linux-kernel - Re: [PATCH] mm: vmscan: fix memcg-aware shrinkers not called on global reclaim

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Message-ID: <20160801151508.GB7603@cmpxchg.org>
Date:	Mon, 1 Aug 2016 11:15:08 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Michal Hocko <mhocko@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Minchan Kim <minchan@...nel.org>,
	Rik van Riel <riel@...riel.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmscan: fix memcg-aware shrinkers not called on
 global reclaim

On Mon, Aug 01, 2016 at 04:03:10PM +0300, Vladimir Davydov wrote:
> We must call shrink_slab() for each memory cgroup on both global and
> memcg reclaim in shrink_node_memcg(). Commit d71df22b55099 accidentally
> changed that so that now shrink_slab() is only called with memcg != NULL
> on memcg reclaim. As a result, memcg-aware shrinkers (including
> dentry/inode) are never invoked on global reclaim. Fix that.
> 
> Fixes: d71df22b55099 ("mm, vmscan: begin reclaiming pages on a per-node basis")
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Ouch, I missed that in the review. The change looked so obviously
correct, but the shrink_slab() interface is a little deceiving. It
would be great if shrink_slab() could handle root_mem_cgroup/NULL and
then we'd always call it from inside the loop. But AFAICS we need the
global call to get the cumulative scanned/lru_pages ratio from all of
the memcgs reclaimed... grr. Oh well.

This fix looks correct to me, anyway.

Acked-by: Johannes Weiner <hannes@...xchg.org>