Re: [PATCH] mm: memcontrol: fix occasional OOMs due to proportional memory.low reclaim

Michal Hocko
2021-08-19T15:01:38Z

On Tue 17-08-21 14:05:06, Johannes Weiner wrote:
> We've noticed occasional OOM killing when memory.low settings are in
> effect for cgroups. This is unexpected and undesirable as memory.low
> is supposed to express non-OOMing memory priorities between cgroups.
>
> The reason for this is proportional memory.low reclaim. When cgroups
> are below their memory.low threshold, reclaim passes them over in the
> first round, and then retries if it couldn't find pages anywhere else.
> But when cgroups are slighly above their memory.low setting, page scan
> force is scaled down and diminished in proportion to the overage, to
> the point where it can cause reclaim to fail as well - only in that
> case we currently don't retry, and instead trigger OOM.
>
> To fix this, hook proportional reclaim into the same retry logic we
> have in place for when cgroups are skipped entirely. This way if
> reclaim fails and some cgroups were scanned with dimished pressure,
> we'll try another full-force cycle before giving up and OOMing.
>
> Reported-by: Leon Yang <lnyng@fb.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.com>

Although I have to say that the code is quite tricky and it deserves
more comments. See below.

[...]
> @@ -2576,6 +2578,15 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> * hard protection.
> */
> unsigned long cgroup_size = mem_cgroup_size(memcg);
> + unsigned long protection;
> +
> + /* memory.low scaling, make sure we retry before OOM */
> + if (!sc->memcg_low_reclaim && low > min) {
> + protection = low;
> + sc->memcg_low_skipped = 1;
> + } else {
> + protection = min;
> + }

Just by looking at this in isolation one could be really curious how
does this not break the low memory protection altogether. The logic is
spread over 3 different places.

Would something like the following be more understandable?

/*
* Low limit protected memcgs are already excluded at
* a higher level (shrink_node_memcgs) but scaling
* down the reclaim target can result in hard to
* reclaim and premature OOM. We do not have a full
* picture here so we cannot really judge this
* sutuation here but pro-actively flag this scenario
* and let do_try_to_free_pages to retry if
* there is no progress.
*/
>
> /* Avoid TOCTOU with earlier protection check */
> cgroup_size = max(cgroup_size, protection);
> --
> 2.32.0

--
Michal Hocko
SUSE Labs