linux-kernel - Re: [PATCH] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo (resend)

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Date:	Fri, 22 Aug 2008 15:49:41 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	hsweeten@...ionengravers.com,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo (resend)

On (21/08/08 17:56), Russell King - ARM Linux didst pronounce:
> On Thu, Aug 21, 2008 at 09:34:00AM -0700, Andrew Morton wrote:
> > On Thu, 21 Aug 2008 14:28:05 +0100 Mel Gorman <mel@....ul.ie> wrote:
> > > This patch lets architectures say when FLATMEM can have holes in the
> > > memmap. Rather than an expensive check for valid memory, /proc/pagetypeinfo
> > > will confirm that the page linkages are still valid by checking page->zone
> > > is still the expected zone. The lookup of page_zone is safe as there is a
> > > limited range of memory that is accessed when calling page_zone.  Even if
> > > page_zone happens to return the correct zone, the impact is that the counters
> > > in /proc/pagetypeinfo are slightly off but fragmentation monitoring is
> > > unlikely to be relevant on an embedded system.
> > 
> > Sounds like this might fix an oops.  Does it?
> > 

Yes, it does. Sorry for not being clear on that.

> > The patch applies to 2.6.25 and to 2.6.26.  Should it be backported?
> 

It wouldn't hurt. It's not a critical functionality failure and only affects
ARM but being able to generate oops from userspace is a bit of a loss.

> The only concern there is with this patch is that we're still walking
> over the memory, which could contain anything.  We could be unlucky
> and end up with page_zone(page) == zone.
> 

If you do, the impact is that the counters are slightly off which is not
that big of a deal. To avoid doing it, information would have to be kept
around that might end up being larger than the memmap freed.

> It'll do as a stop gap, but I think the real solution is to switch over
> to using sparsemem, and get rid of ARMs private version (which even
> pre-dates discontigmem.)  That first assumes that we have sparsemem
> working on ARM - however folk seem to prefer discontigmem over
> sparsemem so I don't know what state sparsemem on ARM is in. :(
> 

No idea. I know that SPARSEMEM would be preferred as a memory model as
it is a lot less arch-specific than DISCONTIG is.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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