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}