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
16static 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(v: &c->min_usage);
27 if (protected != old_protected) {
28 old_protected = atomic_long_xchg(v: &c->min_usage, new: protected);
29 delta = protected - old_protected;
30 if (delta)
31 atomic_long_add(i: delta, v: &c->parent->children_min_usage);
32 }
33
34 protected = min(usage, READ_ONCE(c->low));
35 old_protected = atomic_long_read(v: &c->low_usage);
36 if (protected != old_protected) {
37 old_protected = atomic_long_xchg(v: &c->low_usage, new: protected);
38 delta = protected - old_protected;
39 if (delta)
40 atomic_long_add(i: delta, v: &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 */
49void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
50{
51 long new;
52
53 new = atomic_long_sub_return(i: nr_pages, v: &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(v: &counter->usage, i: new);
59 }
60 propagate_protected_usage(c: counter, usage: 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 */
70void 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(i: nr_pages, v: &c->usage);
78 propagate_protected_usage(c, usage: 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 */
97bool 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(i: nr_pages, v: &c->usage);
120 if (new > c->max) {
121 atomic_long_sub(i: nr_pages, v: &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, usage: 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
141failed:
142 for (c = counter; c != *fail; c = c->parent)
143 page_counter_cancel(counter: 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 */
153void 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(counter: 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 */
171int 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 */
210void 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, usage: atomic_long_read(v: &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 */
227void 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, usage: atomic_long_read(v: &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 */
246int 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(ptr: buf, retptr: &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}
265

source code of linux/mm/page_counter.c