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