3.16.7-ckt5 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Konstantin Khlebnikov <redacted>

commit b800c91a0517071156e772d4fb329ad33590da62 upstream.

Fix for BUG_ON(anon_vma->degree) splashes in unlink_anon_vmas() ("kernel
BUG at mm/rmap.c:399!") caused by commit 7a3ef208e662 ("mm: prevent
endless growth of anon_vma hierarchy")

Anon_vma_clone() is usually called for a copy of source vma in
destination argument.  If source vma has anon_vma it should be already
in dst->anon_vma.  NULL in dst->anon_vma is used as a sign that it's
called from anon_vma_fork().  In this case anon_vma_clone() finds
anon_vma for reusing.

Vma_adjust() calls it differently and this breaks anon_vma reusing
logic: anon_vma_clone() links vma to old anon_vma and updates degree
counters but vma_adjust() overrides vma->anon_vma right after that.  As
a result final unlink_anon_vmas() decrements degree for wrong anon_vma.

This patch assigns ->anon_vma before calling anon_vma_clone().

Signed-off-by: Konstantin Khlebnikov <redacted>
Reported-and-tested-by: Chris Clayton <redacted>
Reported-and-tested-by: Oded Gabbay <redacted>
Reported-and-tested-by: Chih-Wei Huang <redacted>
Acked-by: Rik van Riel <redacted>
Acked-by: Vlastimil Babka <redacted>
Cc: Daniel Forrest <redacted>
Cc: Michal Hocko <redacted>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luis Henriques <redacted>
---
 mm/mmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 9f7b97613ca4..5f0712551402 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -749,10 +749,12 @@ again:			remove_next = 1 + (end > next->vm_end);
 		if (exporter && exporter->anon_vma && !importer->anon_vma) {
 			int error;
 
+			importer->anon_vma = exporter->anon_vma;
 			error = anon_vma_clone(importer, exporter);
-			if (error)
+			if (error) {
+				importer->anon_vma = NULL;
 				return error;
-			importer->anon_vma = exporter->anon_vma;
+			}
 		}
 	}
 

-- 
2.1.4