linux-kernel - Re: [patch] block: properly handle flush/fua requests in blk_insert_cloned

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Date:	Tue, 9 Aug 2011 18:13:34 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <jaxboe@...ionio.com>,
	Mike Snitzer <msnitzer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [patch] block: properly handle flush/fua requests in
 blk_insert_cloned_request

Hello,

On Tue, Aug 09, 2011 at 11:53:51AM -0400, Jeff Moyer wrote:
> Tejun Heo <tj@...nel.org> writes:
> > I'm a bit confused.  We still need ELEVATOR_INSERT_FLUSH fix for
> > insertion paths, right?  Or is blk_insert_cloned_request() supposed to
> > used only by request based dm which lives under the elevator?  If so,
> > it would be great to make that explicit in the comment.  Maybe just
> > renaming it to blk_insert_dm_cloned_request() would be better as it
> > wouldn't be safe for other cases anyway.
> 
> request-based dm is the only caller at present.  I'm not a fan of
> renaming the function, but I'm more than willing to comment it.

I'm still confused and don't think the patch is correct (you can't
turn off REQ_FUA without decomposing it to data + post flush).

Going through flush machinery twice is okay and I think is the right
thing to do.  At the upper queue, the request is decomposed to member
requests.  After decomposition, it's either REQ_FLUSH w/o data or data
request w/ or w/o REQ_FUA.  When the decomposed request reaches lower
queue, the lower queue will then either short-circuit it, execute
as-is or decompose data w/ REQ_FUA into data + REQ_FLUSH sequence.

AFAICS, the breakages are...

* ELEVATOR_INSERT_FLUSH not used properly from insert paths.

* Short circuit not kicking in for the dm requests. (the above and the
  policy patch should solve this, right?)

* BUG(!rq->bio || ...) in blk_insert_flush().  I think we can lift
  this restriction for empty REQ_FLUSH but also dm can just send down
  requests with empty bio.

Thanks.

-- 
tejun
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