Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

Andrey Ryabinin
2018-01-11T12:21:33Z

On 01/11/2018 01:42 PM, Michal Hocko wrote:
> On Wed 10-01-18 15:43:17, Andrey Ryabinin wrote:
> [...]
>> @@ -2506,15 +2480,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
>> if (!ret)
>> break;
>>
>> - try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, !memsw);
>> -
>> - curusage = page_counter_read(counter);
>> - /* Usage is reduced ? */
>> - if (curusage >= oldusage)
>> - retry_count--;
>> - else
>> - oldusage = curusage;
>> - } while (retry_count);
>> + usage = page_counter_read(counter);
>> + if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
>> + GFP_KERNEL, !memsw)) {
>
> If the usage drops below limit in the meantime then you get underflow
> and reclaim the whole memcg. I do not think this is a good idea. This
> can also lead to over reclaim. Why don't you simply stick with the
> original SWAP_CLUSTER_MAX (aka 1 for try_to_free_mem_cgroup_pages)?
>

Because, if new limit is gigabytes bellow the current usage, retrying to set
new limit after reclaiming only 32 pages seems unreasonable.

So, I made this:

From: Andrey Ryabinin <aryabinin-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
Subject: mm-memcg-try-harder-to-decrease-limit_in_bytes-fix

Protect from overreclaim if usage become lower than limit.

Signed-off-by: Andrey Ryabinin <aryabinin-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
---
mm/memcontrol.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4671ae8a8b1a..6120bb619547 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2455,7 +2455,7 @@ static DEFINE_MUTEX(memcg_limit_mutex);
static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
unsigned long limit, bool memsw)
{
- unsigned long usage;
+ unsigned long nr_pages;
bool enlarge = false;
int ret;
bool limits_invariant;
@@ -2487,8 +2487,8 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
if (!ret)
break;

- usage = page_counter_read(counter);
- if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
+ nr_pages = max_t(long, 1, page_counter_read(counter) - limit);
+ if (!try_to_free_mem_cgroup_pages(memcg, nr_pages,
GFP_KERNEL, !memsw)) {
ret = -EBUSY;
break;
--
2.13.6