linux-kernel - Re: [PATCH 24/32] dlm: dlm workqueues aren't used in memory reclaim path

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Message-ID: <20110103144410.GT18831@htj.dyndns.org>
Date:	Mon, 3 Jan 2011 15:44:10 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Christine Caulfield <ccaulfie@...hat.com>,
	David Teigland <teigland@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH 24/32] dlm: dlm workqueues aren't used in memory
 reclaim path

Hello,

On Mon, Jan 03, 2011 at 02:39:19PM +0000, Steven Whitehouse wrote:
> No, the issue arises in case an inode is being ejected from cache with a
> 0 link count (and this need deallocation). At this point the dlm is
> required in order to coordinate which node will perform the
> deallocation, but if it blocks on memory reclaim (via the fs, so
> GFP_NOFS is ok, but GFP_KERNEL is not) then it can land up calling back
> into the fs via a shrink of the dcache which can potentially then also
> call back into the dlm.

I see.  Thanks for the explanation.

> Since there is no workqueue option to indicate that GFP_NOFS is ok, but
> GFP_KERNEL is not, I think in the workqueue case we must mark it as
> WQ_MEM_RECLAIM. So far as I can tell thats a limitation of task creation
> which was the reason for the pre-allocated rescuer thread. I'd rather be
> safe than sorry in this case,

Yeap, in such case, WQ_MEM_RECLAIM seems like the right choice.  It
may call into GFP_KERNEL path again, but the dependency chain isn't
hard.  It will be broken pretty soon and rescuer will kick in.

As the case is rather special, it might be a good idea to explain why
WQ_MEM_RECLAIM is necessary for future references?

Thank you.

-- 
tejun
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