mm/page_counter.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Lockless hierarchical page accounting & limiting
   4  *
   5  * Copyright (C) 2014 Red Hat, Inc., Johannes Weiner
   6  */
   7
   8 #include <linux/page_counter.h>
   9 #include <linux/atomic.h>
  10 #include <linux/kernel.h>
  11 #include <linux/string.h>
  12 #include <linux/sched.h>
  13 #include <linux/bug.h>
  14 #include <asm/page.h>
  15
  16 static void propagate_protected_usage(struct page_counter *c,
  17                                       unsigned long usage)
  18 {
  19         unsigned long protected, old_protected;
  20         long delta;
  21
  22         if (!c->parent)
  23                 return;
  24
  25         protected = min(usage, READ_ONCE(c->min));
  26         old_protected = atomic_long_read(&c->min_usage);
  27         if (protected != old_protected) {
  28                 old_protected = atomic_long_xchg(&c->min_usage, protected);
  29                 delta = protected - old_protected;
  30                 if (delta)
  31                         atomic_long_add(delta, &c->parent->children_min_usage);
  32         }
  33
  34         protected = min(usage, READ_ONCE(c->low));
  35         old_protected = atomic_long_read(&c->low_usage);
  36         if (protected != old_protected) {
  37                 old_protected = atomic_long_xchg(&c->low_usage, protected);
  38                 delta = protected - old_protected;
  39                 if (delta)
  40                         atomic_long_add(delta, &c->parent->children_low_usage);
  41         }
  42 }
  43
  44 /**
  45  * page_counter_cancel - take pages out of the local counter
  46  * @counter: counter
  47  * @nr_pages: number of pages to cancel
  48  */
  49 void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
  50 {
  51         long new;
  52
  53         new = atomic_long_sub_return(nr_pages, &counter->usage);
  54         /* More uncharges than charges? */
  55         if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
  56                       new, nr_pages)) {
  57                 new = 0;
  58                 atomic_long_set(&counter->usage, new);
  59         }
  60         propagate_protected_usage(counter, new);
  61 }
  62
  63 /**
  64  * page_counter_charge - hierarchically charge pages
  65  * @counter: counter
  66  * @nr_pages: number of pages to charge
  67  *
  68  * NOTE: This does not consider any configured counter limits.
  69  */
  70 void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
  71 {
  72         struct page_counter *c;
  73
  74         for (c = counter; c; c = c->parent) {
  75                 long new;
  76
  77                 new = atomic_long_add_return(nr_pages, &c->usage);
  78                 propagate_protected_usage(c, new);
  79                 /*
  80                  * This is indeed racy, but we can live with some
  81                  * inaccuracy in the watermark.
  82                  */
  83                 if (new > READ_ONCE(c->watermark))
  84                         WRITE_ONCE(c->watermark, new);
  85         }
  86 }
  87
  88 /**
  89  * page_counter_try_charge - try to hierarchically charge pages
  90  * @counter: counter
  91  * @nr_pages: number of pages to charge
  92  * @fail: points first counter to hit its limit, if any
  93  *
  94  * Returns %true on success, or %false and @fail if the counter or one
  95  * of its ancestors has hit its configured limit.
  96  */
  97 bool page_counter_try_charge(struct page_counter *counter,
  98                              unsigned long nr_pages,
  99                              struct page_counter **fail)
 100 {
 101         struct page_counter *c;
 102
 103         for (c = counter; c; c = c->parent) {
 104                 long new;
 105                 /*
 106                  * Charge speculatively to avoid an expensive CAS.  If
 107                  * a bigger charge fails, it might falsely lock out a
 108                  * racing smaller charge and send it into reclaim
 109                  * early, but the error is limited to the difference
 110                  * between the two sizes, which is less than 2M/4M in
 111                  * case of a THP locking out a regular page charge.
 112                  *
 113                  * The atomic_long_add_return() implies a full memory
 114                  * barrier between incrementing the count and reading
 115                  * the limit.  When racing with page_counter_set_max(),
 116                  * we either see the new limit or the setter sees the
 117                  * counter has changed and retries.
 118                  */
 119                 new = atomic_long_add_return(nr_pages, &c->usage);
 120                 if (new > c->max) {
 121                         atomic_long_sub(nr_pages, &c->usage);
 122                         /*
 123                          * This is racy, but we can live with some
 124                          * inaccuracy in the failcnt which is only used
 125                          * to report stats.
