[PATCH] mm: page_alloc: place zone id check before VM_BUG_ON_PAGE check

[PATCH] mm: page_alloc: place zone id check before VM_BUG_ON_PAGE check

[Weijie Yang]

If the free page and its buddy has different zone id, the current
zone->lock cann't prevent buddy page getting allocated, this could
trigger VM_BUG_ON_PAGE in a very tiny chance:

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)

This patch fixes this issue by placing the zone id check before
the VM_BUG_ON_PAGE check.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
---
 mm/page_alloc.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 616a2c9..491d055 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -488,17 +488,15 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
 		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
@@ -507,6 +505,8 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
 		if (page_zone_id(page) != page_zone_id(buddy))
 			return 0;
 
+		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
 		return 1;
 	}
 	return 0;
-- 
1.7.10.4


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: page_alloc: place zone id check before VM_BUG_ON_PAGE check

[Mel Gorman]

On Tue, Dec 09, 2014 at 03:40:35PM +0800, Weijie Yang wrote:
> If the free page and its buddy has different zone id, the current
> zone->lock cann't prevent buddy page getting allocated, this could
> trigger VM_BUG_ON_PAGE in a very tiny chance:
> 

Under what circumstances can a buddy page be allocated without the
zone->lock? Any parallel allocation from that zone that takes place will
be from the per-cpu allocator and should not be affected by this. Have
you actually hit this race?

-- 
Mel Gorman
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: page_alloc: place zone id check before VM_BUG_ON_PAGE check

[Weijie Yang]

On Tue, Dec 9, 2014 at 5:59 PM, Mel Gorman <mgorman@suse.de> wrote:
> On Tue, Dec 09, 2014 at 03:40:35PM +0800, Weijie Yang wrote:
>> If the free page and its buddy has different zone id, the current
>> zone->lock cann't prevent buddy page getting allocated, this could
>> trigger VM_BUG_ON_PAGE in a very tiny chance:
>>
>
> Under what circumstances can a buddy page be allocated without the
> zone->lock? Any parallel allocation from that zone that takes place will
> be from the per-cpu allocator and should not be affected by this. Have
> you actually hit this race?

My description maybe not clear, if the free page and its buddy is not
at the same zone, the holding zone->lock cann't prevent buddy page
getting allocated.
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.

I found it when review the code, not a running test.
However, if we cann't remove the zone_id check statement, I think
we should handle this rare race.

If I miss something or make a mistake, please let me know.

Thanks

> --
> Mel Gorman
> SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: page_alloc: place zone id check before VM_BUG_ON_PAGE check

[Mel Gorman]

On Wed, Dec 10, 2014 at 09:38:42PM +0800, Weijie Yang wrote:
> On Tue, Dec 9, 2014 at 5:59 PM, Mel Gorman <mgorman@suse.de> wrote:
> > On Tue, Dec 09, 2014 at 03:40:35PM +0800, Weijie Yang wrote:
> >> If the free page and its buddy has different zone id, the current
> >> zone->lock cann't prevent buddy page getting allocated, this could
> >> trigger VM_BUG_ON_PAGE in a very tiny chance:
> >>
> >
> > Under what circumstances can a buddy page be allocated without the
> > zone->lock? Any parallel allocation from that zone that takes place will
> > be from the per-cpu allocator and should not be affected by this. Have
> > you actually hit this race?
> 
> My description maybe not clear, if the free page and its buddy is not
> at the same zone, the holding zone->lock cann't prevent buddy page
> getting allocated.

You're right, the description is not clear but now I see your
point. Thanks.

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>