitmap_zero(a, MAX_SWAPFILES);
122 bitmap_zero(b, MAX_SWAPFILES);
123
124 spin_lock(&swap_lock);
125 plist_for_each_entry(si, &swap_active_head, list) {
126 if (!WARN_ON(!si->frontswap_map))
127 set_bit(si->type, a);
128 }
129 spin_unlock(&swap_lock);
130
131 /* the new ops needs to know the currently active swap devices */
132 for_each_set_bit(i, a, MAX_SWAPFILES)
133 ops->init(i);
134
135 /*
136 * Setting frontswap_ops must happen after the ops->init() calls
137 * above; cmpxchg implies smp_mb() which will ensure the init is
138 * complete at this point.
139 */
140 do {
141 ops->next = frontswap_ops;
142 } while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
143
144 static_branch_inc(&frontswap_enabled_key);
145
146 spin_lock(&swap_lock);
147 plist_for_each_entry(si, &swap_active_head, list) {
148 if (si->frontswap_map)
149 set_bit(si->type, b);
150 }
151 spin_unlock(&swap_lock);
152
153 /*
154 * On the very unlikely chance that a swap device was added or
155 * removed between setting the "a" list bits and the ops init
156 * calls, we re-check and do init or invalidate for any changed
157 * bits.
158 */
159 if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
160 for (i = 0; i < MAX_SWAPFILES; i++) {
161 if (!test_bit(i, a) && test_bit(i, b))
162 ops->init(i);
163 else if (test_bit(i, a) && !test_bit(i, b))
164 ops->invalidate_area(i);
165 }
166 }
167}
168EXPORT_SYMBOL(frontswap_register_ops);
169
170/*
171 * Enable/disable frontswap writethrough (see above).
172 */
173void frontswap_writethrough(bool enable)
174{
175 frontswap_writethrough_enabled = enable;
176}
177EXPORT_SYMBOL(frontswap_writethrough);
178
179/*
180 * Enable/disable frontswap exclusive gets (see above).
181 */
182void frontswap_tmem_exclusive_gets(bool enable)
183{
184 frontswap_tmem_exclusive_gets_enabled = enable;
185}
186EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
187
188/*
189 * Called when a swap device is swapon'd.
190 */
191void __frontswap_init(unsigned type, unsigned long *map)
192{
193 struct swap_info_struct *sis = swap_info[type];
194 struct frontswap_ops *ops;
195
196 VM_BUG_ON(sis == NULL);
197
198 /*
199 * p->frontswap is a bitmap that we MUST have to figure out which page
200 * has gone in frontswap. Without it there is no point of continuing.
201 */
202 if (WARN_ON(!map))
203 return;
204 /*
205 * Irregardless of whether the frontswap backend has been loaded
206 * before this function or it will be later, we _MUST_ have the
207 * p->frontswap set to something valid to work properly.
208 */
209 frontswap_map_set(sis, map);
210
211 for_each_frontswap_ops(ops)
212 ops->init(type);
213}
214EXPORT_SYMBOL(__frontswap_init);
215
216bool __frontswap_test(struct swap_info_struct *sis,
217 pgoff_t offset)
218{
219 if (sis->frontswap_map)
220 return test_bit(offset, sis->frontswap_map);
221 return false;
222}
223EXPORT_SYMBOL(__frontswap_test);
224
225static inline void __frontswap_set(struct swap_info_struct *sis,
226 pgoff_t offset)
227{
228 set_bit(offset, sis->frontswap_map);
229 atomic_inc(&sis->frontswap_pages);
230}
231
232static inline void __frontswap_clear(struct swap_info_struct *sis,
233 pgoff_t offset)
234{
235 clear_bit(offset, sis->frontswap_map);
236 atomic_dec(&sis->frontswap_pages);
237}
238
239/*
240 * "Store" data from a page to frontswap and associate it with the page's
241 * swaptype and offset. Page must be locked and in the swap cache.
242 * If frontswap already contains a page with matching swaptype and
243 * offset, the frontswap implementation may either overwrite the data and
244 * return success or invalidate the page from frontswap and return failure.
