[Patch v2 1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy

[Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>]

Before commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma
hierarchy"), anon_vma_clone() doesn't change dst->anon_vma. While after
this commit, anon_vma_clone() will try to reuse an exist one on forking.

But this commit go a little bit further for the case not forking.
anon_vma_clone() is called from __vma_split(), __split_vma(), copy_vma()
and anon_vma_fork(). For the first three places, the purpose here is get
a copy of src and we don't expect to touch dst->anon_vma even it is
NULL. While after that commit, it is possible to reuse an anon_vma when
dst->anon_vma is NULL. This is not we intend to have.

This patch stop reuse anon_vma for non-fork cases.

Fix commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma
hierarchy")

Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
---
 include/linux/rmap.h | 3 ++-
 mm/mmap.c            | 6 +++---
 mm/rmap.c            | 7 ++++---
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 988d176472df..963e6ab09b9b 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -142,7 +142,8 @@ static inline void anon_vma_unlock_read(struct anon_vma *anon_vma)
 void anon_vma_init(void);	/* create anon_vma_cachep */
 int  __anon_vma_prepare(struct vm_area_struct *);
 void unlink_anon_vmas(struct vm_area_struct *);
-int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *);
+int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
+		   bool reuse);
 int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
 
 static inline int anon_vma_prepare(struct vm_area_struct *vma)
diff --git a/mm/mmap.c b/mm/mmap.c
index 93f221785956..21e94f8ac4c7 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -791,7 +791,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 			int error;
 
 			importer->anon_vma = exporter->anon_vma;
-			error = anon_vma_clone(importer, exporter);
+			error = anon_vma_clone(importer, exporter, false);
 			if (error)
 				return error;
 		}
@@ -2666,7 +2666,7 @@ int __split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (err)
 		goto out_free_vma;
 
-	err = anon_vma_clone(new, vma);
+	err = anon_vma_clone(new, vma, false);
 	if (err)
 		goto out_free_mpol;
 
@@ -3247,7 +3247,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 		new_vma->vm_pgoff = pgoff;
 		if (vma_dup_policy(vma, new_vma))
 			goto out_free_vma;
-		if (anon_vma_clone(new_vma, vma))
+		if (anon_vma_clone(new_vma, vma, false))
 			goto out_free_mempol;
 		if (new_vma->vm_file)
 			get_file(new_vma->vm_file);
diff --git a/mm/rmap.c b/mm/rmap.c
index d9a23bb773bf..f729e4013613 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -258,7 +258,8 @@ static inline void unlock_anon_vma_root(struct anon_vma *root)
  * good chance of avoiding scanning the whole hierarchy when it searches where
  * page is mapped.
  */
-int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
+int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
+		   bool reuse)
 {
 	struct anon_vma_chain *avc, *pavc;
 	struct anon_vma *root = NULL;
@@ -286,7 +287,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 		 * will always reuse it. Root anon_vma is never reused:
 		 * it has self-parent reference and at least one child.
 		 */
-		if (!dst->anon_vma && anon_vma != src->anon_vma &&
+		if (reuse && !dst->anon_vma && anon_vma != src->anon_vma &&
 				anon_vma->degree < 2)
 			dst->anon_vma = anon_vma;
 	}
@@ -329,7 +330,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 	 * First, attach the new VMA to the parent VMA's anon_vmas,
 	 * so rmap can find non-COWed pages in child processes.
 	 */
-	error = anon_vma_clone(vma, pvma);
+	error = anon_vma_clone(vma, pvma, true);
 	if (error)
 		return error;
 
-- 
2.17.1