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 if (c->min || atomic_long_read(&c->min_usage)) {
26 if (usage <= c->min)
27 protected = usage;
28 else
29 protected = 0;
30
31 old_protected = atomic_long_xchg(&c->min_usage, protected);
32 delta = protected - old_protected;
33 if (delta)
34 atomic_long_add(delta, &c->parent->children_min_usage);
35 }
36
37 if (c->low || atomic_long_read(&c->low_usage)) {
38 if (usage <= c->low)
39 protected = usage;
40 else
41 protected = 0;
42
43 old_protected = atomic_long_xchg(&c->low_usage, protected);
44 delta = protected - old_protected;
45 if (delta)
46 atomic_long_add(delta, &c->parent->children_low_usage);
47 }
48}
49
50/**
51 * page_counter_cancel - take pages out of the local counter
52 * @counter: counter
53 * @nr_pages: number of pages to cancel
54 */
55void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
56{
57 long new;
58
59 new = atomic_long_sub_return(nr_pages, &counter->usage);
60 propagate_protected_usage(counter, new);
61 /* More uncharges than charges? */
62 WARN_ON_ONCE(new < 0);
63}
64
65/**
66 * page_counter_charge - hierarchically charge pages
67 * @counter: counter
68 * @nr_pages: number of pages to charge
69 *
70 * NOTE: This does not consider any configured counter limits.
71 */
72void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
73{
74 struct page_counter *c;
75
76 for (c = counter; c; c = c->parent) {
77 long new;
78
79 new = atomic_long_add_return(nr_pages, &c->usage);
80 propagate_protected_usage(counter, new);
81 /*
82 * This is indeed racy, but we can live with some
83 * inaccuracy in the watermark.
84 */
85 if (new > c->watermark)
86 c->watermark = new;
87 }
88}
89
90/**
91 * page_counter_try_charge - try to hierarchically charge pages
92 * @counter: counter
93 * @nr_pages: number of pages to charge
94 * @fail: points first counter to hit its limit, if any
95 *
96 * Returns %true on success, or %false and @fail if the counter or one
97 * of its ancestors has hit its configured limit.
98 */
99bool page_counter_try_charge(struct page_counter *counter,
100 unsigned long nr_pages,
101 struct page_counter **fail)
102{
103 struct page_counter *c;
104
105 for (c = counter; c; c = c->parent) {
106 long new;
107 /*
108 * Charge speculatively to avoid an expensive CAS. If
109 * a bigger charge fails, it might falsely lock out a
110 * racing smaller charge and send it into reclaim
111 * early, but the error is limited to the difference
112 * between the two sizes, which is less than 2M/4M in
113 * case of a THP locking out a regular page charge.
114 *
115 * The atomic_long_add_return() implies a full memory
116 * barrier between incrementing the count and reading
117 * the limit. When racing with page_counter_limit(),
118 * we either see the new limit or the setter sees the
119 * counter has changed and retries.
120 */
121 new = atomic_long_add_return(nr_pages, &c->usage);
122 if (new > c->max) {
123 atomic_long_sub(nr_pages, &c->usage);
124 propagate_protected_usage(counter, new);
125 /*
126 * This is racy, but we can live with some
127 * inaccuracy in the failcnt.
128 */
129 c->failcnt++;
130 *fail = c;
131 goto failed;
132 }
133 propagate_protected_usage(counter, new);
134 /*
135 * Just like with failcnt, we can live with some
136 * inaccuracy in the watermark.
137 */
138 if (new > c->watermark)
139 c->watermark = new;
140 }
141 return true;
142
143failed:
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 */
155void 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 */
173int 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 = atomic_long_read(&counter->usage);
191
192 if (usage > nr_pages)
193 return -EBUSY;
194
195 old = xchg(&counter->max, nr_pages);
196
197 if (atomic_long_read(&counter->usage) <= 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 */
212void page_counter_set_min(struct page_counter *counter, unsigned long nr_pages)
213{
214 struct page_counter *c;
215
216 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 */
229void page_counter_set_low(struct page_counter *counter, unsigned long nr_pages)
230{
231 struct page_counter *c;
232
233 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 */
248int 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}
267