// SPDX-License-Identifier: GPL-2.0
/*
* 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>
static bool track_protection(struct page_counter *c)
{
return c->protection_support;
}
static void propagate_protected_usage(struct page_counter *c,
unsigned long usage)
{
unsigned long protected, old_protected;
long delta;
if (!c->parent)
return;
protected = min(usage, READ_ONCE(c->min));
old_protected = atomic_long_read(&c->min_usage);
if (protected != old_protected) {
old_protected = atomic_long_xchg(&c->min_usage, protected);
delta = protected - old_protected;
if (delta)
atomic_long_add(delta, &c->parent->children_min_usage);
}
protected = min(usage, READ_ONCE(c->low));
old_protected = atomic_long_read(&c->low_usage);
if (protected != old_protected) {
old_protected = atomic_long_xchg(&c->low_usage, protected);
delta = protected - old_protected;
if (delta)
atomic_long_add(delta, &c->parent->children_low_usage);
}
}
/**
* page_counter_cancel - take pages out of the local counter
* @counter: counter
* @nr_pages: number of pages to cancel
*/
void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
{
long new;
new = atomic_long_sub_return(nr_pages, &counter->usage);
/* More uncharges than charges? */
if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
new, nr_pages)) {
new = 0;
atomic_long_set(&counter->usage, new);
}
if (track_protection(counter))
propagate_protected_usage(counter, new);
}
/**
* 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;
bool protection = track_protection(counter);
for (c = counter; c; c = c->parent) {
long new;
new = atomic_long_add_return(nr_pages, &c->usage);
if (protection)
propagate_protected_usage(c, new);
/*
* This is indeed racy, but we can live with some
* inaccuracy in the watermark.
*
* Notably, we have two watermarks to allow for both a globally
* visible peak and one that can be reset at a smaller scope.
*
* Since we reset both watermarks when the global reset occurs,
* we can guarantee that watermark >= local_watermark, so we
* don't need to do both comparisons every time.
*
* On systems with branch predictors, the inner condition should
* be almost free.
*/
if (new > READ_ONCE(c->local_watermark)) {
WRITE_ONCE(c->local_watermark, new);
if (new > READ_ONCE(c->watermark))
WRITE_ONCE(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 %true on success, or %false and @fail if the counter or one
* of its ancestors has hit its configured limit.
*/
bool page_counter_try_charge(struct page_counter *counter,
unsigned long nr_pages,
struct page_counter **fail)
{
struct page_counter *c;
bool protection = track_protection(counter);
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_set_max(),
* we either see the new limit or the setter sees the
* counter has changed and retries.
*/
new = atomic_long_add_return(nr_pages, &c->usage);
if (new > c->max) {
atomic_long_sub(nr_pages, &c->usage);
/*
* This is racy, but we can live with some
* inaccuracy in the failcnt which is only used
* to report stats.
*/
data_race(c->failcnt++);
*fail = c;
goto failed;
}
if (protection)
propagate_protected_usage(c, new);
/* see comment on page_counter_charge */
if (new > READ_ONCE(c->local_watermark)) {
WRITE_ONCE(c->local_watermark, new);
if (new > READ_ONCE(c->watermark))
WRITE_ONCE(c->watermark, new);
}
}
return true;
failed:
for (c = counter; c != *fail; c = c->parent)
page_counter_cancel(c, nr_pages);
return false;
}
/**
* page_counter_uncharge - hierarchically uncharge pages
* @counter: counter
* @nr_pages: number of pages to uncharge
*/
void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
{
struct page_counter *c;
for (c = counter; c; c = c->parent)
page_counter_cancel(c, nr_pages);
}
/**
* page_counter_set_max - set the maximum number of pages allowed
* @counter: counter
* @nr_pages: 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_set_max(struct page_counter *counter, unsigned long nr_pages)
{
for (;;) {
unsigned long old;
long usage;
/*
* 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.
*/
usage = page_counter_read(counter);
if (usage > nr_pages)
return -EBUSY;
old = xchg(&counter->max, nr_pages);
if (page_counter_read(counter) <= usage || nr_pages >= old)
return 0;
counter->max = old;
cond_resched();
}
}
/**
* page_counter_set_min - set the amount of protected memory
* @counter: counter
* @nr_pages: value to set
*
* The caller must serialize invocations on the same counter.
*/
void page_counter_set_min(struct page_counter *counter, unsigned long nr_pages)
{
struct page_counter *c;
WRITE_ONCE(counter->min, nr_pages);
for (c = counter; c; c = c->parent)
propagate_protected_usage(c, atomic_long_read(&c->usage));
}
/**
* page_counter_set_low - set the amount of protected memory
* @counter: counter
* @nr_pages: value to set
*
* The caller must serialize invocations on the same counter.
*/
void page_counter_set_low(struct page_counter *counter, unsigned long nr_pages)
{
struct page_counter *c;
WRITE_ONCE(counter->low, nr_pages);
for (c = counter; c; c = c->parent)
propagate_protected_usage(c, atomic_long_read(&c->usage));
}
/**
* page_counter_memparse - memparse() for page counter limits
* @buf: string to parse
* @max: string meaning maximum possible value
* @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, const char *max,
unsigned long *nr_pages)
{
char *end;
u64 bytes;
if (!strcmp(buf, max)) {
*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;
}
#ifdef CONFIG_MEMCG
/*
* This function calculates an individual page counter's effective
* protection which is derived from its own memory.min/low, its
* parent's and siblings' settings, as well as the actual memory
* distribution in the tree.
