1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Frontswap frontend
4 *
5 * This code provides the generic "frontend" layer to call a matching
6 * "backend" driver implementation of frontswap. See
7 * Documentation/vm/frontswap.rst for more information.
8 *
9 * Copyright (C) 2009-2012 Oracle Corp. All rights reserved.
10 * Author: Dan Magenheimer
11 */
12
13 #include <linux/mman.h>
14 #include <linux/swap.h>
15 #include <linux/swapops.h>
16 #include <linux/security.h>
17 #include <linux/module.h>
18 #include <linux/debugfs.h>
19 #include <linux/frontswap.h>
20 #include <linux/swapfile.h>
21
22 DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);
23
24 /*
25 * frontswap_ops are added by frontswap_register_ops, and provide the
26 * frontswap "backend" implementation functions. Multiple implementations
27 * may be registered, but implementations can never deregister. This
28 * is a simple singly-linked list of all registered implementations.
29 */
30 static struct frontswap_ops *frontswap_ops __read_mostly;
31
32 #define for_each_frontswap_ops(ops) \
33 for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
34
35 /*
36 * If enabled, frontswap_store will return failure even on success. As
37 * a result, the swap subsystem will always write the page to swap, in
38 * effect converting frontswap into a writethrough cache. In this mode,
39 * there is no direct reduction in swap writes, but a frontswap backend
40 * can unilaterally "reclaim" any pages in use with no data loss, thus
41 * providing increases control over maximum memory usage due to frontswap.
42 */
43 static bool frontswap_writethrough_enabled __read_mostly;
44
45 /*
46 * If enabled, the underlying tmem implementation is capable of doing
47 * exclusive gets, so frontswap_load, on a successful tmem_get must
48 * mark the page as no longer in frontswap AND mark it dirty.
49 */
50 static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
51
52 #ifdef CONFIG_DEBUG_FS
53 /*
54 * Counters available via /sys/kernel/debug/frontswap (if debugfs is
55 * properly configured). These are for information only so are not protected
56 * against increment races.
57 */
58 static u64 frontswap_loads;
59 static u64 frontswap_succ_stores;
60 static u64 frontswap_failed_stores;
61 static u64 frontswap_invalidates;
62
inc_frontswap_loads(void)63 static inline void inc_frontswap_loads(void) {
64 data_race(frontswap_loads++);
65 }
inc_frontswap_succ_stores(void)66 static inline void inc_frontswap_succ_stores(void) {
67 data_race(frontswap_succ_stores++);
68 }
inc_frontswap_failed_stores(void)69 static inline void inc_frontswap_failed_stores(void) {
70 data_race(frontswap_failed_stores++);
71 }
inc_frontswap_invalidates(void)72 static inline void inc_frontswap_invalidates(void) {
73 data_race(frontswap_invalidates++);
74 }
75 #else
inc_frontswap_loads(void)76 static inline void inc_frontswap_loads(void) { }
inc_frontswap_succ_stores(void)77 static inline void inc_frontswap_succ_stores(void) { }
inc_frontswap_failed_stores(void)78 static inline void inc_frontswap_failed_stores(void) { }
inc_frontswap_invalidates(void)79 static inline void inc_frontswap_invalidates(void) { }
80 #endif
81
82 /*
83 * Due to the asynchronous nature of the backends loading potentially
84 * _after_ the swap system has been activated, we have chokepoints
85 * on all frontswap functions to not call the backend until the backend
86 * has registered.
87 *
88 * This would not guards us against the user deciding to call swapoff right as
89 * we are calling the backend to initialize (so swapon is in action).
90 * Fortunately for us, the swapon_mutex has been taken by the callee so we are
91 * OK. The other scenario where calls to frontswap_store (called via
92 * swap_writepage) is racing with frontswap_invalidate_area (called via
93 * swapoff) is again guarded by the swap subsystem.
94 *
95 * While no backend is registered all calls to frontswap_[store|load|
96 * invalidate_area|invalidate_page] are ignored or fail.
97 *
98 * The time between the backend being registered and the swap file system
99 * calling the backend (via the frontswap_* functions) is indeterminate as
100 * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
101 * That is OK as we are comfortable missing some of these calls to the newly
102 * registered backend.
103 *
104 * Obviously the opposite (unloading the backend) must be done after all
105 * the frontswap_[store|load|invalidate_area|invalidate_page] start
106 * ignoring or failing the requests. However, there is currently no way
107 * to unload a backend once it is registered.
