linux-kernel - Re: [PATCH] block: fix residual byte count handling

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Date:	Mon, 03 Mar 2008 22:38:55 +0900
From:	Tejun Heo <htejun@...il.com>
To:	FUJITA Tomonori <tomof@....org>
CC:	fujita.tomonori@....ntt.co.jp, jens.axboe@...cle.com,
	James.Bottomley@...senPartnership.com, efault@....de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
	jgarzik@...ox.com
Subject: Re: [PATCH] block: fix residual byte count handling

FUJITA Tomonori wrote:
> On Mon, 03 Mar 2008 18:21:13 +0900
> Tejun Heo <htejun@...il.com> wrote:
> 
>> FUJITA Tomonori wrote:
>>>>> I can't see what changing the meaning of rq->data_len (and
>>>>> investigating all the block drivers) gives us.
>>>> No matter which way you go, you change the meaning of rq->data_len and
>>>> you MUST inspect rq->data_len usage whichever way you go.
>>> The patch doens't change that rq->data_len means the true data
>>> length. But yeah, it breaks rq->data_len == sum(sg). So it might break
>>> some drivers.
>> Yeah, that's what I was saying.  You end up breaking one of the two
>> assumptions.  As sglist is getting modified for any driver if it has DMA
>> alignment set, whether rq->data_len is adjusted together or not, sglist
>> and data_len usages have to be audited.
> 
> My patch (well, James' original approach) doesn't affect drivers that
> don't use drain buffer. rq->data_len still means the true data length
> and rq->data_len is equal to sum(sg) for them. So right now we need to
> audit only libata.

Your patch does change sglist for any driver which sets DMA alignment.
You'll definitely need to audit more than libata.

> But your patch changes the meaning of rq->data_len. It affects all the
> drivers. So it breaks non libata stuff, like the SMP handler. We need
> to audit all the drivers.

With both patches applied, sglist and data_len are adjusted only for
libata, so only drivers which explicitly requested buffer size
manipulation (currently only libata) need to be audited / updated.

-- 
tejun
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