245 */
246int __frontswap_store(struct page *page)
247{
248 int ret = -1;
249 swp_entry_t entry = { .val = page_private(page), };
250 int type = swp_type(entry);
251 struct swap_info_struct *sis = swap_info[type];
252 pgoff_t offset = swp_offset(entry);
253 struct frontswap_ops *ops;
254
255 VM_BUG_ON(!frontswap_ops);
256 VM_BUG_ON(!PageLocked(page));
257 VM_BUG_ON(sis == NULL);
258
259 /*
260 * If a dup, we must remove the old page first; we can't leave the
261 * old page no matter if the store of the new page succeeds or fails,
262 * and we can't rely on the new page replacing the old page as we may
263 * not store to the same implementation that contains the old page.
264 */
265 if (__frontswap_test(sis, offset)) {
266 __frontswap_clear(sis, offset);
267 for_each_frontswap_ops(ops)
268 ops->invalidate_page(type, offset);
269 }
270
271 /* Try to store in each implementation, until one succeeds. */
272 for_each_frontswap_ops(ops) {
273 ret = ops->store(type, offset, page);
274 if (!ret) /* successful store */
275 break;
276 }
277 if (ret == 0) {
278 __frontswap_set(sis, offset);
279 inc_frontswap_succ_stores();
280 } else {
281 inc_frontswap_failed_stores();
282 }
283 if (frontswap_writethrough_enabled)
284 /* report failure so swap also writes to swap device */
285 ret = -1;
286 return ret;
287}
288EXPORT_SYMBOL(__frontswap_store);
289
290/*
291 * "Get" data from frontswap associated with swaptype and offset that were
292 * specified when the data was put to frontswap and use it to fill the
293 * specified page with data. Page must be locked and in the swap cache.
294 */
295int __frontswap_load(struct page *page)
296{
297 int ret = -1;
298 swp_entry_t entry = { .val = page_private(page), };
299 int type = swp_type(entry);
300 struct swap_info_struct *sis = swap_info[type];
301 pgoff_t offset = swp_offset(entry);
302 struct frontswap_ops *ops;
303
304 VM_BUG_ON(!frontswap_ops);
305 VM_BUG_ON(!PageLocked(page));
306 VM_BUG_ON(sis == NULL);
307
308 if (!__frontswap_test(sis, offset))
309 return -1;
310
311 /* Try loading from each implementation, until one succeeds. */
312 for_each_frontswap_ops(ops) {
313 ret = ops->load(type, offset, page);
314 if (!ret) /* successful load */
315 break;
316 }
317 if (ret == 0) {
318 inc_frontswap_loads();
319 if (frontswap_tmem_exclusive_gets_enabled) {
320 SetPageDirty(page);
321 __frontswap_clear(sis, offset);
322 }
323 }
324 return ret;
325}
326EXPORT_SYMBOL(__frontswap_load);
327
328/*
329 * Invalidate any data from frontswap associated with the specified swaptype
330 * and offset so that a subsequent "get" will fail.
331 */
332void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
333{
334 struct swap_info_struct *sis = swap_info[type];
335 struct frontswap_ops *ops;
336
337 VM_BUG_ON(!frontswap_ops);
338 VM_BUG_ON(sis == NULL);
339
340 if (!__frontswap_test(sis, offset))
341 return;
342
343 for_each_frontswap_ops(ops)
344 ops->invalidate_page(type, offset);
345 __frontswap_clear(sis, offset);
346 inc_frontswap_invalidates();
347}
348EXPORT_SYMBOL(__frontswap_invalidate_page);
349
350/*
351 * Invalidate all data from frontswap associated with all offsets for the
352 * specified swaptype.
