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

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Message-ID: <47CAF614.5050206@cs.wisc.edu>
Date:	Sun, 02 Mar 2008 12:46:44 -0600
From:	Mike Christie <michaelc@...wisc.edu>
To:	FUJITA Tomonori <tomof@....org>
CC:	htejun@...il.com, 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, fujita.tomonori@....ntt.co.jp
Subject: Re: [PATCH] block: fix residual byte count handling

FUJITA Tomonori wrote:
> sum(sg) == rq->data_len is already broken; sg sends such requests
> (though it would be nice if it doesn't).
> 

Actually, I think I was half wrong on that when you asked about 
scsi_debug. The scatterlist that sg.c uses is never seen by the block 
layer or scsi layer. It is just used as a container to hold segments. 
sg.c and st.c use their scatterlist to manage their preallocated 
pages/segments. When they pass it to scsi_execute_async, that function 
will create a request struct and add bios for the pages.

In 2.6.24 and below, sg.c will send a scatterlist length that does not 
match the IO length, and scsi_execute_async will goof up and send a 
rq->data_len that does not match the sum of the bios. That is what I was 
trying to fix in 2.6.24, but the patch got messed up. In 2.6.25-rc2 and 
above that is fixed and scsi_execute_async will catch sg.c doing this 
and set rq->data_len and the bio lengths correctly.

So hopefully that helps any fixes you might have planned.
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