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 {
65 data_race(frontswap_loads++);
66 }
inc_frontswap_succ_stores(void)67 static inline void inc_frontswap_succ_stores(void)
68 {
69 data_race(frontswap_succ_stores++);
70 }
inc_frontswap_failed_stores(void)71 static inline void inc_frontswap_failed_stores(void)
72 {
73 data_race(frontswap_failed_stores++);
74 }
inc_frontswap_invalidates(void)75 static inline void inc_frontswap_invalidates(void)
76 {
77 data_race(frontswap_invalidates++);
78 }
79 #else
inc_frontswap_loads(void)80 static inline void inc_frontswap_loads(void) { }
inc_frontswap_succ_stores(void)81 static inline void inc_frontswap_succ_stores(void) { }
inc_frontswap_failed_stores(void)82 static inline void inc_frontswap_failed_stores(void) { }
inc_frontswap_invalidates(void)83 static inline void inc_frontswap_invalidates(void) { }
84 #endif
85
86 /*
87 * Due to the asynchronous nature of the backends loading potentially
88 * _after_ the swap system has been activated, we have chokepoints
89 * on all frontswap functions to not call the backend until the backend
90 * has registered.
91 *
92 * This would not guards us against the user deciding to call swapoff right as
93 * we are calling the backend to initialize (so swapon is in action).
94 * Fortunately for us, the swapon_mutex has been taken by the callee so we are
95 * OK. The other scenario where calls to frontswap_store (called via
96 * swap_writepage) is racing with frontswap_invalidate_area (called via
97 * swapoff) is again guarded by the swap subsystem.
98 *
99 * While no backend is registered all calls to frontswap_[store|load|
100 * invalidate_area|invalidate_page] are ignored or fail.
101 *
102 * The time between the backend being registered and the swap file system
103 * calling the backend (via the frontswap_* functions) is indeterminate as
104 * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
105 * That is OK as we are comfortable missing some of these calls to the newly
106 * registered backend.
107 *
108 * Obviously the opposite (unloading the backend) must be done after all
109 * the frontswap_[store|load|invalidate_area|invalidate_page] start
110 * ignoring or failing the requests. However, there is currently no way
111 * to unload a backend once it is registered.
112 */
113
114 /*
115 * Register operations for frontswap
116 */
frontswap_register_ops(struct frontswap_ops * ops)117 void frontswap_register_ops(struct frontswap_ops *ops)
118 {
119 DECLARE_BITMAP(a, MAX_SWAPFILES);
120 DECLARE_BITMAP(b, MAX_SWAPFILES);
121 struct swap_info_struct *si;
122 unsigned int i;
123
124 bitmap_zero(a, MAX_SWAPFILES);
125 bitmap_zero(b, MAX_SWAPFILES);
126
127 spin_lock(&swap_lock);
128 plist_for_each_entry(si, &swap_active_head, list) {
129 if (!WARN_ON(!si->frontswap_map))
130 set_bit(si->type, a);
131 }
132 spin_unlock(&swap_lock);
133
134 /* the new ops needs to know the currently active swap devices */
135 for_each_set_bit(i, a, MAX_SWAPFILES)
136 ops->init(i);
137
138 /*
139 * Setting frontswap_ops must happen after the ops->init() calls
140 * above; cmpxchg implies smp_mb() which will ensure the init is
141 * complete at this point.
142 */
143 do {
144 ops->next = frontswap_ops;
145 } while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
146
147 static_branch_inc(&frontswap_enabled_key);
148
149 spin_lock(&swap_lock);
150 plist_for_each_entry(si, &swap_active_head, list) {
151 if (si->frontswap_map)
152 set_bit(si->type, b);
153 }
154 spin_unlock(&swap_lock);
155
156 /*
157 * On the very unlikely chance that a swap device was added or
158 * removed between setting the "a" list bits and the ops init
159 * calls, we re-check and do init or invalidate for any changed
160 * bits.
