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         /* More uncharges than charges? */
  56         if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
  57                       new, nr_pages)) {
  58                 new = 0;
  59                 atomic_long_set(&counter->usage, new);
  60         }
  61         propagate_protected_usage(counter, new);
  62 }
  63
  64 /**
  65  * page_counter_charge - hierarchically charge pages
  66  * @counter: counter
  67  * @nr_pages: number of pages to charge
  68  *
  69  * NOTE: This does not consider any configured counter limits.
  70  */
  71 void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
  72 {
  73         struct page_counter *c;
  74
  75         for (c = counter; c; c = c->parent) {
  76                 long new;
  77
  78                 new = atomic_long_add_return(nr_pages, &c->usage);
  79                 propagate_protected_usage(c, new);
  80                 /*
  81                  * This is indeed racy, but we can live with some
  82                  * inaccuracy in the watermark.
  83                  */
  84                 if (new > READ_ONCE(c->watermark))
  85                         WRITE_ONCE(c->watermark, new);
  86         }
  87 }
  88
  89 /**
  90  * page_counter_try_charge - try to hierarchically charge pages
  91  * @counter: counter
  92  * @nr_pages: number of pages to charge
  93  * @fail: points first counter to hit its limit, if any
  94  *
  95  * Returns %true on success, or %false and @fail if the counter or one
  96  * of its ancestors has hit its configured limit.
  97  */
  98 bool page_counter_try_charge(struct page_counter *counter,
  99                              unsigned long nr_pages,
 100                              struct page_counter **fail)
 101 {
 102         struct page_counter *c;
 103
 104         for (c = counter; c; c = c->parent) {
 105                 long new;
 106                 /*
 107                  * Charge speculatively to avoid an expensive CAS.  If
 108                  * a bigger charge fails, it might falsely lock out a
 109                  * racing smaller charge and send it into reclaim
 110                  * early, but the error is limited to the difference
 111                  * between the two sizes, which is less than 2M/4M in
 112                  * case of a THP locking out a regular page charge.
 113                  *
 114                  * The atomic_long_add_return() implies a full memory
 115                  * barrier between incrementing the count and reading
 116                  * the limit.  When racing with page_counter_set_max(),
 117                  * we either see the new limit or the setter sees the
 118                  * counter has changed and retries.
 119                  */
 120                 new = atomic_long_add_return(nr_pages, &c->usage);
 121                 if (new > c->max) {
 122                         atomic_long_sub(nr_pages, &c->usage);
 123                         propagate_protected_usage(c, new);
 124                         /*
 125                          * This is racy, but we can live with some
 126                          * inaccuracy in the failcnt which is only used
 127                          * to report stats.
 128                          */
 129                         data_race(c->failcnt++);
 130                         *fail = c;
 131                         goto failed;
 132                 }
 133                 propagate_protected_usage(c, new);
 134                 /*
 135                  * Just like with failcnt, we can live with some
 136                  * inaccuracy in the watermark.
 137                  */
 138                 if (new > READ_ONCE(c->watermark))
 139                         WRITE_ONCE(c->watermark, new);
 140         }
 141         return true;
 142
 143 failed:
 144         for (c = counter; c != *fail; c = c->parent)
 145                 page_counter_cancel(c, nr_pages);
 146
 147         return false;
 148 }
 149
 150 /**
 151  * page_counter_uncharge - hierarchically uncharge pages
 152  * @counter: counter
 153  * @nr_pages: number of pages to uncharge
 154  */
 155 void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
 156 {
 157         struct page_counter *c;
 158
 159         for (c = counter; c; c = c->parent)
 160                 page_counter_cancel(c, nr_pages);
 161 }
 162
 163 /**
 164  * page_counter_set_max - set the maximum number of pages allowed
 165  * @counter: counter
 166  * @nr_pages: limit to set
 167  *
 168  * Returns 0 on success, -EBUSY if the current number of pages on the
 169  * counter already exceeds the specified limit.
 170  *
 171  * The caller must serialize invocations on the same counter.
 172  */
 173 int page_counter_set_max(struct page_counter *counter, unsigned long nr_pages)
 174 {
 175         for (;;) {
 176                 unsigned long old;
 177                 long usage;
 178
 179                 /*
 180                  * Update the limit while making sure that it's not
 181                  * below the concurrently-changing counter value.
 182                  *
 183                  * The xchg implies two full memory barriers before
 184                  * and after, so the read-swap-read is ordered and
 185                  * ensures coherency with page_counter_try_charge():
 186                  * that function modifies the count before checking
 187                  * the limit, so if it sees the old limit, we see the
 188                  * modified counter and retry.
 189                  */
 190                 usage = page_counter_read(counter);
 191
 192                 if (usage > nr_pages)
 193                         return -EBUSY;
 194
 195                 old = xchg(&counter->max, nr_pages);
 196
 197                 if (page_counter_read(counter) <= usage)
 198                         return 0;
 199
 200                 counter->max = old;
 201                 cond_resched();
 202         }
 203 }
 204
 205 /**
 206  * page_counter_set_min - set the amount of protected memory
 207  * @counter: counter
 208  * @nr_pages: value to set
 209  *
 210  * The caller must serialize invocations on the same counter.
 211  */
 212 void page_counter_set_min(struct page_counter *counter, unsigned long nr_pages)
 213 {
 214         struct page_counter *c;
 215
 216         WRITE_ONCE(counter->min, nr_pages);
 217
 218         for (c = counter; c; c = c->parent)
 219                 propagate_protected_usage(c, atomic_long_read(&c->usage));
 220 }
 221
 222 /**
 223  * page_counter_set_low - set the amount of protected memory
 224  * @counter: counter
 225  * @nr_pages: value to set
 226  *
 227  * The caller must serialize invocations on the same counter.
 228  */
 229 void page_counter_set_low(struct page_counter *counter, unsigned long nr_pages)
 230 {
 231         struct page_counter *c;
 232
 233         WRITE_ONCE(counter->low, nr_pages);
 234
 235         for (c = counter; c; c = c->parent)
 236                 propagate_protected_usage(c, atomic_long_read(&c->usage));
 237 }
 238
 239 /**
 240  * page_counter_memparse - memparse() for page counter limits
 241  * @buf: string to parse
 242  * @max: string meaning maximum possible value
 243  * @nr_pages: returns the result in number of pages
 244  *
 245  * Returns -EINVAL, or 0 and @nr_pages on success.  @nr_pages will be
 246  * limited to %PAGE_COUNTER_MAX.
 247  */
 248 int page_counter_memparse(const char *buf, const char *max,
 249                           unsigned long *nr_pages)
 250 {
 251         char *end;
 252         u64 bytes;
 253
 254         if (!strcmp(buf, max)) {
 255                 *nr_pages = PAGE_COUNTER_MAX;
 256                 return 0;
 257         }
 258
 259         bytes = memparse(buf, &end);
 260         if (*end != '\0')
 261                 return -EINVAL;
 262
 263         *nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
 264
 265         return 0;
 266 }