[Patch v4 1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy

[Wei Yang <richardw.yang@linux.intel.com>]

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@linux.intel.com>

---
v4:
  * check dst->anon_vma in each iteration
v3:
  * use dst->anon_vma and src->anon_vma to get reuse state
    pointed by Konstantin Khlebnikov
---
 mm/rmap.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index d9a23bb773bf..c34414567474 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -250,13 +250,19 @@ static inline void unlock_anon_vma_root(struct anon_vma *root)
  * Attach the anon_vmas from src to dst.
  * Returns 0 on success, -ENOMEM on failure.
  *
- * If dst->anon_vma is NULL this function tries to find and reuse existing
- * anon_vma which has no vmas and only one child anon_vma. This prevents
- * degradation of anon_vma hierarchy to endless linear chain in case of
- * constantly forking task. On the other hand, an anon_vma with more than one
- * child isn't reused even if there was no alive vma, thus rmap walker has a
- * good chance of avoiding scanning the whole hierarchy when it searches where
- * page is mapped.
+ * anon_vma_clone() is called by __vma_split(), __split_vma(), copy_vma() and
+ * anon_vma_fork(). The first three want an exact copy of src, while the last
+ * one, anon_vma_fork(), may try to reuse an existing anon_vma to prevent
+ * endless growth of anon_vma. Since dst->anon_vma is set to NULL before call,
+ * we can identify this case by checking (!dst->anon_vma && src->anon_vma).
+ *
+ * If (!dst->anon_vma && src->anon_vma) is true, this function tries to find
+ * and reuse existing anon_vma which has no vmas and only one child anon_vma.
+ * This prevents degradation of anon_vma hierarchy to endless linear chain in
+ * case of constantly forking task. On the other hand, an anon_vma with more
+ * than one child isn't reused even if there was no alive vma, thus rmap
+ * walker has a 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)
 {
@@ -286,8 +292,8 @@ 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 &&
-				anon_vma->degree < 2)
+		if (!dst->anon_vma && src->anon_vma &&
+		    anon_vma != src->anon_vma && anon_vma->degree < 2)
 			dst->anon_vma = anon_vma;
 	}
 	if (dst->anon_vma)
-- 
2.17.1