*
* The following rules apply to the effective protection values:
*
* 1. At the first level of reclaim, effective protection is equal to
* the declared protection in memory.min and memory.low.
*
* 2. To enable safe delegation of the protection configuration, at
* subsequent levels the effective protection is capped to the
* parent's effective protection.
*
* 3. To make complex and dynamic subtrees easier to configure, the
* user is allowed to overcommit the declared protection at a given
* level. If that is the case, the parent's effective protection is
* distributed to the children in proportion to how much protection
* they have declared and how much of it they are utilizing.
*
* This makes distribution proportional, but also work-conserving:
* if one counter claims much more protection than it uses memory,
* the unused remainder is available to its siblings.
*
* 4. Conversely, when the declared protection is undercommitted at a
* given level, the distribution of the larger parental protection
* budget is NOT proportional. A counter's protection from a sibling
* is capped to its own memory.min/low setting.
*
* 5. However, to allow protecting recursive subtrees from each other
* without having to declare each individual counter's fixed share
* of the ancestor's claim to protection, any unutilized -
* "floating" - protection from up the tree is distributed in
* proportion to each counter's *usage*. This makes the protection
* neutral wrt sibling cgroups and lets them compete freely over
* the shared parental protection budget, but it protects the
* subtree as a whole from neighboring subtrees.
*
* Note that 4. and 5. are not in conflict: 4. is about protecting
* against immediate siblings whereas 5. is about protecting against
* neighboring subtrees.
*/
static unsigned long effective_protection(unsigned long usage,
unsigned long parent_usage,
unsigned long setting,
unsigned long parent_effective,
unsigned long siblings_protected,
bool recursive_protection)
{
unsigned long protected;
unsigned long ep;
protected = min(usage, setting);
/*
* If all cgroups at this level combined claim and use more
* protection than what the parent affords them, distribute
* shares in proportion to utilization.
*
* We are using actual utilization rather than the statically
* claimed protection in order to be work-conserving: claimed
* but unused protection is available to siblings that would
* otherwise get a smaller chunk than what they claimed.
*/
if (siblings_protected > parent_effective)
return protected * parent_effective / siblings_protected;
/*
* Ok, utilized protection of all children is within what the
* parent affords them, so we know whatever this child claims
* and utilizes is effectively protected.
*
* If there is unprotected usage beyond this value, reclaim
* will apply pressure in proportion to that amount.
*
* If there is unutilized protection, the cgroup will be fully
* shielded from reclaim, but we do return a smaller value for
* protection than what the group could enjoy in theory. This
* is okay. With the overcommit distribution above, effective
* protection is always dependent on how memory is actually
* consumed among the siblings anyway.
*/
ep = protected;
/*
* If the children aren't claiming (all of) the protection
* afforded to them by the parent, distribute the remainder in
* proportion to the (unprotected) memory of each cgroup. That
* way, cgroups that aren't explicitly prioritized wrt each
* other compete freely over the allowance, but they are
* collectively protected from neighboring trees.
*
* We're using unprotected memory for the weight so that if
* some cgroups DO claim explicit protection, we don't protect
* the same bytes twice.
*
* Check both usage and parent_usage against the respective
* protected values. One should imply the other, but they
* aren't read atomically - make sure the division is sane.
*/
if (!recursive_protection)
return ep;
if (parent_effective > siblings_protected &&
parent_usage > siblings_protected &&
usage > protected) {
unsigned long unclaimed;
unclaimed = parent_effective - siblings_protected;
unclaimed *= usage - protected;
unclaimed /= parent_usage - siblings_protected;
ep += unclaimed;
}
return ep;
}
/**
* page_counter_calculate_protection - check if memory consumption is in the normal range
* @root: the top ancestor of the sub-tree being checked
* @counter: the page_counter the counter to update
* @recursive_protection: Whether to use memory_recursiveprot behavior.
*
* Calculates elow/emin thresholds for given page_counter.
*
* WARNING: This function is not stateless! It can only be used as part
* of a top-down tree iteration, not for isolated queries.
*/
void page_counter_calculate_protection(struct page_counter *root,
struct page_counter *counter,
bool recursive_protection)
{
unsigned long usage, parent_usage;
struct page_counter *parent = counter->parent;
/*
* Effective values of the reclaim targets are ignored so they
* can be stale. Have a look at mem_cgroup_protection for more
* details.
* TODO: calculation should be more robust so that we do not need
* that special casing.
*/
if (root == counter)
return;
usage = page_counter_read(counter);
if (!usage)
return;
if (parent == root) {
counter->emin = READ_ONCE(counter->min);
counter->elow = READ_ONCE(counter->low);
return;
}
parent_usage = page_counter_read(parent);
WRITE_ONCE(counter->emin, effective_protection(usage, parent_usage,
READ_ONCE(counter->min),
READ_ONCE(parent->emin),
atomic_long_read(&parent->children_min_usage),
recursive_protection));
WRITE_ONCE(counter->elow, effective_protection(usage, parent_usage,
READ_ONCE(counter->low),
READ_ONCE(parent->elow),
atomic_long_read(&parent->children_low_usage),
recursive_protection));
}
#endif /* CONFIG_MEMCG */
