> 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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