Re: [PATCH] mm/rmap.c: reuse mergeable anon_vma as parent when fork

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

On Fri, Oct 04, 2019 at 07:33:53PM +0300, Konstantin Khlebnikov wrote:
>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>
>

Thanks


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Wei Yang
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