linux-kernel - Re: scsi_cmnd data

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Date:	Thu, 9 Sep 2010 10:00:28 +0200
From:	Christof Schmitt <christof.schmitt@...ibm.com>
To:	Anil kumar <anils_r@...oo.com>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: scsi_cmnd data_buffer checksum

On Wed, Sep 08, 2010 at 08:36:32PM -0700, Anil kumar wrote:
> 
> I am writing a checksum calculation of scsi_cmnd data buffer in the driver. 
> 
> I calculate the checksum of the scsi_cmd data buffer(request_buffer) in driver queuecommand.
> 
> Now when the command is completed from the hardware and before driver sends it back to mid-layer, I calculate the checksum again of the same scsi_cmd data_buffer again.
> 
> Sometimes the checksums don't match. I mean somehow looks like OS changed the scsi_cmd data_buffer(request_buffer) in the meantime when driver is working on the command.
> I print the address of the scsi_cmd data_buffer (virtual address) and its same and the contents of the buffer is also same during both the calculations.
> 
> Can this happen?

Yes. While a write I/O is being processed in Linux or in flight, the
data buffers can change. This causes problems for other checksums as
well; i ran into this when looking at the DIF/DIX checksums for SCSI
commands.

At the moment, this problem can be avoided e.g. by running only direct
I/O on the xfs filesystem. There have been discussions about this, a
short summary is here, look for "stable pages":
http://lwn.net/Articles/399148/

Christof
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