From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] mm: memcontrol: fix occasional OOMs due to proportional memory.low reclaim Date: Fri, 20 Aug 2021 17:44:54 +0200 Message-ID: References: <20210817180506.220056-1-hannes@cmpxchg.org> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1629474295; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ogYunvpZVJ/3GqmU2H+7Y5oXqBsYG05vVrhz9e8RiVA=; b=lp6617sFBwYD2Nii4xorvJxfjYSV8LrclKGB2WTnYrAWOtQilQXMmz/haWyv08xcbodjuG XYmUagb/VEJLqdlM0wrNEZzi1NgMpx5AweF9G0PDh7fS1trGoqV3CCCSy8q6aEsZ2bLdYa p7TbUYkg28fW0xG3j7ydNOkRrrnhMWM= Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Johannes Weiner 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 On Thu 19-08-21 16:38:59, Johannes Weiner wrote: > On Thu, Aug 19, 2021 at 05:01:38PM +0200, Michal Hocko wrote: [...] > > 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; Yes, this makes the situation more explicit. I still see some advantage to be explicit about those other layers as this will be easier to follow the code but I will certainly not insist. Andrew has already sent your original patch to Linus so this will need to go as a separate patch. For that Acked-by: Michal Hocko Thanks! -- Michal Hocko SUSE Labs