161 */
162 if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
163 for (i = 0; i < MAX_SWAPFILES; i++) {
164 if (!test_bit(i, a) && test_bit(i, b))
165 ops->init(i);
166 else if (test_bit(i, a) && !test_bit(i, b))
167 ops->invalidate_area(i);
168 }
169 }
170 }
171 EXPORT_SYMBOL(frontswap_register_ops);
172
173 /*
174 * Enable/disable frontswap writethrough (see above).
175 */
frontswap_writethrough(bool enable)176 void frontswap_writethrough(bool enable)
177 {
178 frontswap_writethrough_enabled = enable;
179 }
180 EXPORT_SYMBOL(frontswap_writethrough);
181
182 /*
183 * Enable/disable frontswap exclusive gets (see above).
184 */
frontswap_tmem_exclusive_gets(bool enable)185 void frontswap_tmem_exclusive_gets(bool enable)
186 {
187 frontswap_tmem_exclusive_gets_enabled = enable;
188 }
189 EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
190
191 /*
192 * Called when a swap device is swapon'd.
193 */
__frontswap_init(unsigned type,unsigned long * map)194 void __frontswap_init(unsigned type, unsigned long *map)
195 {
196 struct swap_info_struct *sis = swap_info[type];
197 struct frontswap_ops *ops;
198
199 VM_BUG_ON(sis == NULL);
200
201 /*
202 * p->frontswap is a bitmap that we MUST have to figure out which page
203 * has gone in frontswap. Without it there is no point of continuing.
204 */
205 if (WARN_ON(!map))
206 return;
207 /*
208 * Irregardless of whether the frontswap backend has been loaded
209 * before this function or it will be later, we _MUST_ have the
210 * p->frontswap set to something valid to work properly.
211 */
212 frontswap_map_set(sis, map);
213
214 for_each_frontswap_ops(ops)
215 ops->init(type);
216 }
217 EXPORT_SYMBOL(__frontswap_init);
218
__frontswap_test(struct swap_info_struct * sis,pgoff_t offset)219 bool __frontswap_test(struct swap_info_struct *sis,
220 pgoff_t offset)
221 {
222 if (sis->frontswap_map)
223 return test_bit(offset, sis->frontswap_map);
224 return false;
225 }
226 EXPORT_SYMBOL(__frontswap_test);
227
__frontswap_set(struct swap_info_struct * sis,pgoff_t offset)228 static inline void __frontswap_set(struct swap_info_struct *sis,
229 pgoff_t offset)
230 {
231 set_bit(offset, sis->frontswap_map);
232 atomic_inc(&sis->frontswap_pages);
233 }
234
__frontswap_clear(struct swap_info_struct * sis,pgoff_t offset)235 static inline void __frontswap_clear(struct swap_info_struct *sis,
236 pgoff_t offset)
237 {
238 clear_bit(offset, sis->frontswap_map);
239 atomic_dec(&sis->frontswap_pages);
240 }
241
242 /*
243 * "Store" data from a page to frontswap and associate it with the page's
244 * swaptype and offset. Page must be locked and in the swap cache.
245 * If frontswap already contains a page with matching swaptype and
246 * offset, the frontswap implementation may either overwrite the data and
247 * return success or invalidate the page from frontswap and return failure.
248 */
__frontswap_store(struct page * page)249 int __frontswap_store(struct page *page)
250 {
251 int ret = -1;
252 swp_entry_t entry = { .val = page_private(page), };
253 int type = swp_type(entry);
254 struct swap_info_struct *sis = swap_info[type];
255 pgoff_t offset = swp_offset(entry);
256 struct frontswap_ops *ops;
257
258 VM_BUG_ON(!frontswap_ops);
259 VM_BUG_ON(!PageLocked(page));
260 VM_BUG_ON(sis == NULL);
261
262 /*
263 * If a dup, we must remove the old page first; we can't leave the
264 * old page no matter if the store of the new page succeeds or fails,
265 * and we can't rely on the new page replacing the old page as we may
266 * not store to the same implementation that contains the old page.
