[PATCH 2/7] 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.

This change also removes an optimization which avoided taking anon_vma
lock during brk adjustments. We could probably make that optimization
work again, but the following anon rmap change would break it,
so I kept things as simple as possible here.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 mm/mmap.c |   14 ++++++--------
 1 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index cebc346ba0db..5e64c7dfc090 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -570,14 +570,12 @@ again:			remove_next = 1 + (end > next->vm_end);
 
 	vma_adjust_trans_huge(vma, start, end, adjust_next);
 
-	/*
-	 * When changing only vma->vm_end, we don't really need anon_vma
-	 * lock. This is a fairly rare case by itself, but the anon_vma
-	 * lock may be shared between many sibling processes.  Skipping
-	 * the lock for brk adjustments makes a difference sometimes.
-	 */
-	if (vma->anon_vma && (importer || start != vma->vm_start)) {
-		anon_vma = vma->anon_vma;
+	anon_vma = vma->anon_vma;
+	if (!anon_vma && adjust_next)
+		anon_vma = next->anon_vma;
+	if (anon_vma) {
+		VM_BUG_ON(adjust_next && next->anon_vma &&
+			  anon_vma != next->anon_vma);
 		anon_vma_lock(anon_vma);
 	}
 
-- 
1.7.7.3

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