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/*
 24 * frontswap_ops are added by frontswap_register_ops, and provide the
 25 * frontswap "backend" implementation functions.  Multiple implementations
 26 * may be registered, but implementations can never deregister.  This
 27 * is a simple singly-linked list of all registered implementations.
 28 */
 29static struct frontswap_ops *frontswap_ops __read_mostly;
 30
 31#define for_each_frontswap_ops(ops)		\
 32	for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
 33
 34/*
 35 * If enabled, frontswap_store will return failure even on success.  As
 36 * a result, the swap subsystem will always write the page to swap, in
 37 * effect converting frontswap into a writethrough cache.  In this mode,
 38 * there is no direct reduction in swap writes, but a frontswap backend
 39 * can unilaterally "reclaim" any pages in use with no data loss, thus
 40 * providing increases control over maximum memory usage due to frontswap.
 41 */
 42static bool frontswap_writethrough_enabled __read_mostly;
 43
 44/*
 45 * If enabled, the underlying tmem implementation is capable of doing
 46 * exclusive gets, so frontswap_load, on a successful tmem_get must
 47 * mark the page as no longer in frontswap AND mark it dirty.
 48 */
 49static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
 50
 51#ifdef CONFIG_DEBUG_FS
 52/*
 53 * Counters available via /sys/kernel/debug/frontswap (if debugfs is
 54 * properly configured).  These are for information only so are not protected
 55 * against increment races.
 56 */
 57static u64 frontswap_loads;
 58static u64 frontswap_succ_stores;
 59static u64 frontswap_failed_stores;
 60static u64 frontswap_invalidates;
 61
 62static inline void inc_frontswap_loads(void) {
 63	frontswap_loads++;
 64}
 65static inline void inc_frontswap_succ_stores(void) {
 66	frontswap_succ_stores++;
 67}
 68static inline void inc_frontswap_failed_stores(void) {
 69	frontswap_failed_stores++;
 70}
 71static inline void inc_frontswap_invalidates(void) {
 72	frontswap_invalidates++;
 73}
 74#else
 75static inline void inc_frontswap_loads(void) { }
 76static inline void inc_frontswap_succ_stores(void) { }
 77static inline void inc_frontswap_failed_stores(void) { }
 78static inline void inc_frontswap_invalidates(void) { }
 79#endif
 80
 81/*
 82 * Due to the asynchronous nature of the backends loading potentially
 83 * _after_ the swap system has been activated, we have chokepoints
 84 * on all frontswap functions to not call the backend until the backend
 85 * has registered.
 86 *
 87 * This would not guards us against the user deciding to call swapoff right as
 88 * we are calling the backend to initialize (so swapon is in action).
 89 * Fortunatly for us, the swapon_mutex has been taked by the callee so we are
 90 * OK. The other scenario where calls to frontswap_store (called via
 91 * swap_writepage) is racing with frontswap_invalidate_area (called via
 92 * swapoff) is again guarded by the swap subsystem.
 93 *
 94 * While no backend is registered all calls to frontswap_[store|load|
 95 * invalidate_area|invalidate_page] are ignored or fail.
 96 *
 97 * The time between the backend being registered and the swap file system
 98 * calling the backend (via the frontswap_* functions) is indeterminate as
 99 * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
100 * That is OK as we are comfortable missing some of these calls to the newly
101 * registered backend.
102 *
103 * Obviously the opposite (unloading the backend) must be done after all
104 * the frontswap_[store|load|invalidate_area|invalidate_page] start
105 * ignoring or failing the requests.  However, there is currently no way
106 * to unload a backend once it is registered.
