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

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Message-Id: <20080306135627P.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 06 Mar 2008 13:56:27 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	htejun@...il.com
Cc:	fujita.tomonori@....ntt.co.jp, tomof@....org, efault@....de,
	jens.axboe@...cle.com, James.Bottomley@...senPartnership.com,
	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] block: fix residual byte count handling

On Wed, 05 Mar 2008 09:44:01 +0900
Tejun Heo <htejun@...il.com> wrote:

> >> Things going the other way is fine with me but I at least want to hear a
> >> valid rationale.  Till now all I got is "because that's the true size"
> >> which doesn't really make much sense to me.
> > 
> > Most of users of request structure care about only the real data
> > length, don't care about padding and drain length. Why do they bother
> > to use a helper function to get the real data length?
> 
> I think this is where the difference comes from.  To me it seems
> internal usage seems more wide-spread and more delicate and not too many
> care about the true size and when they do only in well defined places.
> Maybe it comes from the difference between your most and my most.

I don't think that they only in well defined places.

If you see scsi mid-layer (and LLDs), you can find several places that
use rq->data_len as the true data length.

Breaking rq->data_len == the true data length theoretically
wrong. Even if it affects only libata now, it will hurt us, I think.
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