linux-kernel - Re: Oops in 3.7-rc8 isolate_free_pages

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Date:	Thu, 6 Dec 2012 20:28:45 +0100
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Jan Kara <jack@...e.cz>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Oops in 3.7-rc8 isolate_free_pages_block()

> Actually, looking at it some more, I think that two-liner patch had
> *ANOTHER* bug.
> 
> Because the other line seems buggy as well.
> 
> Instead of
> 
>         end_pfn = ALIGN(pfn + pageblock_nr_pages, pageblock_nr_pages);
> 
> I think it should be
> 
>         end_pfn = ALIGN(pfn+1, pageblock_nr_pages);
> 
> instead. ALIGN() already aligns upwards (but the "+1" is needed in
> case pfn is already at a pageblock_nr_pages boundary, at which point
> ALIGN() would have just returned that same boundary.

Ah, and now the two callers treat the pointers the same way.

> Hmm? Mel, please confirm. And Henrik, it might be good to test that
> doubly-fixed patch. Because reading the patch and trying to fix bugs
> in it that way is *not* the same as actually verifying it ;)

Confirmed, working. I also checked 3.6, but could not trigger the
original problem there. The code also looks different, so it makes
sense. To be explicit, this is what I tested on top of v3.7-rc8:

---
 mm/compaction.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 9eef558..ff1c483 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -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;
-- 
1.8.0.1

Hopefully, that's a wrap. :-)

Henrik
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