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

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Date:	Thu, 28 Feb 2008 16:35:43 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Tejun Heo <htejun@...il.com>
Cc:	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-ide@...r.kernel.org,
	linux-scsi@...r.kernel.org, Jeff Garzik <jgarzik@...ox.com>
Subject: Re: [PATCH] block: fix residual byte count handling

On Thu, Feb 28 2008, Tejun Heo wrote:
> rq->raw_data_len introduced for block layer padding and draining
> (commit 6b00769fe1502b4ad97bb327ef7ac971b208bfb5) broke residual byte
> count handling.  Block drivers modify rq->data_len to notify residual
> byte count to the block layer which blindly reported unmodified
> rq->raw_data_len to userland.
> 
> To keep block drivers dealing only with rq->data_len, this should be
> handled inside block layer.  However, how much extra buffer was
> appened is lost after rq->data_len is modified.
> 
> This patch replaces rq->raw_data_len with rq->extra_len and add
> blk_rq_raw_data_len() helper to calculate raw data size from
> rq->data_len and rq->extra_len.  The helper returns correct raw
> residual byte count when called on a rq whose data_len is modified to
> carry residual byte count.
> 
> This problem was reported and diagnosed by Mike Galbraith.

Tejun, this patch isn't much cleaner at all. It really shows the pain of
these two seperate, yet related, variables.

-- 
Jens Axboe

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