linux-kernel - Re: [PATCH] blk: missing add of padded bytes to io completion byte count

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Message-ID: <20080306134138.GF17940@kernel.dk>
Date:	Thu, 6 Mar 2008 14:41:39 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	James.Bottomley@...senPartnership.com, htejun@...il.com,
	bharrosh@...asas.com, efault@....de, tomof@....org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
	jgarzik@...ox.com, bzolnier@...il.com
Subject: Re: [PATCH] blk: missing add of padded bytes to io completion byte   count

On Thu, Mar 06 2008, FUJITA Tomonori wrote:
> On Wed, 05 Mar 2008 09:21:24 -0600
> James Bottomley <James.Bottomley@...senPartnership.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@...il.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?

-- 
Jens Axboe

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