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 <[email protected]>
---
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
In function __anon_vma_prepare(), we will try to find anon_vma if it is
possible to reuse it. While on fork, the logic is different.
Since commit 5beb49305251 ("mm: change anon_vma linking to fix
multi-process server scalability issue"), function anon_vma_clone()
tries to allocate new anon_vma for child process. But the logic here
will allocate a new anon_vma for each vma, even in parent this vma
is mergeable and share the same anon_vma with its sibling. This may do
better for scalability issue, while it is not necessary to do so
especially after interval tree is used.
Commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
tries to reuse some anon_vma by counting child anon_vma and attached
vmas. While for those mergeable anon_vmas, we can just reuse it and not
necessary to go through the logic.
After this change, kernel build test reduces 20% anon_vma allocation.
Do the same kernel build test, it shows run time in sys reduced 11.5%.
Origin:
real 2m50.467s
user 17m52.002s
sys 1m51.953s
real 2m48.662s
user 17m55.464s
sys 1m50.553s
real 2m51.143s
user 17m59.687s
sys 1m53.600s
Patched:
real 2m40.080s
user 17m4.644s
sys 1m39.321s
real 2m39.967s
user 17m2.445s
sys 1m38.850s
real 2m40.581s
user 17m1.975s
sys 1m39.065s
Signed-off-by: Wei Yang <[email protected]>
---
mm/rmap.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/mm/rmap.c b/mm/rmap.c
index fc0aba7fb9b9..0dd5f8b04a48 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -269,6 +269,18 @@ 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;
+ struct vm_area_struct *prev = dst->vm_prev, *pprev = src->vm_prev;
+
+ /*
+ * If parent share anon_vma with its vm_prev, keep this sharing in in
+ * child.
+ *
+ * 1. Parent has vm_prev, which implies we have vm_prev.
+ * 2. Parent and its vm_prev have the same anon_vma.
+ */
+ if (reuse && pprev && pprev->anon_vma == src->anon_vma)
+ dst->anon_vma = prev->anon_vma;
+
list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
struct anon_vma *anon_vma;
--
2.17.1
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 <[email protected]>
>
>---
>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
Help you, Help me