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

[Wei Yang]

On Fri, Oct 11, 2019 at 11:04:16AM +0300, Konstantin Khlebnikov wrote:
>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@xxxxxxxxxxxxxxx>
>
>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.

Yes, agree with you.

Thanks for your comment :-)

>
>Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>
>> 
>> ---
>> 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)
>> 

-- 
Wei Yang
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