The patch titled
Subject: mm/page_alloc.c: place zone_id check before VM_BUG_ON_PAGE check
has been added to the -mm tree. Its filename is
mm-page_alloc-place-zone_id-check-before-vm_bug_on_page-check.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-page_alloc-place-zone_id-check-before-vm_bug_on_page-check.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-page_alloc-place-zone_id-check-before-vm_bug_on_page-check.patch
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------------------------------------------------------
From: Weijie Yang <weijie.yang@xxxxxxxxxxx>
Subject: mm/page_alloc.c: place zone_id check before VM_BUG_ON_PAGE check
If the freeing page and its buddy page are not at the same zone, the
current holding zone->lock for the freeing page cann't prevent buddy page
getting allocated, this could trigger VM_BUG_ON_PAGE in page_is_buddy() at
a very tiny chance, such as:
cpu 0: cpu 1:
hold zone_1 lock
check page and it buddy
PageBuddy(buddy) is true hold zone_2 lock
page_order(buddy) == order is true alloc buddy
trigger VM_BUG_ON_PAGE(page_count(buddy) != 0)
zone_1->lock prevents the freeing page getting allocated
zone_2->lock prevents the buddy page getting allocated
they are not the same zone->lock.
If we can't remove the zone_id check statement, it's better handle this
rare race. This patch fixes this by placing the zone_id check before the
VM_BUG_ON_PAGE check.
Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff -puN mm/page_alloc.c~mm-page_alloc-place-zone_id-check-before-vm_bug_on_page-check mm/page_alloc.c
--- a/mm/page_alloc.c~mm-page_alloc-place-zone_id-check-before-vm_bug_on_page-check
+++ a/mm/page_alloc.c
@@ -552,17 +552,15 @@ static inline int page_is_buddy(struct p
return 0;
if (page_is_guard(buddy) && page_order(buddy) == order) {
- VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
-
if (page_zone_id(page) != page_zone_id(buddy))
return 0;
+ VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
return 1;
}
if (PageBuddy(buddy) && page_order(buddy) == order) {
- VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
-
/*
* zone check is done late to avoid uselessly
* calculating zone/node ids for pages that could
@@ -571,6 +569,8 @@ static inline int page_is_buddy(struct p
if (page_zone_id(page) != page_zone_id(buddy))
return 0;
+ VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
return 1;
}
return 0;
_
Patches currently in -mm which might be from weijie.yang@xxxxxxxxxxx are
origin.patch
mm-page_alloc-place-zone_id-check-before-vm_bug_on_page-check.patch
mm-page_isolation-check-pfn-validity-before-access.patch
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