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

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Message-Id: <20080821093400.fe02a76f.akpm@linux-foundation.org>
Date:	Thu, 21 Aug 2008 09:34:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org, hsweeten@...ionengravers.com
Subject: Re: [PATCH] Skip memory holes in FLATMEM when reading
 /proc/pagetypeinfo (resend)

On Thu, 21 Aug 2008 14:28:05 +0100 Mel Gorman <mel@....ul.ie> wrote:

> This is a resend. The last patch went to Russell King with lkml cc'd as I
> wasn't subscribed to the linux-arm list. However, I haven't heard it being
> picked up so trying linux-arm this time.
> 
> ===
> 
> Ordinarily, memory holes in flatmem still have a valid memmap and is safe
> to use. However, an architecture (ARM) frees up the memmap backing memory
> holes on the assumption it is never used. /proc/pagetypeinfo reads the
> whole range of pages in a zone believing that the memmap is valid and that
> pfn_valid will return false if it is not. On ARM, freeing the memmap breaks
> the page->zone linkages even though pfn_valid() returns true and the kernel
> can oops shortly afterwards due to accessing a bogus struct zone *.
> 
> 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?

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

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