linux-kernel - Re: scsi_cmnd data

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Date:	Thu, 9 Sep 2010 02:09:28 -0700 (PDT)
From:	Anil kumar <anils_r@...oo.com>
To:	Christof Schmitt <christof.schmitt@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: scsi_cmnd data_buffer checksum

I quickly tried copying the data buffer to local buffers as follows:

In Queuecommand:

cmd->local_write_buf = pci_alloc_consistent(...);
cmd->write_buf = (u8 *)(kmap_atomic(sg->page, KM_IRQ0) + sg->offset);

memcpy(cmd->local_write_buf, cmd->write_buf, scsibufflen(scsi_cmnd));

Now I calculate checksums of cmd->write_buf and my local cmd->local_write_buf

and the checksum fails. Am I doing something wrong here?

--- On Thu, 9/9/10, Christof Schmitt <christof.schmitt@...ibm.com> wrote:

> From: Christof Schmitt <christof.schmitt@...ibm.com>
> Subject: Re: scsi_cmnd data_buffer checksum
> To: "Anil kumar" <anils_r@...oo.com>
> Cc: linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
> Date: Thursday, September 9, 2010, 4:51 AM
> On Thu, Sep 09, 2010 at 01:35:02AM
> -0700, Anil kumar wrote:
> > Hi Christof,
> > 
> > Thanks for the response.
> > 
> > I am running mkfs.ext3 command.
> > 
> > I am doing the following in the driver for write(10):
> > 
> > Queuecommand:
> > 
> > sg = scsi_sglist(cmd->scsi_cmd);
> > cmd->write_buf = (u8 *)(kmap_atomic(sg->page,
> KM_IRQ0) + sg->offset);
> > Calculate checksum for write_buf
> > 
> > Write Done:
> > Calculate checksum for cmd->write_buf
> > 
> > and checksums don't match. I am wondering how come OS
> changed the cmd->write_buf when I have not even unmapped
> the buffer. Is filesystem changing this cmd->write_buf
> pages when driver/HW is working on it?
> 
> Yes, the driver has direct access to the data. Usually, the
> data is
> not copied for I/O requests. The driver gets one sg list
> that points
> to the data pages of file system (or whatever the data
> source is).
> When the filesystem decides to change the data, this single
> data
> buffer is changed.
> 
> > Is there anyway I can avoid this. How about if we
> allocate a local buffer(kmalloc/pci_alloc_consistent) and
> memcpy kmap_atomic to that local buffer and then calculate
> checksum on that local buffer. Will this help?
> 
> Sure, you can create copies of data buffers in the driver,
> calculate
> the checksum of the copy and submit the data copy with the
> checksum to
> the hardware controller. This is usually not done for
> performance
> reasons, and you probably should keep a mempool to be able
> to issue
> I/Os when memory is low.
> 
> Christof
> 


      
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