Commit e80d6a24 authored Aug 14, 2008 by Mel Gorman Committed by Russell King Aug 27, 2008

[ARM] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo

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.

Reported-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

parent f1bcf7e3

arch/arm/Kconfig

+5 −0

Original line number	Diff line number	Diff line
		@@ -810,6 +810,11 @@ config OABI_COMPAT
		UNPREDICTABLE (in fact it can be predicted that it won't work
		at all). If in doubt say Y.

		config ARCH_FLATMEM_HAS_HOLES
		bool
		default y
		depends on FLATMEM

		config ARCH_DISCONTIGMEM_ENABLE
		bool
		default (ARCH_LH7A40X && !LH7A40X_CONTIGMEM)

mm/vmstat.c

+18 −1

Original line number	Diff line number	Diff line
		@@ -516,8 +516,25 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
		continue;

		page = pfn_to_page(pfn);
		#ifdef CONFIG_ARCH_FLATMEM_HAS_HOLES
		/*
		* Ordinarily, memory holes in flatmem still have a valid
		* memmap for the PFN range. However, an architecture for
		* embedded systems (e.g. ARM) can free up the memmap backing
		* holes to save memory on the assumption the memmap is
		* never used. The page_zone linkages are then broken even
		* though pfn_valid() returns true. Skip the page if the
		* linkages are broken. Even if this test passed, the impact
		* is that the counters for the movable type are off but
		* fragmentation monitoring is likely meaningless on small
		* systems.
		*/
		if (page_zone(page) != zone)
		continue;
		#endif
		mtype = get_pageblock_migratetype(page);

		if (mtype < MIGRATE_TYPES)
		count[mtype]++;
		}

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