267 */
268 if (__frontswap_test(sis, offset)) {
269 __frontswap_clear(sis, offset);
270 for_each_frontswap_ops(ops)
271 ops->invalidate_page(type, offset);
272 }
273
274 /* Try to store in each implementation, until one succeeds. */
275 for_each_frontswap_ops(ops) {
276 ret = ops->store(type, offset, page);
277 if (!ret) /* successful store */
278 break;
279 }
280 if (ret == 0) {
281 __frontswap_set(sis, offset);
282 inc_frontswap_succ_stores();
283 } else {
284 inc_frontswap_failed_stores();
285 }
286 if (frontswap_writethrough_enabled)
287 /* report failure so swap also writes to swap device */
288 ret = -1;
289 return ret;
290 }
291 EXPORT_SYMBOL(__frontswap_store);
292
293 /*
294 * "Get" data from frontswap associated with swaptype and offset that were
295 * specified when the data was put to frontswap and use it to fill the
296 * specified page with data. Page must be locked and in the swap cache.
297 */
__frontswap_load(struct page * page)298 int __frontswap_load(struct page *page)
299 {
300 int ret = -1;
301 swp_entry_t entry = { .val = page_private(page), };
302 int type = swp_type(entry);
303 struct swap_info_struct *sis = swap_info[type];
304 pgoff_t offset = swp_offset(entry);
305 struct frontswap_ops *ops;
306
307 VM_BUG_ON(!frontswap_ops);
308 VM_BUG_ON(!PageLocked(page));
309 VM_BUG_ON(sis == NULL);
310
311 if (!__frontswap_test(sis, offset))
312 return -1;
313
314 /* Try loading from each implementation, until one succeeds. */
315 for_each_frontswap_ops(ops) {
316 ret = ops->load(type, offset, page);
317 if (!ret) /* successful load */
318 break;
319 }
320 if (ret == 0) {
321 inc_frontswap_loads();
322 if (frontswap_tmem_exclusive_gets_enabled) {
323 SetPageDirty(page);
324 __frontswap_clear(sis, offset);
325 }
326 }
327 return ret;
328 }
329 EXPORT_SYMBOL(__frontswap_load);
330
331 /*
332 * Invalidate any data from frontswap associated with the specified swaptype
333 * and offset so that a subsequent "get" will fail.
334 */
__frontswap_invalidate_page(unsigned type,pgoff_t offset)335 void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
336 {
337 struct swap_info_struct *sis = swap_info[type];
338 struct frontswap_ops *ops;
339
340 VM_BUG_ON(!frontswap_ops);
341 VM_BUG_ON(sis == NULL);
342
343 if (!__frontswap_test(sis, offset))
344 return;
345
346 for_each_frontswap_ops(ops)
347 ops->invalidate_page(type, offset);
348 __frontswap_clear(sis, offset);
349 inc_frontswap_invalidates();
350 }
351 EXPORT_SYMBOL(__frontswap_invalidate_page);
352
353 /*
354 * Invalidate all data from frontswap associated with all offsets for the
355 * specified swaptype.
356 */
__frontswap_invalidate_area(unsigned type)357 void __frontswap_invalidate_area(unsigned type)
358 {
359 struct swap_info_struct *sis = swap_info[type];
360 struct frontswap_ops *ops;
361
362 VM_BUG_ON(!frontswap_ops);
363 VM_BUG_ON(sis == NULL);
364
365 if (sis->frontswap_map == NULL)
366 return;
367
368 for_each_frontswap_ops(ops)
369 ops->invalidate_area(type);
370 atomic_set(&sis->frontswap_pages, 0);
371 bitmap_zero(sis->frontswap_map, sis->max);
372 }
373 EXPORT_SYMBOL(__frontswap_invalidate_area);
374
__frontswap_curr_pages(void)375 static unsigned long __frontswap_curr_pages(void)
376 {
377 unsigned long totalpages = 0;
378 struct swap_info_struct *si = NULL;
379
380 assert_spin_locked(&swap_lock);
381 plist_for_each_entry(si, &swap_active_head, list)
382 totalpages += atomic_read(&si->frontswap_pages);
383 return totalpages;
384 }
385
__frontswap_unuse_pages(unsigned long total,unsigned long * unused,int * swapid)386 static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
387 int *swapid)
388 {
389 int ret = -EINVAL;
390 struct swap_info_struct *si = NULL;
391 int si_frontswap_pages;
392 unsigned long total_pages_to_unuse = total;
393 unsigned long pages = 0, pages_to_unuse = 0;
394
395 assert_spin_locked(&swap_lock);
396 plist_for_each_entry(si, &swap_active_head, list) {
397 si_frontswap_pages = atomic_read(&si->frontswap_pages);
398 if (total_pages_to_unuse < si_frontswap_pages) {
399 pages = pages_to_unuse = total_pages_to_unuse;
400 } else {
401 pages = si_frontswap_pages;
402 pages_to_unuse = 0; /* unuse all */
403 }
404 /* ensure there is enough RAM to fetch pages from frontswap */
405 if (security_vm_enough_memory_mm(current->mm, pages)) {
406 ret = -ENOMEM;
407 continue;
408 }
409 vm_unacct_memory(pages);
410 *unused = pages_to_unuse;
411 *swapid = si->type;
412 ret = 0;
413 break;
414 }
415
416 return ret;
417 }
418
419 /*
420 * Used to check if it's necessary and feasible to unuse pages.
