// SPDX-License-Identifier: GPL-2.0
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/*
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* Lockless hierarchical page accounting & limiting
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*
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* Copyright (C) 2014 Red Hat, Inc., Johannes Weiner
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*/
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#include <linux/page_counter.h>
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#include <linux/atomic.h>
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#include <linux/kernel.h>
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#include <linux/string.h>
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#include <linux/sched.h>
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#include <linux/bug.h>
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#include <asm/page.h>
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static void propagate_protected_usage(struct page_counter *c,
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unsigned long usage)
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{
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unsigned long protected, old_protected;
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unsigned long low, min;
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long delta;
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if (!c->parent)
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return;
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min = READ_ONCE(c->min);
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if (min || atomic_long_read(&c->min_usage)) {
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protected = min(usage, min);
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old_protected = atomic_long_xchg(&c->min_usage, protected);
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delta = protected - old_protected;
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if (delta)
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atomic_long_add(delta, &c->parent->children_min_usage);
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}
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low = READ_ONCE(c->low);
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if (low || atomic_long_read(&c->low_usage)) {
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protected = min(usage, low);
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old_protected = atomic_long_xchg(&c->low_usage, protected);
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delta = protected - old_protected;
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if (delta)
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atomic_long_add(delta, &c->parent->children_low_usage);
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}
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}
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/**
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* page_counter_cancel - take pages out of the local counter
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* @counter: counter
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* @nr_pages: number of pages to cancel
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*/
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void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
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{
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long new;
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new = atomic_long_sub_return(nr_pages, &counter->usage);
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propagate_protected_usage(counter, new);
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/* More uncharges than charges? */
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WARN_ON_ONCE(new < 0);
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}
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/**
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* page_counter_charge - hierarchically charge pages
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* @counter: counter
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* @nr_pages: number of pages to charge
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*
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* NOTE: This does not consider any configured counter limits.
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*/
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void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
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{
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struct page_counter *c;
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for (c = counter; c; c = c->parent) {
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long new;
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new = atomic_long_add_return(nr_pages, &c->usage);
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propagate_protected_usage(c, new);
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/*
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* This is indeed racy, but we can live with some
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* inaccuracy in the watermark.
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*/
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if (new > READ_ONCE(c->watermark))
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WRITE_ONCE(c->watermark, new);
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}
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}
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/**
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* page_counter_try_charge - try to hierarchically charge pages
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* @counter: counter
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* @nr_pages: number of pages to charge
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* @fail: points first counter to hit its limit, if any
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*
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* Returns %true on success, or %false and @fail if the counter or one
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* of its ancestors has hit its configured limit.
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*/
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bool page_counter_try_charge(struct page_counter *counter,
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unsigned long nr_pages,
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struct page_counter **fail)
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{
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struct page_counter *c;
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for (c = counter; c; c = c->parent) {
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long new;
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/*
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* Charge speculatively to avoid an expensive CAS. If
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* a bigger charge fails, it might falsely lock out a
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* racing smaller charge and send it into reclaim
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* early, but the error is limited to the difference
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* between the two sizes, which is less than 2M/4M in
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* case of a THP locking out a regular page charge.
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*
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* The atomic_long_add_return() implies a full memory
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* barrier between incrementing the count and reading
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* the limit. When racing with page_counter_set_max(),
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* we either see the new limit or the setter sees the
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* counter has changed and retries.
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*/
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new = atomic_long_add_return(nr_pages, &c->usage);
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if (new > c->max) {
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atomic_long_sub(nr_pages, &c->usage);
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propagate_protected_usage(c, new);
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/*
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* This is racy, but we can live with some
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* inaccuracy in the failcnt which is only used
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* to report stats.
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*/
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data_race(c->failcnt++);
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*fail = c;
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goto failed;
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}
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propagate_protected_usage(c, new);
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/*
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* Just like with failcnt, we can live with some
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* inaccuracy in the watermark.
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*/
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if (new > READ_ONCE(c->watermark))
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WRITE_ONCE(c->watermark, new);
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}
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return true;
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failed:
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for (c = counter; c != *fail; c = c->parent)
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page_counter_cancel(c, nr_pages);
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return false;
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}
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/**
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* page_counter_uncharge - hierarchically uncharge pages
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* @counter: counter
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* @nr_pages: number of pages to uncharge
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*/
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void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
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{
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struct page_counter *c;
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for (c = counter; c; c = c->parent)
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page_counter_cancel(c, nr_pages);
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}
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/**
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* page_counter_set_max - set the maximum number of pages allowed
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* @counter: counter
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* @nr_pages: limit to set
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*
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* Returns 0 on success, -EBUSY if the current number of pages on the
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* counter already exceeds the specified limit.
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*
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* The caller must serialize invocations on the same counter.
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*/
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int page_counter_set_max(struct page_counter *counter, unsigned long nr_pages)
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{
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for (;;) {
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unsigned long old;
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long usage;
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/*
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* Update the limit while making sure that it's not
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* below the concurrently-changing counter value.
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*
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* The xchg implies two full memory barriers before
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* and after, so the read-swap-read is ordered and
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* ensures coherency with page_counter_try_charge():
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* that function modifies the count before checking
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* the limit, so if it sees the old limit, we see the
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* modified counter and retry.
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*/
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usage = atomic_long_read(&counter->usage);
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if (usage > nr_pages)
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return -EBUSY;
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old = xchg(&counter->max, nr_pages);
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if (atomic_long_read(&counter->usage) <= usage)
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return 0;
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counter->max = old;
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cond_resched();
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}
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}
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/**
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* page_counter_set_min - set the amount of protected memory
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* @counter: counter
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* @nr_pages: value to set
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*
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* The caller must serialize invocations on the same counter.
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*/
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void page_counter_set_min(struct page_counter *counter, unsigned long nr_pages)
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{
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struct page_counter *c;
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WRITE_ONCE(counter->min, nr_pages);
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for (c = counter; c; c = c->parent)
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propagate_protected_usage(c, atomic_long_read(&c->usage));
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}
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/**
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* page_counter_set_low - set the amount of protected memory
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* @counter: counter
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* @nr_pages: value to set
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*
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* The caller must serialize invocations on the same counter.
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*/
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void page_counter_set_low(struct page_counter *counter, unsigned long nr_pages)
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{
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struct page_counter *c;
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WRITE_ONCE(counter->low, nr_pages);
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for (c = counter; c; c = c->parent)
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propagate_protected_usage(c, atomic_long_read(&c->usage));
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}
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/**
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* page_counter_memparse - memparse() for page counter limits
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* @buf: string to parse
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* @max: string meaning maximum possible value
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* @nr_pages: returns the result in number of pages
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*
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* Returns -EINVAL, or 0 and @nr_pages on success. @nr_pages will be
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* limited to %PAGE_COUNTER_MAX.
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*/
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int page_counter_memparse(const char *buf, const char *max,
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unsigned long *nr_pages)
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{
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char *end;
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u64 bytes;
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if (!strcmp(buf, max)) {
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*nr_pages = PAGE_COUNTER_MAX;
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return 0;
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}
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bytes = memparse(buf, &end);
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if (*end != '\0')
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return -EINVAL;
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*nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
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return 0;
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}
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