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/*
* Lockless hierarchical page accounting & limiting
*
* Copyright (C) 2014 Red Hat, Inc., Johannes Weiner
*/
#include <linux/page_counter.h>
#include <linux/atomic.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/sched.h>
#include <linux/bug.h>
#include <asm/page.h>
/**
* page_counter_cancel - take pages out of the local counter
* @counter: counter
* @nr_pages: number of pages to cancel
*
* Returns whether there are remaining pages in the counter.
*/
int page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
{
long new;
new = atomic_long_sub_return(nr_pages, &counter->count);
/* More uncharges than charges? */
WARN_ON_ONCE(new < 0);
return new > 0;
}
/**
* page_counter_charge - hierarchically charge pages
* @counter: counter
* @nr_pages: number of pages to charge
*
* NOTE: This does not consider any configured counter limits.
*/
void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
{
struct page_counter *c;
for (c = counter; c; c = c->parent) {
long new;
new = atomic_long_add_return(nr_pages, &c->count);
/*
* This is indeed racy, but we can live with some
* inaccuracy in the watermark.
*/
if (new > c->watermark)
c->watermark = new;
}
}
/**
* page_counter_try_charge - try to hierarchically charge pages
* @counter: counter
* @nr_pages: number of pages to charge
* @fail: points first counter to hit its limit, if any
*
* Returns 0 on success, or -ENOMEM and @fail if the counter or one of
* its ancestors has hit its configured limit.
*/
int page_counter_try_charge(struct page_counter *counter,
unsigned long nr_pages,
struct page_counter **fail)
{
struct page_counter *c;
for (c = counter; c; c = c->parent) {
long new;
/*
* Charge speculatively to avoid an expensive CAS. If
* a bigger charge fails, it might falsely lock out a
* racing smaller charge and send it into reclaim
* early, but the error is limited to the difference
* between the two sizes, which is less than 2M/4M in
* case of a THP locking out a regular page charge.
*
* The atomic_long_add_return() implies a full memory
* barrier between incrementing the count and reading
* the limit. When racing with page_counter_limit(),
* we either see the new limit or the setter sees the
* counter has changed and retries.
*/
new = atomic_long_add_return(nr_pages, &c->count);
if (new > c->limit) {
atomic_long_sub(nr_pages, &c->count);
/*
* This is racy, but we can live with some
* inaccuracy in the failcnt.
*/
c->failcnt++;
*fail = c;
goto failed;
}
/*
* Just like with failcnt, we can live with some
* inaccuracy in the watermark.
*/
if (new > c->watermark)
c->watermark = new;
}
return 0;
failed:
for (c = counter; c != *fail; c = c->parent)
page_counter_cancel(c, nr_pages);
return -ENOMEM;
}
/**
* page_counter_uncharge - hierarchically uncharge pages
* @counter: counter
* @nr_pages: number of pages to uncharge
*
* Returns whether there are remaining charges in @counter.
*/
int page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
{
struct page_counter *c;
int ret = 1;
for (c = counter; c; c = c->parent) {
int remainder;
remainder = page_counter_cancel(c, nr_pages);
if (c == counter && !remainder)
ret = 0;
}
return ret;
}
/**
* page_counter_limit - limit the number of pages allowed
* @counter: counter
* @limit: limit to set
*
* Returns 0 on success, -EBUSY if the current number of pages on the
* counter already exceeds the specified limit.
*
* The caller must serialize invocations on the same counter.
*/
int page_counter_limit(struct page_counter *counter, unsigned long limit)
{
for (;;) {
unsigned long old;
long count;
/*
* Update the limit while making sure that it's not
* below the concurrently-changing counter value.
*
* The xchg implies two full memory barriers before
* and after, so the read-swap-read is ordered and
* ensures coherency with page_counter_try_charge():
* that function modifies the count before checking
* the limit, so if it sees the old limit, we see the
* modified counter and retry.
*/
count = atomic_long_read(&counter->count);
if (count > limit)
return -EBUSY;
old = xchg(&counter->limit, limit);
if (atomic_long_read(&counter->count) <= count)
return 0;
counter->limit = old;
cond_resched();
}
}
/**
* page_counter_memparse - memparse() for page counter limits
* @buf: string to parse
* @nr_pages: returns the result in number of pages
*
* Returns -EINVAL, or 0 and @nr_pages on success. @nr_pages will be
* limited to %PAGE_COUNTER_MAX.
*/
int page_counter_memparse(const char *buf, unsigned long *nr_pages)
{
char unlimited[] = "-1";
char *end;
u64 bytes;
if (!strncmp(buf, unlimited, sizeof(unlimited))) {
*nr_pages = PAGE_COUNTER_MAX;
return 0;
}
bytes = memparse(buf, &end);
if (*end != '\0')
return -EINVAL;
*nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
return 0;
}
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