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