 126                          */
 127                         data_race(c->failcnt++);
 128                         *fail = c;
 129                         goto failed;
 130                 }
 131                 propagate_protected_usage(c, new);
 132                 /*
 133                  * Just like with failcnt, we can live with some
 134                  * inaccuracy in the watermark.
 135                  */
 136                 if (new > READ_ONCE(c->watermark))
 137                         WRITE_ONCE(c->watermark, new);
 138         }
 139         return true;
 140
 141 failed:
 142         for (c = counter; c != *fail; c = c->parent)
 143                 page_counter_cancel(c, nr_pages);
 144
 145         return false;
 146 }
 147
 148 /**
 149  * page_counter_uncharge - hierarchically uncharge pages
 150  * @counter: counter
 151  * @nr_pages: number of pages to uncharge
 152  */
 153 void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
 154 {
 155         struct page_counter *c;
 156
 157         for (c = counter; c; c = c->parent)
 158                 page_counter_cancel(c, nr_pages);
 159 }
 160
 161 /**
 162  * page_counter_set_max - set the maximum number of pages allowed
 163  * @counter: counter
 164  * @nr_pages: limit to set
 165  *
 166  * Returns 0 on success, -EBUSY if the current number of pages on the
 167  * counter already exceeds the specified limit.
 168  *
 169  * The caller must serialize invocations on the same counter.
 170  */
 171 int page_counter_set_max(struct page_counter *counter, unsigned long nr_pages)
 172 {
 173         for (;;) {
 174                 unsigned long old;
 175                 long usage;
 176
 177                 /*
 178                  * Update the limit while making sure that it's not
 179                  * below the concurrently-changing counter value.
 180                  *
 181                  * The xchg implies two full memory barriers before
 182                  * and after, so the read-swap-read is ordered and
 183                  * ensures coherency with page_counter_try_charge():
 184                  * that function modifies the count before checking
 185                  * the limit, so if it sees the old limit, we see the
 186                  * modified counter and retry.
 187                  */
 188                 usage = page_counter_read(counter);
 189
 190                 if (usage > nr_pages)
 191                         return -EBUSY;
 192
 193                 old = xchg(&counter->max, nr_pages);
 194
 195                 if (page_counter_read(counter) <= usage || nr_pages >= old)
 196                         return 0;
 197
 198                 counter->max = old;
 199                 cond_resched();
 200         }
 201 }
 202
 203 /**
 204  * page_counter_set_min - set the amount of protected memory
 205  * @counter: counter
 206  * @nr_pages: value to set
 207  *
 208  * The caller must serialize invocations on the same counter.
 209  */
 210 void page_counter_set_min(struct page_counter *counter, unsigned long nr_pages)
 211 {
 212         struct page_counter *c;
 213
 214         WRITE_ONCE(counter->min, nr_pages);
 215
 216         for (c = counter; c; c = c->parent)
 217                 propagate_protected_usage(c, atomic_long_read(&c->usage));
 218 }
 219
 220 /**
 221  * page_counter_set_low - set the amount of protected memory
 222  * @counter: counter
 223  * @nr_pages: value to set
 224  *
 225  * The caller must serialize invocations on the same counter.
 226  */
 227 void page_counter_set_low(struct page_counter *counter, unsigned long nr_pages)
 228 {
 229         struct page_counter *c;
 230
 231         WRITE_ONCE(counter->low, nr_pages);
 232
 233         for (c = counter; c; c = c->parent)
 234                 propagate_protected_usage(c, atomic_long_read(&c->usage));
 235 }
 236
 237 /**
 238  * page_counter_memparse - memparse() for page counter limits
 239  * @buf: string to parse
 240  * @max: string meaning maximum possible value
 241  * @nr_pages: returns the result in number of pages
 242  *
 243  * Returns -EINVAL, or 0 and @nr_pages on success.  @nr_pages will be
 244  * limited to %PAGE_COUNTER_MAX.
 245  */
 246 int page_counter_memparse(const char *buf, const char *max,
 247                           unsigned long *nr_pages)
 248 {
 249         char *end;
 250         u64 bytes;
 251
 252         if (!strcmp(buf, max)) {
 253                 *nr_pages = PAGE_COUNTER_MAX;
 254                 return 0;
 255         }
 256
 257         bytes = memparse(buf, &end);
 258         if (*end != '\0')
 259                 return -EINVAL;
 260
 261         *nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
 262
 263         return 0;
 264 }