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