[PATCH 1/2] mm: rmap: explicitly reset vma->anon_vma in unlink_anon_vmas()

[Li Xinhai <lixinhai.lxh@gmail.com>]

In case the vma will continue to be used after unlink its relevant
anon_vma, we need to reset the vma->anon_vma pointer to NULL. So,
later when fault happen within this vma again, a new anon_vma will be
prepared.

By this way, the vma will only be checked for reverse mapping of pages
which been fault in after the unlink_anon_vmas call.

Currently, the mremap with MREMAP_DONTUNMAP scenario will continue use
the vma after moved its page table entries to a new vma. For other
scenarios, the vma itself will be freed after call unlink_anon_vmas.

Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
---
 mm/rmap.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 08c56aaf72eb..31ef76180c53 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -413,8 +413,15 @@ 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 would still be needed after unlink, and anon_vma will be prepared
+		 * when handle fault.
+		 */
+		vma->anon_vma = NULL;
+	}
 	unlock_anon_vma_root(root);
 
 	/*
-- 
2.18.4