107 */
108
109/*
110 * Register operations for frontswap
111 */
112void frontswap_register_ops(struct frontswap_ops *ops)
113{
114	DECLARE_BITMAP(a, MAX_SWAPFILES);
115	DECLARE_BITMAP(b, MAX_SWAPFILES);
116	struct swap_info_struct *si;
117	unsigned int i;
118
119	bitmap_zero(a, MAX_SWAPFILES);
120	bitmap_zero(b, MAX_SWAPFILES);
121
122	spin_lock(&swap_lock);
123	plist_for_each_entry(si, &swap_active_head, list) {
124		if (!WARN_ON(!si->frontswap_map))
125			set_bit(si->type, a);
126	}
127	spin_unlock(&swap_lock);
128
129	/* the new ops needs to know the currently active swap devices */
130	for_each_set_bit(i, a, MAX_SWAPFILES)
131		ops->init(i);
132
133	/*
134	 * Setting frontswap_ops must happen after the ops->init() calls
135	 * above; cmpxchg implies smp_mb() which will ensure the init is
136	 * complete at this point.
137	 */
138	do {
139		ops->next = frontswap_ops;
140	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
141
142	spin_lock(&swap_lock);
143	plist_for_each_entry(si, &swap_active_head, list) {
144		if (si->frontswap_map)
145			set_bit(si->type, b);
146	}
147	spin_unlock(&swap_lock);
148
149	/*
150	 * On the very unlikely chance that a swap device was added or
151	 * removed between setting the "a" list bits and the ops init
152	 * calls, we re-check and do init or invalidate for any changed
153	 * bits.
154	 */
155	if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
156		for (i = 0; i < MAX_SWAPFILES; i++) {
157			if (!test_bit(i, a) && test_bit(i, b))
158				ops->init(i);
159			else if (test_bit(i, a) && !test_bit(i, b))
160				ops->invalidate_area(i);
161		}
162	}
163}
164EXPORT_SYMBOL(frontswap_register_ops);
165
166/*
167 * Enable/disable frontswap writethrough (see above).
168 */
169void frontswap_writethrough(bool enable)
170{
171	frontswap_writethrough_enabled = enable;
172}
173EXPORT_SYMBOL(frontswap_writethrough);
174
175/*
176 * Enable/disable frontswap exclusive gets (see above).
177 */
178void frontswap_tmem_exclusive_gets(bool enable)
179{
180	frontswap_tmem_exclusive_gets_enabled = enable;
181}
182EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
183
184/*
185 * Called when a swap device is swapon'd.
186 */
187void __frontswap_init(unsigned type, unsigned long *map)
188{
189	struct swap_info_struct *sis = swap_info[type];
190	struct frontswap_ops *ops;
191
192	BUG_ON(sis == NULL);
193
194	/*
195	 * p->frontswap is a bitmap that we MUST have to figure out which page
196	 * has gone in frontswap. Without it there is no point of continuing.
197	 */
198	if (WARN_ON(!map))
199		return;
200	/*
201	 * Irregardless of whether the frontswap backend has been loaded
202	 * before this function or it will be later, we _MUST_ have the
203	 * p->frontswap set to something valid to work properly.
204	 */
205	frontswap_map_set(sis, map);
206
207	for_each_frontswap_ops(ops)
208		ops->init(type);
209}
210EXPORT_SYMBOL(__frontswap_init);
211
212bool __frontswap_test(struct swap_info_struct *sis,
213				pgoff_t offset)
214{
215	if (sis->frontswap_map)
216		return test_bit(offset, sis->frontswap_map);
217	return false;
218}
219EXPORT_SYMBOL(__frontswap_test);
220
221static inline void __frontswap_set(struct swap_info_struct *sis,
222				   pgoff_t offset)
223{
224	set_bit(offset, sis->frontswap_map);
225	atomic_inc(&sis->frontswap_pages);
226}
227
228static inline void __frontswap_clear(struct swap_info_struct *sis,
229				     pgoff_t offset)
230{
231	clear_bit(offset, sis->frontswap_map);
232	atomic_dec(&sis->frontswap_pages);
233}
234
235/*
236 * "Store" data from a page to frontswap and associate it with the page's
237 * swaptype and offset.  Page must be locked and in the swap cache.
238 * If frontswap already contains a page with matching swaptype and
239 * offset, the frontswap implementation may either overwrite the data and
240 * return success or invalidate the page from frontswap and return failure.
