[PATCH] mm: fix potential anon_vma locking issue in mprotect()

[Michel Lespinasse <walken@google.com>]

This change fixes an anon_vma locking issue in the following situation:
- vma has no anon_vma
- next has an anon_vma
- vma is being shrunk / next is being expanded, due to an mprotect call

We need to take next's anon_vma lock to avoid races with rmap users
(such as page migration) while next is being expanded.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 mm/mmap.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 3edfcdfa42d9..6fd7afa0e651 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -578,8 +578,12 @@ again:			remove_next = 1 + (end > next->vm_end);
 	 */
 	if (vma->anon_vma && (importer || start != vma->vm_start)) {
 		anon_vma = vma->anon_vma;
+		VM_BUG_ON(adjust_next && next->anon_vma &&
+			  anon_vma != next->anon_vma);
+	} else if (adjust_next && next->anon_vma)
+		anon_vma = next->anon_vma;
+	if (anon_vma)
 		anon_vma_lock(anon_vma);
-	}
 
 	if (root) {
 		flush_dcache_mmap_lock(mapping);
-- 
1.7.7.3

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