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[98.15.154.102]) by smtp.gmail.com with ESMTPSA id z6sm372234qtq.78.2021.08.19.13.37.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Aug 2021 13:37:20 -0700 (PDT) Date: Thu, 19 Aug 2021 16:38:59 -0400 From: Johannes Weiner To: Michal Hocko Cc: Andrew Morton , Leon Yang , Chris Down , Roman Gushchin , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] mm: memcontrol: fix occasional OOMs due to proportional memory.low reclaim Message-ID: References: <20210817180506.220056-1-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 19, 2021 at 05:01:38PM +0200, Michal Hocko wrote: > 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 > > Signed-off-by: Johannes Weiner > > Acked-by: Michal Hocko Thanks > > 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. You're right, it's a bit too terse. > 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. > */ I've been drafting around with this, but it seems to say the same thing as the comment I put into struct scan_control already: /* * Cgroup memory below memory.low is protected as long as we * don't threaten to OOM. If any cgroup is reclaimed at * reduced force or passed over entirely due to its memory.low * setting (memcg_low_skipped), and nothing is reclaimed as a * result, then go back back for one more cycle that reclaims * the protected memory (memcg_low_reclaim) to avert OOM. */ How about a brief version of this with a pointer to the original? diff --git a/mm/vmscan.c b/mm/vmscan.c index 701106e1829c..c32d686719d5 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2580,7 +2580,12 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc, unsigned long cgroup_size = mem_cgroup_size(memcg); unsigned long protection; - /* memory.low scaling, make sure we retry before OOM */ + /* + * Soft protection must not cause reclaim failure. Let + * the upper level know if we skipped pages during the + * first pass, so it can retry if necessary. See the + * struct scan_control definition of those flags. + */ if (!sc->memcg_low_reclaim && low > min) { protection = low; sc->memcg_low_skipped = 1; @@ -2853,16 +2858,16 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc) if (mem_cgroup_below_min(memcg)) { /* - * Hard protection. - * If there is no reclaimable memory, OOM. + * Hard protection. Always respected. If there is not + * enough reclaimable memory elsewhere, it's an OOM. */ continue; } else if (mem_cgroup_below_low(memcg)) { /* - * Soft protection. - * Respect the protection only as long as - * there is an unprotected supply - * of reclaimable memory from other cgroups. + * Soft protection must not cause reclaim failure. Let + * the upper level know if we skipped pages during the + * first pass, so it can retry if necessary. See the + * struct scan_control definition of those flags. */ if (!sc->memcg_low_reclaim) { sc->memcg_low_skipped = 1;