241 */
242int __frontswap_store(struct page *page)
243{
244	int ret = -1;
245	swp_entry_t entry = { .val = page_private(page), };
246	int type = swp_type(entry);
247	struct swap_info_struct *sis = swap_info[type];
248	pgoff_t offset = swp_offset(entry);
249	struct frontswap_ops *ops;
250
251	/*
252	 * Return if no backend registed.
253	 * Don't need to inc frontswap_failed_stores here.
254	 */
255	if (!frontswap_ops)
256		return -1;
257
258	BUG_ON(!PageLocked(page));
259	BUG_ON(sis == NULL);
260
261	/*
262	 * If a dup, we must remove the old page first; we can't leave the
263	 * old page no matter if the store of the new page succeeds or fails,
264	 * and we can't rely on the new page replacing the old page as we may
265	 * not store to the same implementation that contains the old page.
266	 */
267	if (__frontswap_test(sis, offset)) {
268		__frontswap_clear(sis, offset);
269		for_each_frontswap_ops(ops)
270			ops->invalidate_page(type, offset);
271	}
272
273	/* Try to store in each implementation, until one succeeds. */
274	for_each_frontswap_ops(ops) {
275		ret = ops->store(type, offset, page);
276		if (!ret) /* successful store */
277			break;
278	}
279	if (ret == 0) {
280		__frontswap_set(sis, offset);
281		inc_frontswap_succ_stores();
282	} else {
283		inc_frontswap_failed_stores();
284	}
285	if (frontswap_writethrough_enabled)
286		/* report failure so swap also writes to swap device */
287		ret = -1;
288	return ret;
289}
290EXPORT_SYMBOL(__frontswap_store);
291
292/*
293 * "Get" data from frontswap associated with swaptype and offset that were
294 * specified when the data was put to frontswap and use it to fill the
295 * specified page with data. Page must be locked and in the swap cache.
296 */
297int __frontswap_load(struct page *page)
298{
299	int ret = -1;
300	swp_entry_t entry = { .val = page_private(page), };
301	int type = swp_type(entry);
302	struct swap_info_struct *sis = swap_info[type];
303	pgoff_t offset = swp_offset(entry);
304	struct frontswap_ops *ops;
305
306	if (!frontswap_ops)
307		return -1;
308
309	BUG_ON(!PageLocked(page));
310	BUG_ON(sis == NULL);
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}
329EXPORT_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 */
335void __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	if (!frontswap_ops)
341		return;
342
343	BUG_ON(sis == NULL);
344	if (!__frontswap_test(sis, offset))
345		return;
346
347	for_each_frontswap_ops(ops)
348		ops->invalidate_page(type, offset);
349	__frontswap_clear(sis, offset);
350	inc_frontswap_invalidates();
351}
352EXPORT_SYMBOL(__frontswap_invalidate_page);
353
354/*
355 * Invalidate all data from frontswap associated with all offsets for the
356 * specified swaptype.
