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