108 */
109
110 /*
111 * Register operations for frontswap
112 */
frontswap_register_ops(struct frontswap_ops * ops)113 void frontswap_register_ops(struct frontswap_ops *ops)
114 {
115 DECLARE_BITMAP(a, MAX_SWAPFILES);
116 DECLARE_BITMAP(b, MAX_SWAPFILES);
117 struct swap_info_struct *si;
118 unsigned int i;
119
120 bitmap_zero(a, MAX_SWAPFILES);
121 bitmap_zero(b, MAX_SWAPFILES);
122
123 spin_lock(&swap_lock);
124 plist_for_each_entry(si, &swap_active_head, list) {
125 if (!WARN_ON(!si->frontswap_map))
126 set_bit(si->type, a);
127 }
128 spin_unlock(&swap_lock);
129
130 /* the new ops needs to know the currently active swap devices */
131 for_each_set_bit(i, a, MAX_SWAPFILES)
132 ops->init(i);
133
134 /*
135 * Setting frontswap_ops must happen after the ops->init() calls
136 * above; cmpxchg implies smp_mb() which will ensure the init is
137 * complete at this point.
138 */
139 do {
140 ops->next = frontswap_ops;
141 } while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
142
143 static_branch_inc(&frontswap_enabled_key);
144
145 spin_lock(&swap_lock);
146 plist_for_each_entry(si, &swap_active_head, list) {
147 if (si->frontswap_map)
148 set_bit(si->type, b);
149 }
150 spin_unlock(&swap_lock);
151
152 /*
153 * On the very unlikely chance that a swap device was added or
154 * removed between setting the "a" list bits and the ops init
155 * calls, we re-check and do init or invalidate for any changed
156 * bits.
157 */
158 if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
159 for (i = 0; i < MAX_SWAPFILES; i++) {
160 if (!test_bit(i, a) && test_bit(i, b))
161 ops->init(i);
162 else if (test_bit(i, a) && !test_bit(i, b))
163 ops->invalidate_area(i);
164 }
165 }
166 }
167 EXPORT_SYMBOL(frontswap_register_ops);
168
169 /*
170 * Enable/disable frontswap writethrough (see above).
171 */
frontswap_writethrough(bool enable)172 void frontswap_writethrough(bool enable)
173 {
174 frontswap_writethrough_enabled = enable;
175 }
176 EXPORT_SYMBOL(frontswap_writethrough);
177
178 /*
179 * Enable/disable frontswap exclusive gets (see above).
180 */
frontswap_tmem_exclusive_gets(bool enable)181 void frontswap_tmem_exclusive_gets(bool enable)
182 {
183 frontswap_tmem_exclusive_gets_enabled = enable;
184 }
185 EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
186
187 /*
188 * Called when a swap device is swapon'd.
189 */
__frontswap_init(unsigned type,unsigned long * map)190 void __frontswap_init(unsigned type, unsigned long *map)
191 {
192 struct swap_info_struct *sis = swap_info[type];
193 struct frontswap_ops *ops;
194
195 VM_BUG_ON(sis == NULL);
196
197 /*
198 * p->frontswap is a bitmap that we MUST have to figure out which page
199 * has gone in frontswap. Without it there is no point of continuing.
200 */
201 if (WARN_ON(!map))
202 return;
203 /*
204 * Irregardless of whether the frontswap backend has been loaded
205 * before this function or it will be later, we _MUST_ have the
206 * p->frontswap set to something valid to work properly.
207 */
208 frontswap_map_set(sis, map);
209
210 for_each_frontswap_ops(ops)
211 ops->init(type);
212 }
213 EXPORT_SYMBOL(__frontswap_init);
214
__frontswap_test(struct swap_info_struct * sis,pgoff_t offset)215 bool __frontswap_test(struct swap_info_struct *sis,
216 pgoff_t offset)
217 {
218 if (sis->frontswap_map)
219 return test_bit(offset, sis->frontswap_map);
220 return false;
221 }
222 EXPORT_SYMBOL(__frontswap_test);
223
__frontswap_set(struct swap_info_struct * sis,pgoff_t offset)224 static inline void __frontswap_set(struct swap_info_struct *sis,
225 pgoff_t offset)
226 {
227 set_bit(offset, sis->frontswap_map);
228 atomic_inc(&sis->frontswap_pages);
229 }
230
__frontswap_clear(struct swap_info_struct * sis,pgoff_t offset)231 static inline void __frontswap_clear(struct swap_info_struct *sis,
232 pgoff_t offset)
233 {
234 clear_bit(offset, sis->frontswap_map);
235 atomic_dec(&sis->frontswap_pages);
236 }
237
238 /*
239 * "Store" data from a page to frontswap and associate it with the page's
240 * swaptype and offset. Page must be locked and in the swap cache.
