xref: /openbmc/linux/mm/page_counter.c (revision 7e6f7d24) 
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 	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  */
55 void 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  */
72 void 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  */
99 bool 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 
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 = 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  */
212 void 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  */
229 void 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  */
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 }
267