Re: [Patch v3 1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy

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

On Fri, Oct 11, 2019 at 10:58:40AM +0800, Wei Yang wrote:
>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>
>
>---
>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;

What a shame.

dst->anon_vma would be changed in the loop, so we only need to assign it when
dst->anon_vma == NULL.

> 	}
> 	if (dst->anon_vma)
>-- 
>2.17.1

-- 
Wei Yang
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