[Patch v3 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>

---
v3:
  * use dst->anon_vma and src->anon_vma to get reuse state
    pointed by Konstantin Khlebnikov
---
 mm/rmap.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index d9a23bb773bf..fc0aba7fb9b9 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -250,7 +250,13 @@ 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_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 (reuse = !dst->anon_vma && src->anon_vma).
+ *
+ * If reuse 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
@@ -262,6 +268,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 {
 	struct anon_vma_chain *avc, *pavc;
 	struct anon_vma *root = NULL;
+	bool reuse = !dst->anon_vma && src->anon_vma;
 
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma;
@@ -286,8 +293,7 @@ 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 &&
-				anon_vma->degree < 2)
+		if (reuse && anon_vma != src->anon_vma && anon_vma->degree < 2)
 			dst->anon_vma = anon_vma;
 	}
 	if (dst->anon_vma)
-- 
2.17.1