When vma->anon_vma is not set, page fault handler will set it by either
reusing anon_vma of an adjacent VMA if VMAs are compatible or by
allocating a new one. find_mergeable_anon_vma() walks VMA tree to find
a compatible adjacent VMA and that requires not only the faulting VMA
to be stable but also the tree structure and other VMAs inside that tree.
Therefore locking just the faulting VMA is not enough for this search.
Fall back to taking mmap_lock when vma->anon_vma is not set. This
situation happens only on the first page fault and should not affect
overall performance.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/memory.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 5e1c124552a1..13369ff15ec1 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -5242,6 +5242,10 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma_is_anonymous(vma))
 		goto inval;
 
+	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
+	if (!vma->anon_vma)
+		goto inval;
+
 	if (!vma_start_read(vma))
 		goto inval;
 

-- 
2.39.1