1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Manage cache of swap slots to be used for and returned from
4 * swap.
5 *
6 * Copyright(c) 2016 Intel Corporation.
7 *
8 * Author: Tim Chen <tim.c.chen@linux.intel.com>
9 *
10 * We allocate the swap slots from the global pool and put
11 * it into local per cpu caches. This has the advantage
12 * of no needing to acquire the swap_info lock every time
13 * we need a new slot.
14 *
15 * There is also opportunity to simply return the slot
16 * to local caches without needing to acquire swap_info
17 * lock. We do not reuse the returned slots directly but
18 * move them back to the global pool in a batch. This
19 * allows the slots to coaellesce and reduce fragmentation.
20 *
21 * The swap entry allocated is marked with SWAP_HAS_CACHE
22 * flag in map_count that prevents it from being allocated
23 * again from the global pool.
24 *
25 * The swap slots cache is protected by a mutex instead of
26 * a spin lock as when we search for slots with scan_swap_map,
27 * we can possibly sleep.
28 */
29
30 #include <linux/swap_slots.h>
31 #include <linux/cpu.h>
32 #include <linux/cpumask.h>
33 #include <linux/vmalloc.h>
34 #include <linux/mutex.h>
35 #include <linux/mm.h>
36
37 static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
38 static bool swap_slot_cache_active;
39 bool swap_slot_cache_enabled;
40 static bool swap_slot_cache_initialized;
41 static DEFINE_MUTEX(swap_slots_cache_mutex);
42 /* Serialize swap slots cache enable/disable operations */
43 static DEFINE_MUTEX(swap_slots_cache_enable_mutex);
44
45 static void __drain_swap_slots_cache(unsigned int type);
46 static void deactivate_swap_slots_cache(void);
47 static void reactivate_swap_slots_cache(void);
48
49 #define use_swap_slot_cache (swap_slot_cache_active && \
50 swap_slot_cache_enabled && swap_slot_cache_initialized)
51 #define SLOTS_CACHE 0x1
52 #define SLOTS_CACHE_RET 0x2
53
deactivate_swap_slots_cache(void)54 static void deactivate_swap_slots_cache(void)
55 {
56 mutex_lock(&swap_slots_cache_mutex);
57 swap_slot_cache_active = false;
58 __drain_swap_slots_cache(SLOTS_CACHE|SLOTS_CACHE_RET);
59 mutex_unlock(&swap_slots_cache_mutex);
60 }
61
reactivate_swap_slots_cache(void)62 static void reactivate_swap_slots_cache(void)
63 {
64 mutex_lock(&swap_slots_cache_mutex);
65 swap_slot_cache_active = true;
66 mutex_unlock(&swap_slots_cache_mutex);
67 }
68
69 /* Must not be called with cpu hot plug lock */
disable_swap_slots_cache_lock(void)70 void disable_swap_slots_cache_lock(void)
71 {
72 mutex_lock(&swap_slots_cache_enable_mutex);
73 swap_slot_cache_enabled = false;
74 if (swap_slot_cache_initialized) {
75 /* serialize with cpu hotplug operations */
76 get_online_cpus();
77 __drain_swap_slots_cache(SLOTS_CACHE|SLOTS_CACHE_RET);
78 put_online_cpus();
79 }
80 }
81
__reenable_swap_slots_cache(void)82 static void __reenable_swap_slots_cache(void)
83 {
84 swap_slot_cache_enabled = has_usable_swap();
85 }
86
reenable_swap_slots_cache_unlock(void)87 void reenable_swap_slots_cache_unlock(void)
88 {
89 __reenable_swap_slots_cache();
90 mutex_unlock(&swap_slots_cache_enable_mutex);
91 }
92
check_cache_active(void)93 static bool check_cache_active(void)
94 {
95 long pages;
96
97 if (!swap_slot_cache_enabled || !swap_slot_cache_initialized)
98 return false;
99
100 pages = get_nr_swap_pages();
101 if (!swap_slot_cache_active) {
102 if (pages > num_online_cpus() *
103 THRESHOLD_ACTIVATE_SWAP_SLOTS_CACHE)
104 reactivate_swap_slots_cache();
105 goto out;
106 }
107
108 /* if global pool of slot caches too low, deactivate cache */
109 if (pages < num_online_cpus() * THRESHOLD_DEACTIVATE_SWAP_SLOTS_CACHE)
110 deactivate_swap_slots_cache();
111 out:
112 return swap_slot_cache_active;
113 }
114
alloc_swap_slot_cache(unsigned int cpu)115 static int alloc_swap_slot_cache(unsigned int cpu)
116 {
117 struct swap_slots_cache *cache;
118 swp_entry_t *slots, *slots_ret;
119
120 /*
121 * Do allocation outside swap_slots_cache_mutex
122 * as kvzalloc could trigger reclaim and get_swap_page,
123 * which can lock swap_slots_cache_mutex.
