linux-kernel - Re: [PATCH 2/8] cfq-iosched: fix async oom queue handling

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Message-ID: <x49r3pmhwim.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 08 Jun 2015 14:42:41 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, vgoyal@...hat.com,
	avanzini.arianna@...il.com
Subject: Re: [PATCH 2/8] cfq-iosched: fix async oom queue handling

Tejun Heo <tj@...nel.org> writes:

> Async cfqq's (cfq_queue's) are shared across cfq_data.  When
> cfq_get_queue() obtains a new queue from cfq_find_alloc_queue(), it
> stashes the pointer in cfq_data and reuses it from then on; however,
> the function doesn't consider that cfq_find_alloc_queue() may return
> the oom_cfqq under memory pressure and installs the returned queue
> unconditionally.
>
> If the oom_cfqq is installed as an async cfqq, cfq_set_request() will
> continue calling cfq_get_queue() hoping to replace it with a proper
> queue; however, cfq_get_queue() will keep returning the cached queue
> for the slot - the oom_cfqq.
>
> Fix it by skipping caching if the queue is the oom one.

Good catch.

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
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