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Re: [PATCH 02/17] xfs: skip writeback from reclaim context

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 02/17] xfs: skip writeback from reclaim context
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 2 Jun 2010 14:39:57 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20100531160859.184576507@xxxxxxxxxxxxxxxxxxxxxx>
References: <20100531160727.842750532@xxxxxxxxxxxxxxxxxxxxxx> <20100531160859.184576507@xxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon, May 31, 2010 at 12:07:29PM -0400, Christoph Hellwig wrote:
> Allowing writeback from reclaim context causes massive problems with stack
> overflows as we can call into the writeback code which tends to be a heavy
> stack user both in the generic code and XFS from random contexts that
> perform memory allocations.
> 
> Follow the example of btrfs (and in slightly different form ext4) and refuse
> to write out data from reclaim context.  This issue should really be handled
> by the VM so that we can tune better for this case, but until we get it
> sorted out there we have to hack around this in each filesystem with a
> complex writeback path.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Yup, I agree that we have to do this for the moment. It hasn't
caused any problems in my usual OOM testing, so it seems safe
to do.

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>

Also worth thinking about is if should be checked in
xfs_vm_releasepage() as well to avoid the same stack issues if it
triggers allocation...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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