357 */
358void __frontswap_invalidate_area(unsigned type)
359{
360	struct swap_info_struct *sis = swap_info[type];
361	struct frontswap_ops *ops;
362
363	if (!frontswap_ops)
364		return;
365
366	BUG_ON(sis == NULL);
367	if (sis->frontswap_map == NULL)
368		return;
369
370	for_each_frontswap_ops(ops)
371		ops->invalidate_area(type);
372	atomic_set(&sis->frontswap_pages, 0);
373	bitmap_zero(sis->frontswap_map, sis->max);
374}
375EXPORT_SYMBOL(__frontswap_invalidate_area);
376
377static unsigned long __frontswap_curr_pages(void)
378{
379	unsigned long totalpages = 0;
380	struct swap_info_struct *si = NULL;
381
382	assert_spin_locked(&swap_lock);
383	plist_for_each_entry(si, &swap_active_head, list)
384		totalpages += atomic_read(&si->frontswap_pages);
385	return totalpages;
386}
387
388static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
389					int *swapid)
390{
391	int ret = -EINVAL;
392	struct swap_info_struct *si = NULL;
393	int si_frontswap_pages;
394	unsigned long total_pages_to_unuse = total;
395	unsigned long pages = 0, pages_to_unuse = 0;
396
397	assert_spin_locked(&swap_lock);
398	plist_for_each_entry(si, &swap_active_head, list) {
399		si_frontswap_pages = atomic_read(&si->frontswap_pages);
400		if (total_pages_to_unuse < si_frontswap_pages) {
401			pages = pages_to_unuse = total_pages_to_unuse;
402		} else {
403			pages = si_frontswap_pages;
404			pages_to_unuse = 0; /* unuse all */
405		}
406		/* ensure there is enough RAM to fetch pages from frontswap */
407		if (security_vm_enough_memory_mm(current->mm, pages)) {
408			ret = -ENOMEM;
409			continue;
410		}
411		vm_unacct_memory(pages);
412		*unused = pages_to_unuse;
413		*swapid = si->type;
414		ret = 0;
415		break;
416	}
417
418	return ret;
419}
420
421/*
422 * Used to check if it's necessory and feasible to unuse pages.
423 * Return 1 when nothing to do, 0 when need to shink pages,
424 * error code when there is an error.
425 */
426static int __frontswap_shrink(unsigned long target_pages,
427				unsigned long *pages_to_unuse,
428				int *type)
429{
430	unsigned long total_pages = 0, total_pages_to_unuse;
431
432	assert_spin_locked(&swap_lock);
433
434	total_pages = __frontswap_curr_pages();
435	if (total_pages <= target_pages) {
436		/* Nothing to do */
437		*pages_to_unuse = 0;
438		return 1;
439	}
440	total_pages_to_unuse = total_pages - target_pages;
441	return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
442}
443
444/*
445 * Frontswap, like a true swap device, may unnecessarily retain pages
446 * under certain circumstances; "shrink" frontswap is essentially a
447 * "partial swapoff" and works by calling try_to_unuse to attempt to
448 * unuse enough frontswap pages to attempt to -- subject to memory
449 * constraints -- reduce the number of pages in frontswap to the
450 * number given in the parameter target_pages.
451 */
452void frontswap_shrink(unsigned long target_pages)
453{
454	unsigned long pages_to_unuse = 0;
455	int uninitialized_var(type), ret;
456
457	/*
458	 * we don't want to hold swap_lock while doing a very
459	 * lengthy try_to_unuse, but swap_list may change
460	 * so restart scan from swap_active_head each time
461	 */
462	spin_lock(&swap_lock);
463	ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
464	spin_unlock(&swap_lock);
465	if (ret == 0)
466		try_to_unuse(type, true, pages_to_unuse);
467	return;
468}
469EXPORT_SYMBOL(frontswap_shrink);
470
471/*
472 * Count and return the number of frontswap pages across all
473 * swap devices.  This is exported so that backend drivers can
474 * determine current usage without reading debugfs.
475 */
476unsigned long frontswap_curr_pages(void)
477{
478	unsigned long totalpages = 0;
479
480	spin_lock(&swap_lock);
481	totalpages = __frontswap_curr_pages();
482	spin_unlock(&swap_lock);
483
484	return totalpages;
485}
486EXPORT_SYMBOL(frontswap_curr_pages);
487
488static int __init init_frontswap(void)
489{
490#ifdef CONFIG_DEBUG_FS
491	struct dentry *root = debugfs_create_dir("frontswap", NULL);
492	if (root == NULL)
493		return -ENXIO;
494	debugfs_create_u64("loads", S_IRUGO, root, &frontswap_loads);
495	debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
496	debugfs_create_u64("failed_stores", S_IRUGO, root,
497				&frontswap_failed_stores);
498	debugfs_create_u64("invalidates", S_IRUGO,
499				root, &frontswap_invalidates);
500#endif
501	return 0;
502}
503
504module_init(init_frontswap);