124 */
125 slots = kvcalloc(SWAP_SLOTS_CACHE_SIZE, sizeof(swp_entry_t),
126 GFP_KERNEL);
127 if (!slots)
128 return -ENOMEM;
129
130 slots_ret = kvcalloc(SWAP_SLOTS_CACHE_SIZE, sizeof(swp_entry_t),
131 GFP_KERNEL);
132 if (!slots_ret) {
133 kvfree(slots);
134 return -ENOMEM;
135 }
136
137 mutex_lock(&swap_slots_cache_mutex);
138 cache = &per_cpu(swp_slots, cpu);
139 if (cache->slots || cache->slots_ret)
140 /* cache already allocated */
141 goto out;
142 if (!cache->lock_initialized) {
143 mutex_init(&cache->alloc_lock);
144 spin_lock_init(&cache->free_lock);
145 cache->lock_initialized = true;
146 }
147 cache->nr = 0;
148 cache->cur = 0;
149 cache->n_ret = 0;
150 /*
151 * We initialized alloc_lock and free_lock earlier. We use
152 * !cache->slots or !cache->slots_ret to know if it is safe to acquire
153 * the corresponding lock and use the cache. Memory barrier below
154 * ensures the assumption.
155 */
156 mb();
157 cache->slots = slots;
158 slots = NULL;
159 cache->slots_ret = slots_ret;
160 slots_ret = NULL;
161 out:
162 mutex_unlock(&swap_slots_cache_mutex);
163 if (slots)
164 kvfree(slots);
165 if (slots_ret)
166 kvfree(slots_ret);
167 return 0;
168 }
169
drain_slots_cache_cpu(unsigned int cpu,unsigned int type,bool free_slots)170 static void drain_slots_cache_cpu(unsigned int cpu, unsigned int type,
171 bool free_slots)
172 {
173 struct swap_slots_cache *cache;
174 swp_entry_t *slots = NULL;
175
176 cache = &per_cpu(swp_slots, cpu);
177 if ((type & SLOTS_CACHE) && cache->slots) {
178 mutex_lock(&cache->alloc_lock);
179 swapcache_free_entries(cache->slots + cache->cur, cache->nr);
180 cache->cur = 0;
181 cache->nr = 0;
182 if (free_slots && cache->slots) {
183 kvfree(cache->slots);
184 cache->slots = NULL;
185 }
186 mutex_unlock(&cache->alloc_lock);
187 }
188 if ((type & SLOTS_CACHE_RET) && cache->slots_ret) {
189 spin_lock_irq(&cache->free_lock);
190 swapcache_free_entries(cache->slots_ret, cache->n_ret);
191 cache->n_ret = 0;
192 if (free_slots && cache->slots_ret) {
193 slots = cache->slots_ret;
194 cache->slots_ret = NULL;
195 }
196 spin_unlock_irq(&cache->free_lock);
197 if (slots)
198 kvfree(slots);
199 }
200 }
201
__drain_swap_slots_cache(unsigned int type)202 static void __drain_swap_slots_cache(unsigned int type)
203 {
204 unsigned int cpu;
205
206 /*
207 * This function is called during
208 * 1) swapoff, when we have to make sure no
209 * left over slots are in cache when we remove
210 * a swap device;
211 * 2) disabling of swap slot cache, when we run low
212 * on swap slots when allocating memory and need
213 * to return swap slots to global pool.
214 *
215 * We cannot acquire cpu hot plug lock here as
216 * this function can be invoked in the cpu
217 * hot plug path:
218 * cpu_up -> lock cpu_hotplug -> cpu hotplug state callback
219 * -> memory allocation -> direct reclaim -> get_swap_page
220 * -> drain_swap_slots_cache
221 *
222 * Hence the loop over current online cpu below could miss cpu that
223 * is being brought online but not yet marked as online.
224 * That is okay as we do not schedule and run anything on a
225 * cpu before it has been marked online. Hence, we will not
226 * fill any swap slots in slots cache of such cpu.
227 * There are no slots on such cpu that need to be drained.
