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SubjectRe: [PATCH] mm/rmap.c: reuse mergeable anon_vma as parent when fork
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On 04/10/2019 19.06, Wei Yang wrote:
> 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.
>

Makes sense. This might have much bigger effect for scenarios when task
unmaps holes in huge vma as red-zones between allocations and then forks.

Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
> mm/rmap.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d9a23bb773bf..12f6c3d7fd9d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -262,6 +262,17 @@ 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;
> + 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 (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;
>

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