353 */
354void __frontswap_invalidate_area(unsigned type)
355{
356 struct swap_info_struct *sis = swap_info[type];
357 struct frontswap_ops *ops;
358
359 VM_BUG_ON(!frontswap_ops);
360 VM_BUG_ON(sis == NULL);
361
362 if (sis->frontswap_map == NULL)
363 return;
364
365 for_each_frontswap_ops(ops)
366 ops->invalidate_area(type);
367 atomic_set(&sis->frontswap_pages, 0);
368 bitmap_zero(sis->frontswap_map, sis->max);
369}
370EXPORT_SYMBOL(__frontswap_invalidate_area);
371
372static unsigned long __frontswap_curr_pages(void)
373{
374 unsigned long totalpages = 0;
375 struct swap_info_struct *si = NULL;
376
377 assert_spin_locked(&swap_lock);
378 plist_for_each_entry(si, &swap_active_head, list)
379 totalpages += atomic_read(&si->frontswap_pages);
380 return totalpages;
381}
382
383static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
384 int *swapid)
385{
386 int ret = -EINVAL;
387 struct swap_info_struct *si = NULL;
388 int si_frontswap_pages;
389 unsigned long total_pages_to_unuse = total;
390 unsigned long pages = 0, pages_to_unuse = 0;
391
392 assert_spin_locked(&swap_lock);
393 plist_for_each_entry(si, &swap_active_head, list) {
394 si_frontswap_pages = atomic_read(&si->frontswap_pages);
395 if (total_pages_to_unuse < si_frontswap_pages) {
396 pages = pages_to_unuse = total_pages_to_unuse;
397 } else {
398 pages = si_frontswap_pages;
399 pages_to_unuse = 0; /* unuse all */
400 }
401 /* ensure there is enough RAM to fetch pages from frontswap */
402 if (security_vm_enough_memory_mm(current->mm, pages)) {
403 ret = -ENOMEM;
404 continue;
405 }
406 vm_unacct_memory(pages);
407 *unused = pages_to_unuse;
408 *swapid = si->type;
409 ret = 0;
410 break;
411 }
412
413 return ret;
414}
415
416/*
417 * Used to check if it's necessory and feasible to unuse pages.
418 * Return 1 when nothing to do, 0 when need to shink pages,
419 * error code when there is an error.
420 */
421static int __frontswap_shrink(unsigned long target_pages,
422 unsigned long *pages_to_unuse,
423 int *type)
424{
425 unsigned long total_pages = 0, total_pages_to_unuse;
426
427 assert_spin_locked(&swap_lock);
428
429 total_pages = __frontswap_curr_pages();
430 if (total_pages <= target_pages) {
431 /* Nothing to do */
432 *pages_to_unuse = 0;
433 return 1;
434 }
435 total_pages_to_unuse = total_pages - target_pages;
436 return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
437}
438
439/*
440 * Frontswap, like a true swap device, may unnecessarily retain pages
441 * under certain circumstances; "shrink" frontswap is essentially a
442 * "partial swapoff" and works by calling try_to_unuse to attempt to
443 * unuse enough frontswap pages to attempt to -- subject to memory
444 * constraints -- reduce the number of pages in frontswap to the
445 * number given in the parameter target_pages.
446 */
447void frontswap_shrink(unsigned long target_pages)
448{
449 unsigned long pages_to_unuse = 0;
450 int uninitialized_var(type), ret;
451
452 /*
453 * we don't want to hold swap_lock while doing a very
454 * lengthy try_to_unuse, but swap_list may change
455 * so restart scan from swap_active_head each time
456 */
457 spin_lock(&swap_lock);
458 ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
459 spin_unlock(&swap_lock);
460 if (ret == 0)
461 try_to_unuse(type, true, pages_to_unuse);
462 return;
463}
464EXPORT_SYMBOL(frontswap_shrink);
465
466/*
467 * Count and return the number of frontswap pages across all
468 * swap devices. This is exported so that backend drivers can
469 * determine current usage without reading debugfs.
470 */
471unsigned long frontswap_curr_pages(void)
472{
473 unsigned long totalpages = 0;
474
475 spin_lock(&swap_lock);
476 totalpages = __frontswap_curr_pages();
477 spin_unlock(&swap_lock);
478
479 return totalpages;
480}
481EXPORT_SYMBOL(frontswap_curr_pages);
482
483static int __init init_frontswap(void)
484{
485#ifdef CONFIG_DEBUG_FS
486 struct dentry *root = debugfs_create_dir("frontswap", NULL);
487 if (root == NULL)
488 return -ENXIO;
489 debugfs_create_u64("loads", 0444, root, &frontswap_loads);
490 debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
491 debugfs_create_u64("failed_stores", 0444, root,
492 &frontswap_failed_stores);
493 debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);
494#endif
495 return 0;
496}
497
498module_init(init_frontswap);
499