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-rw-r--r--mm/rmap.c24
1 files changed, 15 insertions, 9 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index 0c7b2a9400d4..ca254f5e4337 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -251,13 +251,19 @@ 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)
{
@@ -287,8 +293,8 @@ 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)