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From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 12/15] swap: do not send discards as barriers
Date: Thu, 19 Aug 2010 05:02:21 -0400
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Cc: Christoph Hellwig <hch@infradead.org>, tj@kernel.org,
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To: Hugh Dickins <hughd@google.com>
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On Wed, Aug 18, 2010 at 08:47:52PM -0700, Hugh Dickins wrote:
> (though if you happened to respin, I'd be be glad for those newlines
> between GFP_KERNEL, BLKDEV_IFL_WAIT to go away now)
> 
> This patch could go in any time now: I may want to push it in and
> get it to 35-stable, because it certainly helps against the swap
> discard regression that Nigel reported (though I've not yet seen
> his numbers with this).  However, I don't think it eliminates the
> regression, so I've more testing and experimenting to do before
> deciding about it - the right answer may just be to disable swap
> discard, as you asked for long ago.

If we do get a bit testing I'd be almost inclined to drop BLKDEV_IFL_BARRIER
under the floor for all callers in .36, and also remove BLKDEV_IFL_WAIT,
as it's now unconditional.  That would allow us to simpliy the
synchronous interfaces for the two, and give us an easy way to develop
a proper asynchronous discard interface in the .37 cycle without
interfering with the barrier rework.  Any comments on that idea from the
filesystem crowd?