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Linux/mm/frontswap.c

  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.txt 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 
 23 DEFINE_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  */
 31 static 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  */
 44 static 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  */
 51 static 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  */
 59 static u64 frontswap_loads;
 60 static u64 frontswap_succ_stores;
 61 static u64 frontswap_failed_stores;
 62 static u64 frontswap_invalidates;
 63 
 64 static inline void inc_frontswap_loads(void) {
 65         frontswap_loads++;
 66 }
 67 static inline void inc_frontswap_succ_stores(void) {
 68         frontswap_succ_stores++;
 69 }
 70 static inline void inc_frontswap_failed_stores(void) {
 71         frontswap_failed_stores++;
 72 }
 73 static inline void inc_frontswap_invalidates(void) {
 74         frontswap_invalidates++;
 75 }
 76 #else
 77 static inline void inc_frontswap_loads(void) { }
 78 static inline void inc_frontswap_succ_stores(void) { }
 79 static inline void inc_frontswap_failed_stores(void) { }
 80 static 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  */
114 void 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 }
168 EXPORT_SYMBOL(frontswap_register_ops);
169 
170 /*
171  * Enable/disable frontswap writethrough (see above).
172  */
173 void frontswap_writethrough(bool enable)
174 {
175         frontswap_writethrough_enabled = enable;
176 }
177 EXPORT_SYMBOL(frontswap_writethrough);
178 
179 /*
180  * Enable/disable frontswap exclusive gets (see above).
181  */
182 void frontswap_tmem_exclusive_gets(bool enable)
183 {
184         frontswap_tmem_exclusive_gets_enabled = enable;
185 }
186 EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
187 
188 /*
189  * Called when a swap device is swapon'd.
190  */
191 void __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 }
214 EXPORT_SYMBOL(__frontswap_init);
215 
216 bool __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 }
223 EXPORT_SYMBOL(__frontswap_test);
224 
225 static 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 
232 static 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  */
246 int __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 }
288 EXPORT_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  */
295 int __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 }
326 EXPORT_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  */
332 void __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 }
348 EXPORT_SYMBOL(__frontswap_invalidate_page);
349 
350 /*
351  * Invalidate all data from frontswap associated with all offsets for the
352  * specified swaptype.
353  */
354 void __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 }
370 EXPORT_SYMBOL(__frontswap_invalidate_area);
371 
372 static 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 
383 static 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  */
421 static 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  */
447 void 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 }
464 EXPORT_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  */
471 unsigned 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 }
481 EXPORT_SYMBOL(frontswap_curr_pages);
482 
483 static 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", S_IRUGO, root, &frontswap_loads);
490         debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
491         debugfs_create_u64("failed_stores", S_IRUGO, root,
492                                 &frontswap_failed_stores);
493         debugfs_create_u64("invalidates", S_IRUGO,
494                                 root, &frontswap_invalidates);
495 #endif
496         return 0;
497 }
498 
499 module_init(init_frontswap);
500 

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