Commit 60177d31 authored Dec 06, 2012 by Mel Gorman Committed by Linus Torvalds Dec 06, 2012

mm: compaction: validate pfn range passed to isolate_freepages_block

Commit 0bf380bc ("mm: compaction: check pfn_valid when entering a
new MAX_ORDER_NR_PAGES block during isolation for migration") added a
check for pfn_valid() when isolating pages for migration as the scanner
does not necessarily start pageblock-aligned.

Since commit c89511ab ("mm: compaction: Restart compaction from near
where it left off"), the free scanner has the same problem. This patch
makes sure that the pfn range passed to isolate_freepages_block() is
within the same block so that pfn_valid() checks are unnecessary.

In answer to Henrik's wondering why others have not reported this:
reproducing this requires a large enough hole with the right aligment to
have compaction walk into a PFN range with no memmap. Size and
alignment depends in the memory model - 4M for FLATMEM and 128M for
SPARSEMEM on x86. It needs a "lucky" machine.

Reported-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 04c5decd

mm/compaction.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -713,7 +713,15 @@ static void isolate_freepages(struct zone *zone,

		/* Found a block suitable for isolating free pages from */
		isolated = 0;
		end_pfn = min(pfn + pageblock_nr_pages, zone_end_pfn);

		/*
		* As pfn may not start aligned, pfn+pageblock_nr_page
		* may cross a MAX_ORDER_NR_PAGES boundary and miss
		* a pfn_valid check. Ensure isolate_freepages_block()
		* only scans within a pageblock
		*/
		end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
		end_pfn = min(end_pfn, zone_end_pfn);
		isolated = isolate_freepages_block(cc, pfn, end_pfn,
		freelist, false);
		nr_freepages += isolated;

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