421 * Return 1 when nothing to do, 0 when need to shrink pages,
422 * error code when there is an error.
423 */
__frontswap_shrink(unsigned long target_pages,unsigned long * pages_to_unuse,int * type)424 static int __frontswap_shrink(unsigned long target_pages,
425 unsigned long *pages_to_unuse,
426 int *type)
427 {
428 unsigned long total_pages = 0, total_pages_to_unuse;
429
430 assert_spin_locked(&swap_lock);
431
432 total_pages = __frontswap_curr_pages();
433 if (total_pages <= target_pages) {
434 /* Nothing to do */
435 *pages_to_unuse = 0;
436 return 1;
437 }
438 total_pages_to_unuse = total_pages - target_pages;
439 return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
440 }
441
442 /*
443 * Frontswap, like a true swap device, may unnecessarily retain pages
444 * under certain circumstances; "shrink" frontswap is essentially a
445 * "partial swapoff" and works by calling try_to_unuse to attempt to
446 * unuse enough frontswap pages to attempt to -- subject to memory
447 * constraints -- reduce the number of pages in frontswap to the
448 * number given in the parameter target_pages.
449 */
frontswap_shrink(unsigned long target_pages)450 void frontswap_shrink(unsigned long target_pages)
451 {
452 unsigned long pages_to_unuse = 0;
453 int type, ret;
454
455 /*
456 * we don't want to hold swap_lock while doing a very
457 * lengthy try_to_unuse, but swap_list may change
458 * so restart scan from swap_active_head each time
459 */
460 spin_lock(&swap_lock);
461 ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
462 spin_unlock(&swap_lock);
463 if (ret == 0)
464 try_to_unuse(type, true, pages_to_unuse);
465 return;
466 }
467 EXPORT_SYMBOL(frontswap_shrink);
468
469 /*
470 * Count and return the number of frontswap pages across all
471 * swap devices. This is exported so that backend drivers can
472 * determine current usage without reading debugfs.
473 */
frontswap_curr_pages(void)474 unsigned long frontswap_curr_pages(void)
475 {
476 unsigned long totalpages = 0;
477
478 spin_lock(&swap_lock);
479 totalpages = __frontswap_curr_pages();
480 spin_unlock(&swap_lock);
481
482 return totalpages;
483 }
484 EXPORT_SYMBOL(frontswap_curr_pages);
485
init_frontswap(void)486 static int __init init_frontswap(void)
487 {
488 #ifdef CONFIG_DEBUG_FS
489 struct dentry *root = debugfs_create_dir("frontswap", NULL);
490 if (root == NULL)
491 return -ENXIO;
492 debugfs_create_u64("loads", 0444, root, &frontswap_loads);
493 debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
494 debugfs_create_u64("failed_stores", 0444, root,
495 &frontswap_failed_stores);
496 debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);
497 #endif
498 return 0;
499 }
500
501 module_init(init_frontswap);
502
