Message-Id: <200803201254.m2KCstCa031594@mbox.iij4u.or.jp>
To: jens.axboe@oracle.com
Cc: fujita.tomonori@lab.ntt.co.jp, James.Bottomley@HansenPartnership.com,
       htejun@gmail.com, bharrosh@panasas.com, efault@gmx.de, tomof@acm.org,
       akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       linux-ide@vger.kernel.org, linux-scsi@vger.kernel.org,
       jgarzik@pobox.com, bzolnier@gmail.com
Subject: Re: [PATCH] blk: missing add of padded bytes to io completion byte
   count
From: FUJITA Tomonori <tomof@acm.org>
In-Reply-To: <20080306134138.GF17940@kernel.dk>
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	<20080306134146A.fujita.tomonori@lab.ntt.co.jp>
	<20080306134138.GF17940@kernel.dk>
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Date: Thu, 20 Mar 2008 21:54:51 +0900
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From: Jens Axboe <jens.axboe@oracle.com>
Subject: Re: [PATCH] blk: missing add of padded bytes to io completion byte   count
Date: Thu, 6 Mar 2008 14:41:39 +0100

> On Thu, Mar 06 2008, FUJITA Tomonori wrote:
> > On Wed, 05 Mar 2008 09:21:24 -0600
> > James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> > 
> > > On Wed, 2008-03-05 at 14:51 +0100, Jens Axboe wrote:
> > > > On Wed, Mar 05 2008, Tejun Heo wrote:
> > > > > This is getting insanely subtle.  Let's say there's PIO driver which
> > > > > transfer certain sized chunks at a time and completes request partially
> > > > > after completing each chunk and the driver uses draining to eat up
> > > > > whatever excess data, which seems like a legit use case to me.  But it
> > > > > won't work because __end_that_request_first() will terminate when it
> > > > > reaches reaches the 'true' transfer size.  That's just broken API.  FWIW,
> > > > > 
> > > > > Nacked-by: Tejun Heo <htejun@gmail.com>
> > > > 
> > > > Yeah, I think I may have gone a bit overboard in applying this so
> > > > quickly. It's just not a good interface, silently adding the extra
> > > > length if asked to complete more. It may even happen right now, for a
> > > > driver that does no padding (it probably wont do any harm here either,
> > > > but still).
> > > > 
> > > > I'll try and see if I can come up with something cleaner.
> > > > 
> > > > My basic design paradigm for this is that the _driver_ (or mid layer, if
> > > > SCSI wants to handle it) should care about the padding. So make it easy
> > > > for them to pad, but have it 'unrolled' by completion time. We should
> > > > NOT need any extra_len checks or additions in the block/ directory,
> > > > period.
> > > 
> > > Right, that's why my original proposal was to do nothing for padding
> > > (other than ensure the driver could adjust the length if it wanted to)
> > > and to add an extra element always for draining, which the driver could
> > > ignore.  It basically pushed the use paradigm onto the driver.
> > > 
> > > If we want the use paradigm shared between block and driver, then I
> > > think the best approach is to keep all the bios the same (so not adjust
> > > for padding), but do adjust in the blk_rq_map_sg().  That way we have
> > > the padding and draining unwind information by comparing with the bio.
> > 
> > Adjusting only sg in blk_rq_map_sg (like drain) looks much
> > better. This works with libata for me.
> 
> Looks like a much better solution to me. Anyone have any valid
> objections against moving the padding to the sg map time?

What's the situation with this fix?
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