Re: [Patch v4 1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy

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

On 11/10/2019 10.22, 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>

Yes, reusing heuristic was designed for fork.
But this isn't strictly necessary - any vmas could share anon_vma.
For example all vmas in system could be linked with single anon_vma.

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

> 
> ---
> v4:
>    * check dst->anon_vma in each iteration
> v3:
>    * use dst->anon_vma and src->anon_vma to get reuse state
>      pointed by Konstantin Khlebnikov
> ---
>   mm/rmap.c | 24 +++++++++++++++---------
>   1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d9a23bb773bf..c34414567474 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -250,13 +250,19 @@ 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 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
> - * child isn't reused even if there was no alive vma, thus rmap walker has a
> - * good chance of avoiding scanning the whole hierarchy when it searches where
> - * page is mapped.
> + * 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 checking (!dst->anon_vma && src->anon_vma).
> + *
> + * If (!dst->anon_vma && src->anon_vma) 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 child isn't reused even if there was no alive vma, thus rmap
> + * walker has a good chance of avoiding scanning the whole hierarchy when it
> + * searches where page is mapped.
>    */
>   int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>   {
> @@ -286,8 +292,8 @@ 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 (!dst->anon_vma && src->anon_vma &&
> +		    anon_vma != src->anon_vma && anon_vma->degree < 2)
>   			dst->anon_vma = anon_vma;
>   	}
>   	if (dst->anon_vma)
>