[PATCH, RFC 21/62] mm/rmap: Clear vma->anon_vma on unlink_anon_vmas()

[Kirill A. Shutemov]

If all pages in the VMA got unmapped there's no reason to link it into
original anon VMA hierarchy: it cannot possibly share any pages with
other VMA.

Set vma->anon_vma to NULL on unlink_anon_vmas(). With the change VMA
can be reused. The new anon VMA will be allocated on the first fault.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/rmap.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index b30c7c71d1d9..4ec2aee7baa3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -400,8 +400,10 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
-	if (vma->anon_vma)
+	if (vma->anon_vma) {
 		vma->anon_vma->degree--;
+		vma->anon_vma = NULL;
+	}
 	unlock_anon_vma_root(root);
 
 	/*
-- 
2.20.1