mm/swap_slots.c

   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
  38 static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
  39 static bool     swap_slot_cache_active;
  40 bool    swap_slot_cache_enabled;
  41 static bool     swap_slot_cache_initialized;
  42 static DEFINE_MUTEX(swap_slots_cache_mutex);
  43 /* Serialize swap slots cache enable/disable operations */
  44 static DEFINE_MUTEX(swap_slots_cache_enable_mutex);
  45
  46 static 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
  52 static 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
  60 static 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 */
  68 void 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
  80 static void __reenable_swap_slots_cache(void)
  81 {
  82         swap_slot_cache_enabled = has_usable_swap();
  83 }
  84
  85 void reenable_swap_slots_cache_unlock(void)
  86 {
  87         __reenable_swap_slots_cache();
  88         mutex_unlock(&swap_slots_cache_enable_mutex);
  89 }
  90
  91 static 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();
 109 out:
 110         return swap_slot_cache_active;
 111 }
 112
 113 static 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
 168 static 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
 199 static 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
 230 static 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
 238 void 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();
 254 out_unlock:
 255         mutex_unlock(&swap_slots_cache_enable_mutex);
 256 }
 257
 258 /* called with swap slot cache's alloc lock held */
 259 static 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, 0);
 268
 269         return cache->nr;
 270 }
 271
 272 void free_swap_slot(swp_entry_t entry)
 273 {
 274         struct swap_slots_cache *cache;
 275
 276         /* Large folio swap slot is not covered. */
 277         zswap_invalidate(entry);
 278
 279         cache = raw_cpu_ptr(&swp_slots);
 280         if (likely(use_swap_slot_cache && cache->slots_ret)) {
 281                 spin_lock_irq(&cache->free_lock);
 282                 /* Swap slots cache may be deactivated before acquiring lock */
 283                 if (!use_swap_slot_cache || !cache->slots_ret) {
 284                         spin_unlock_irq(&cache->free_lock);
 285                         goto direct_free;
 286                 }
 287                 if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE) {
 288                         /*
 289                          * Return slots to global pool.
 290                          * The current swap_map value is SWAP_HAS_CACHE.
 291                          * Set it to 0 to indicate it is available for
 292                          * allocation in global pool
 293                          */
 294                         swapcache_free_entries(cache->slots_ret, cache->n_ret);
 295                         cache->n_ret = 0;
 296                 }
 297                 cache->slots_ret[cache->n_ret++] = entry;
 298                 spin_unlock_irq(&cache->free_lock);
 299         } else {
 300 direct_free:
 301                 swapcache_free_entries(&entry, 1);
 302         }
 303 }
 304
 305 swp_entry_t folio_alloc_swap(struct folio *folio)
 306 {
 307         swp_entry_t entry;
 308         struct swap_slots_cache *cache;
 309
 310         entry.val = 0;
 311
 312         if (folio_test_large(folio)) {
 313                 if (IS_ENABLED(CONFIG_THP_SWAP))
 314                         get_swap_pages(1, &entry, folio_order(folio));
 315                 goto out;
 316         }
 317
 318         /*
 319          * Preemption is allowed here, because we may sleep
 320          * in refill_swap_slots_cache().  But it is safe, because
 321          * accesses to the per-CPU data structure are protected by the
 322          * mutex cache->alloc_lock.
 323          *
 324          * The alloc path here does not touch cache->slots_ret
 325          * so cache->free_lock is not taken.
 326          */
 327         cache = raw_cpu_ptr(&swp_slots);
 328
 329         if (likely(check_cache_active() && cache->slots)) {
 330                 mutex_lock(&cache->alloc_lock);
 331                 if (cache->slots) {
 332 repeat:
 333                         if (cache->nr) {
 334                                 entry = cache->slots[cache->cur];
 335                                 cache->slots[cache->cur++].val = 0;
 336                                 cache->nr--;
 337                         } else if (refill_swap_slots_cache(cache)) {
 338                                 goto repeat;
 339                         }
 340                 }
 341                 mutex_unlock(&cache->alloc_lock);
 342                 if (entry.val)
 343                         goto out;
 344         }
 345
 346         get_swap_pages(1, &entry, 0);
 347 out:
 348         if (mem_cgroup_try_charge_swap(folio, entry)) {
 349                 put_swap_folio(folio, entry);
 350                 entry.val = 0;
 351         }
 352         return entry;
 353 }