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