linux-kernel - Re: Kernel 2.6.19.2 New RAID 5 Bug (oops when writing Samba -> RAID5)

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Date:	Mon, 22 Jan 2007 18:44:11 -0700
From:	"Dan Williams" <dan.j.williams@...il.com>
To:	"Neil Brown" <neilb@...e.de>
Cc:	"Chuck Ebbert" <cebbert@...hat.com>,
	"Justin Piszcz" <jpiszcz@...idpixels.com>,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: Kernel 2.6.19.2 New RAID 5 Bug (oops when writing Samba -> RAID5)

On 1/22/07, Neil Brown <neilb@...e.de> wrote:
> On Monday January 22, cebbert@...hat.com wrote:
> > Justin Piszcz wrote:
> > > My .config is attached, please let me know if any other information is
> > > needed and please CC (lkml) as I am not on the list, thanks!
> > >
> > > Running Kernel 2.6.19.2 on a MD RAID5 volume.  Copying files over Samba to
> > > the RAID5 running XFS.
> > >
> > > Any idea what happened here?
> ....
> > >
> > Without digging too deeply, I'd say you've hit the same bug Sami Farin
> > and others
> > have reported starting with 2.6.19: pages mapped with kmap_atomic()
> > become unmapped
> > during memcpy() or similar operations.  Try disabling preempt -- that
> > seems to be the
> > common factor.
>
> That is exactly the conclusion I had just come to (a kmap_atomic page
> must be being unmapped during memcpy).  I wasn't aware that others had
> reported it - thanks for that.
>
> Turning off CONFIG_PREEMPT certainly seems like a good idea.
>
Coming from an ARM background I am not yet versed in the inner
workings of kmap_atomic, but if you have time for a question I am
curious as to why spin_lock(&sh->lock)  is not sufficient pre-emption
protection for copy_data() in this case?

> NeilBrown

Regards,
Dan
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