linux-kernel - Re: [PATCH v2.6.36-rc7] ubd: fix incorrect sector handling during request restart

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Date:	Fri, 15 Oct 2010 12:56:43 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	"jdike@...toit.com" <jdike@...toit.com>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	"user-mode-linux-user@...ts.sourceforge.net" 
	<user-mode-linux-user@...ts.sourceforge.net>,
	"janjaap@....nl" <janjaap@....nl>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"syzop@...nscan.org" <syzop@...nscan.org>,
	Chris Frey <cdfrey@...rsquare.net>, stable <stable@...nel.org>
Subject: Re: [PATCH v2.6.36-rc7] ubd: fix incorrect sector handling during
 request  restart

On 2010-10-15 11:19, Tejun Heo wrote:
> Commit f81f2f7c (ubd: drop unnecessary rq->sector manipulation)
> dropped request->sector manipulation in preparation for global request
> handling cleanup; unfortunately, it incorrectly assumed that the
> updated sector wasn't being used.
> 
> ubd tries to issue as many requests as possible to io_thread.  When
> issuing fails due to memory pressure or other reasons, the device is
> put on the restart list and issuing stops.  On IO completion, devices
> on the restart list are scanned and IO issuing is restarted.
> 
> ubd issues IOs sg-by-sg and issuing can be stopped in the middle of a
> request, so each device on the restart queue needs to remember where
> to restart in its current request.  ubd needs to keep track of the
> issue position itself because,
> 
> * blk_rq_pos(req) is now updated by the block layer to keep track of
>   _completion_ position.
> 
> * Multiple io_req's for the current request may be in flight, so it's
>   difficult to tell where blk_rq_pos(req) currently is.
> 
> Add ubd->rq_pos to keep track of the issue position and use it to
> correctly restart io_req issue.

Thanks Tejun, good to see this one resolved.

-- 
Jens Axboe

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