linux-ext4 - Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP

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Date:	Wed, 01 Jul 2015 10:30:36 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>, Nikolay Borisov <kernel@...p.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>, Marian Marinov <mm@...com>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS
 allocations

On 07/01/2015 09:37 AM, Michal Hocko wrote:

> Fix this issue by limiting the wait to reclaim triggered by __GFP_FS
> allocations to make sure we are not called from filesystem paths which
> might be doing exactly this kind of IO optimizations. The page fault
> path, which is the only path that triggers memcg oom killer since 3.12,
> shouldn't require GFP_NOFS and so we shouldn't reintroduce the premature
> OOM killer issue which was originally addressed by the heuristic.
> 
> As per David Chinner the xfs is doing similar thing since 2.6.15 already
> so ext4 is not the only affected filesystem. Moreover he notes:
> : For example: IO completion might require unwritten extent conversion
> : which executes filesystem transactions and GFP_NOFS allocations. The
> : writeback flag on the pages can not be cleared until unwritten
> : extent conversion completes. Hence memory reclaim cannot wait on
> : page writeback to complete in GFP_NOFS context because it is not
> : safe to do so, memcg reclaim or otherwise.

I remember fixing something like this back in the 2.2
days. Funny how these bugs keep coming back.

> Cc: stable # 3.6+
> Fixes: c3b94f44fcb0 ("memcg: further prevent OOM with too many dirty pages")
> Reported-by: Nikolay Borisov <kernel@...p.com>
> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Reviewed-by: Rik van Riel <riel@...hat.com>

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