241 * If frontswap already contains a page with matching swaptype and
242 * offset, the frontswap implementation may either overwrite the data and
243 * return success or invalidate the page from frontswap and return failure.
244 */
__frontswap_store(struct page * page)245 int __frontswap_store(struct page *page)
246 {
247 int ret = -1;
248 swp_entry_t entry = { .val = page_private(page), };
249 int type = swp_type(entry);
250 struct swap_info_struct *sis = swap_info[type];
251 pgoff_t offset = swp_offset(entry);
252 struct frontswap_ops *ops;
253
254 VM_BUG_ON(!frontswap_ops);
255 VM_BUG_ON(!PageLocked(page));
256 VM_BUG_ON(sis == NULL);
257
258 /*
259 * If a dup, we must remove the old page first; we can't leave the
260 * old page no matter if the store of the new page succeeds or fails,
261 * and we can't rely on the new page replacing the old page as we may
262 * not store to the same implementation that contains the old page.
263 */
264 if (__frontswap_test(sis, offset)) {
265 __frontswap_clear(sis, offset);
266 for_each_frontswap_ops(ops)
267 ops->invalidate_page(type, offset);
268 }
269
270 /* Try to store in each implementation, until one succeeds. */
271 for_each_frontswap_ops(ops) {
272 ret = ops->store(type, offset, page);
273 if (!ret) /* successful store */
274 break;
275 }
276 if (ret == 0) {
277 __frontswap_set(sis, offset);
278 inc_frontswap_succ_stores();
279 } else {
280 inc_frontswap_failed_stores();
281 }
282 if (frontswap_writethrough_enabled)
283 /* report failure so swap also writes to swap device */
284 ret = -1;
285 return ret;
286 }
287 EXPORT_SYMBOL(__frontswap_store);
288
289 /*
290 * "Get" data from frontswap associated with swaptype and offset that were
291 * specified when the data was put to frontswap and use it to fill the
292 * specified page with data. Page must be locked and in the swap cache.
293 */
__frontswap_load(struct page * page)294 int __frontswap_load(struct page *page)
295 {
296 int ret = -1;
297 swp_entry_t entry = { .val = page_private(page), };
298 int type = swp_type(entry);
299 struct swap_info_struct *sis = swap_info[type];
300 pgoff_t offset = swp_offset(entry);
301 struct frontswap_ops *ops;
302
303 VM_BUG_ON(!frontswap_ops);
304 VM_BUG_ON(!PageLocked(page));
305 VM_BUG_ON(sis == NULL);
306
307 if (!__frontswap_test(sis, offset))
308 return -1;
309
310 /* Try loading from each implementation, until one succeeds. */
311 for_each_frontswap_ops(ops) {
312 ret = ops->load(type, offset, page);
313 if (!ret) /* successful load */
314 break;
315 }
316 if (ret == 0) {
317 inc_frontswap_loads();
318 if (frontswap_tmem_exclusive_gets_enabled) {
319 SetPageDirty(page);
320 __frontswap_clear(sis, offset);
321 }
322 }
323 return ret;
324 }
325 EXPORT_SYMBOL(__frontswap_load);
326
327 /*
328 * Invalidate any data from frontswap associated with the specified swaptype
329 * and offset so that a subsequent "get" will fail.
330 */
__frontswap_invalidate_page(unsigned type,pgoff_t offset)331 void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
332 {
333 struct swap_info_struct *sis = swap_info[type];
334 struct frontswap_ops *ops;
335
336 VM_BUG_ON(!frontswap_ops);
337 VM_BUG_ON(sis == NULL);
338
339 if (!__frontswap_test(sis, offset))
340 return;
341
342 for_each_frontswap_ops(ops)
343 ops->invalidate_page(type, offset);
344 __frontswap_clear(sis, offset);
345 inc_frontswap_invalidates();
346 }
347 EXPORT_SYMBOL(__frontswap_invalidate_page);
348
349 /*
350 * Invalidate all data from frontswap associated with all offsets for the
351 * specified swaptype.
