linux-kernel - Re: 2.6.35 Regression: Ages spent discarding blocks that weren't used!

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Date:	Fri, 13 Aug 2010 07:54:24 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Nigel Cunningham <nigel@...onice.net>,
	Mark Lord <kernel@...savvy.com>,
	LKML <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.35 Regression: Ages spent discarding blocks that weren't
 used!

On Fri, Aug 06, 2010 at 03:07:25PM -0700, Hugh Dickins wrote:
> If REQ_SOFTBARRIER means that the device is still free to reorder a
> write, which was issued after discard completion was reported, before
> the discard (so later discarding the data written), then certainly I
> agree with Christoph (now Cc'ed) that the REQ_HARDBARRIER is
> unavoidable there; but if not, then it's not needed for the swap case.
>  I hope to gain a little more enlightenment on such barriers shortly.

REQ_SOFTBARRIER is indeed purely a reordering barrier inside the block
elevator.

> What does seem over the top to me, is for mm/swapfile.c's
> blkdev_issue_discard()s to be asking for both BLKDEV_IFL_WAIT and
> BLKDEV_IFL_BARRIER: those swap discards were originally written just
> to use barriers, without needing to wait for completion in there.  I'd
> be interested to hear if cutting out the BLKDEV_IFL_WAITs makes the
> swap discards behave acceptably again for you - but understand that
> you won't have a chance to try that until later next week.

That does indeed look incorrect to me.  Any kind of explicit waits
usually mean the caller provides ordering.  Getting rid of
BLKDEV_IFL_BARRIER in the swap code ASAP would indeed be beneficial
given that we are trying to get rid of hard barriers completely soon.
Auditing the existing blkdev_issue_discard callers in filesystems
is high on the todo list for me.
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