linux-kernel - Re: Patch "page-allocator: preserve PFN ordering when __GFP

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Message-Id: <200908131122.40423.jbe@pengutronix.de>
Date:	Thu, 13 Aug 2009 11:22:39 +0200
From:	Juergen Beisert <jbe@...gutronix.de>
To:	ben-linux@...ff.org
Cc:	linux-kernel@...r.kernel.org, Mel Gorman <mel@....ul.ie>,
	Arnaud Faucher <arnaud.faucher@...il.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-hotplug@...r.kernel.org
Subject: Re: Patch "page-allocator: preserve PFN ordering when __GFP_COLD is set" fails on my system

On Donnerstag, 13. August 2009, Mel Gorman wrote:
> On Wed, Aug 12, 2009 at 02:40:30PM -0400, Arnaud Faucher wrote:
> > I have a rather similar problem on a driver that I try to keep
> > up-to-date with recent kernel versions
> > (http://code.ximeta.com/trac-ndas/ticket/1110#comment:30). The NDAS
> > hardware is an ethernet-enabled disk controller on one chip, kind of a
> > cheap iSCSI.
> >
> > In my case there is no oops: the symptoms are that the read blocks seem
> > to be swapped or full of garbage.
> >
> > After investigation in the NDAS code, the bug triggers when the driver
> > tries to merge adjacent requests before sending them to the controller.
> > I had to disable this merge in order to restore normal behavior, at the
> > expense of a reduced efficiency.
>
> That is a very interesting point and one I hadn't considered. The point
> of the patch was to help drivers that merge adjacent requests if they
> happen to be physically contiguous. The reported bug that led to the
> patch was a regression of memory not being physically contiguous and
> requests not being merged.
>
> > > After this oops, system startup continues. Then the next oops occurs:
> > >
> > > This one is new, since I try to mount the connected SD card.
> >
> > Mel's buffer overrun theory seems to apply in the NDAS driver case,
> > where the original requests adjacency test seems faulty.
> >
> > May it also be the cause of the SD mounting crash ?
>
> It's a possibility. If it's not an overrun, it's possible that the
> automatic merging code is buggy as well.
>
> Juergen, is the disk controller on your machine capable of merging
> requests? If so, can you disable it and see if the bug still occurs
> please?

Hmmm, hard to say. Maybe the author of this driver can say more.

@Ben: MMC/SD/SDHC driver for the s3c2440-CPU. Can you answer Mel's question? 

Regards,
Juergen

-- 
Pengutronix e.K.                              | Juergen Beisert             |
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