Re: [Patch v4 2/2] mm/rmap.c: reuse mergeable anon_vma as parent when fork

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 11/10/2019 10.22, 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.
> 
> Do the same kernel build test, it shows run time in sys reduced 11.6%.
> 
> 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	2m39.933s
> user	17m1.835s
> sys	1m38.802s
> 
> real	2m39.321s
> user	17m1.634s
> sys	1m39.206s
> 
> real	2m39.575s
> user	17m1.420s
> sys	1m38.845s
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

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

> ---
>   mm/rmap.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c34414567474..2c13e2bfd393 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -268,6 +268,19 @@ 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 (!dst->anon_vma && src->anon_vma &&
> +	    pprev && pprev->anon_vma == src->anon_vma)
> +		dst->anon_vma = prev->anon_vma;
> +

I believe that in present code "prev" cannot be NULL if !dst->anon_vma && src->anon_vma is true.
It would be safer to check this explicitly.

>   
>   	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
>   		struct anon_vma *anon_vma;
>