From: Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS
 allocations
Date: Wed, 01 Jul 2015 10:30:36 -0400
Message-ID: <5593F98C.4010406@redhat.com>
References: <1435677437-16717-1-git-send-email-mhocko@suse.cz> <20150701061731.GB6286@dhcp22.suse.cz> <20150701133715.GA6287@dhcp22.suse.cz>
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Cc: Dave Chinner <david@fromorbit.com>, "Theodore Ts'o" <tytso@mit.edu>,
        Marian Marinov <mm@1h.com>, Hugh Dickins <hughd@google.com>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
        linux-ext4@vger.kernel.org
To: Michal Hocko <mhocko@suse.cz>, Nikolay Borisov <kernel@kyup.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>
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In-Reply-To: <20150701133715.GA6287@dhcp22.suse.cz>
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List-Id: linux-ext4.vger.kernel.org

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@kyup.com>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>

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

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