352 */
__frontswap_invalidate_area(unsigned type)353 void __frontswap_invalidate_area(unsigned type)
354 {
355 struct swap_info_struct *sis = swap_info[type];
356 struct frontswap_ops *ops;
357
358 VM_BUG_ON(!frontswap_ops);
359 VM_BUG_ON(sis == NULL);
360
361 if (sis->frontswap_map == NULL)
362 return;
363
364 for_each_frontswap_ops(ops)
365 ops->invalidate_area(type);
366 atomic_set(&sis->frontswap_pages, 0);
367 bitmap_zero(sis->frontswap_map, sis->max);
368 }
369 EXPORT_SYMBOL(__frontswap_invalidate_area);
370
__frontswap_curr_pages(void)371 static unsigned long __frontswap_curr_pages(void)
372 {
373 unsigned long totalpages = 0;
374 struct swap_info_struct *si = NULL;
375
376 assert_spin_locked(&swap_lock);
377 plist_for_each_entry(si, &swap_active_head, list)
378 totalpages += atomic_read(&si->frontswap_pages);
379 return totalpages;
380 }
381
__frontswap_unuse_pages(unsigned long total,unsigned long * unused,int * swapid)382 static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
383 int *swapid)
384 {
385 int ret = -EINVAL;
386 struct swap_info_struct *si = NULL;
387 int si_frontswap_pages;
388 unsigned long total_pages_to_unuse = total;
389 unsigned long pages = 0, pages_to_unuse = 0;
390
391 assert_spin_locked(&swap_lock);
392 plist_for_each_entry(si, &swap_active_head, list) {
393 si_frontswap_pages = atomic_read(&si->frontswap_pages);
394 if (total_pages_to_unuse < si_frontswap_pages) {
395 pages = pages_to_unuse = total_pages_to_unuse;
396 } else {
397 pages = si_frontswap_pages;
398 pages_to_unuse = 0; /* unuse all */
399 }
400 /* ensure there is enough RAM to fetch pages from frontswap */
401 if (security_vm_enough_memory_mm(current->mm, pages)) {
402 ret = -ENOMEM;
403 continue;
404 }
405 vm_unacct_memory(pages);
406 *unused = pages_to_unuse;
407 *swapid = si->type;
408 ret = 0;
409 break;
410 }
411
412 return ret;
413 }
414
415 /*
416 * Used to check if it's necessary and feasible to unuse pages.
417 * Return 1 when nothing to do, 0 when need to shrink pages,
418 * error code when there is an error.
419 */
__frontswap_shrink(unsigned long target_pages,unsigned long * pages_to_unuse,int * type)420 static int __frontswap_shrink(unsigned long target_pages,
421 unsigned long *pages_to_unuse,
422 int *type)
423 {
424 unsigned long total_pages = 0, total_pages_to_unuse;
425
426 assert_spin_locked(&swap_lock);
427
428 total_pages = __frontswap_curr_pages();
429 if (total_pages <= target_pages) {
430 /* Nothing to do */
431 *pages_to_unuse = 0;
432 return 1;
433 }
434 total_pages_to_unuse = total_pages - target_pages;
435 return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
436 }
437
438 /*
439 * Frontswap, like a true swap device, may unnecessarily retain pages
440 * under certain circumstances; "shrink" frontswap is essentially a
441 * "partial swapoff" and works by calling try_to_unuse to attempt to
442 * unuse enough frontswap pages to attempt to -- subject to memory
443 * constraints -- reduce the number of pages in frontswap to the
444 * number given in the parameter target_pages.
445 */
frontswap_shrink(unsigned long target_pages)446 void frontswap_shrink(unsigned long target_pages)
447 {
448 unsigned long pages_to_unuse = 0;
449 int type, ret;
450
451 /*
452 * we don't want to hold swap_lock while doing a very
453 * lengthy try_to_unuse, but swap_list may change
454 * so restart scan from swap_active_head each time
455 */
456 spin_lock(&swap_lock);
457 ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
458 spin_unlock(&swap_lock);
459 if (ret == 0)
460 try_to_unuse(type, true, pages_to_unuse);
461 return;
462 }
463 EXPORT_SYMBOL(frontswap_shrink);
464
465 /*
466 * Count and return the number of frontswap pages across all
467 * swap devices. This is exported so that backend drivers can
468 * determine current usage without reading debugfs.
469 */
frontswap_curr_pages(void)470 unsigned long frontswap_curr_pages(void)
471 {
472 unsigned long totalpages = 0;
473
474 spin_lock(&swap_lock);
475 totalpages = __frontswap_curr_pages();
476 spin_unlock(&swap_lock);
477
478 return totalpages;
479 }
480 EXPORT_SYMBOL(frontswap_curr_pages);
481
init_frontswap(void)482 static int __init init_frontswap(void)
483 {
484 #ifdef CONFIG_DEBUG_FS
485 struct dentry *root = debugfs_create_dir("frontswap", NULL);
486 if (root == NULL)
487 return -ENXIO;
488 debugfs_create_u64("loads", 0444, root, &frontswap_loads);
489 debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
490 debugfs_create_u64("failed_stores", 0444, root,
491 &frontswap_failed_stores);
492 debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);
493 #endif
494 return 0;
495 }
496
497 module_init(init_frontswap);
498
