mm/swap_slots.c

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