228 */
229 for_each_online_cpu(cpu)
230 drain_slots_cache_cpu(cpu, type, false);
231 }
232
free_slot_cache(unsigned int cpu)233 static int free_slot_cache(unsigned int cpu)
234 {
235 mutex_lock(&swap_slots_cache_mutex);
236 drain_slots_cache_cpu(cpu, SLOTS_CACHE | SLOTS_CACHE_RET, true);
237 mutex_unlock(&swap_slots_cache_mutex);
238 return 0;
239 }
240
enable_swap_slots_cache(void)241 int enable_swap_slots_cache(void)
242 {
243 int ret = 0;
244
245 mutex_lock(&swap_slots_cache_enable_mutex);
246 if (swap_slot_cache_initialized) {
247 __reenable_swap_slots_cache();
248 goto out_unlock;
249 }
250
251 ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "swap_slots_cache",
252 alloc_swap_slot_cache, free_slot_cache);
253 if (WARN_ONCE(ret < 0, "Cache allocation failed (%s), operating "
254 "without swap slots cache.\n", __func__))
255 goto out_unlock;
256
257 swap_slot_cache_initialized = true;
258 __reenable_swap_slots_cache();
259 out_unlock:
260 mutex_unlock(&swap_slots_cache_enable_mutex);
261 return 0;
262 }
263
264 /* called with swap slot cache's alloc lock held */
refill_swap_slots_cache(struct swap_slots_cache * cache)265 static int refill_swap_slots_cache(struct swap_slots_cache *cache)
266 {
267 if (!use_swap_slot_cache || cache->nr)
268 return 0;
269
270 cache->cur = 0;
271 if (swap_slot_cache_active)
272 cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE,
273 cache->slots, 1);
274
275 return cache->nr;
276 }
277
free_swap_slot(swp_entry_t entry)278 int free_swap_slot(swp_entry_t entry)
279 {
280 struct swap_slots_cache *cache;
281
282 cache = raw_cpu_ptr(&swp_slots);
283 if (likely(use_swap_slot_cache && cache->slots_ret)) {
284 spin_lock_irq(&cache->free_lock);
285 /* Swap slots cache may be deactivated before acquiring lock */
286 if (!use_swap_slot_cache || !cache->slots_ret) {
287 spin_unlock_irq(&cache->free_lock);
288 goto direct_free;
289 }
290 if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE) {
291 /*
292 * Return slots to global pool.
293 * The current swap_map value is SWAP_HAS_CACHE.
294 * Set it to 0 to indicate it is available for
295 * allocation in global pool
296 */
297 swapcache_free_entries(cache->slots_ret, cache->n_ret);
298 cache->n_ret = 0;
299 }
300 cache->slots_ret[cache->n_ret++] = entry;
301 spin_unlock_irq(&cache->free_lock);
302 } else {
303 direct_free:
304 swapcache_free_entries(&entry, 1);
305 }
306
307 return 0;
308 }
309
get_swap_page(struct page * page)310 swp_entry_t get_swap_page(struct page *page)
311 {
312 swp_entry_t entry, *pentry;
313 struct swap_slots_cache *cache;
314
315 entry.val = 0;
316
317 if (PageTransHuge(page)) {
318 if (IS_ENABLED(CONFIG_THP_SWAP))
319 get_swap_pages(1, &entry, HPAGE_PMD_NR);
320 goto out;
321 }
322
323 /*
324 * Preemption is allowed here, because we may sleep
325 * in refill_swap_slots_cache(). But it is safe, because
326 * accesses to the per-CPU data structure are protected by the
327 * mutex cache->alloc_lock.
328 *
329 * The alloc path here does not touch cache->slots_ret
330 * so cache->free_lock is not taken.
331 */
332 cache = raw_cpu_ptr(&swp_slots);
333
334 if (likely(check_cache_active() && cache->slots)) {
335 mutex_lock(&cache->alloc_lock);
336 if (cache->slots) {
337 repeat:
338 if (cache->nr) {
339 pentry = &cache->slots[cache->cur++];
340 entry = *pentry;
341 pentry->val = 0;
342 cache->nr--;
343 } else {
344 if (refill_swap_slots_cache(cache))
345 goto repeat;
346 }
347 }
348 mutex_unlock(&cache->alloc_lock);
349 if (entry.val)
350 goto out;
351 }
352
353 get_swap_pages(1, &entry, 1);
354 out:
355 if (mem_cgroup_try_charge_swap(page, entry)) {
356 put_swap_page(page, entry);
357 entry.val = 0;
358 }
359 return entry;
360 }
361
