Message-ID: <47CAF614.5050206@cs.wisc.edu>
Date: Sun, 02 Mar 2008 12:46:44 -0600
From: Mike Christie <michaelc@cs.wisc.edu>
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To: FUJITA Tomonori <tomof@acm.org>
CC: htejun@gmail.com, jens.axboe@oracle.com,
       James.Bottomley@HansenPartnership.com, efault@gmx.de,
       akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       linux-ide@vger.kernel.org, linux-scsi@vger.kernel.org,
       jgarzik@pobox.com, fujita.tomonori@lab.ntt.co.jp
Subject: Re: [PATCH] block: fix residual byte count handling
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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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