Re: [RFC PATCH] mm/mempolicy: add MPOL_PREFERRED_STRICT memory policy

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 10/14/21 15:08, Michal Hocko wrote:
> On Thu 14-10-21 15:00:22, Aneesh Kumar K.V wrote:
>> Michal Hocko <mhocko@suse.com> writes:
>>
>>> On Wed 13-10-21 18:53:55, Aneesh Kumar K.V wrote:
>>>> On 10/13/21 18:46, Andi Kleen wrote:
>>>>>
>>>>>> The difference with MPOL_BIND is the ability to specify a preferred node
>>>>>> which is the first node in the nodemask argument passed.
>>>>>
>>>>> That's always the one with the lowest number. Isn't that quite limiting
>>>>> in practice?
>>>>>
>>>>> It seems if you really want to do that you would need another argument.
>>>>>
>>>> Yes. But that would make it a new syscall. Should we do that?
>>>
>>> Yes, I do not see any reasonable to cram this into the existing syscall.
>>> I am not yet sure what the syscall should look like though. I can see
>>> two usecases, one of the is a very specific node allocation fallback
>>> order requirement and another one is preferrence for a cpu less node
>>> over other nodes. Both are slightly different.
>>
>> How about
>>
>> SYSCALL_DEFINE5(preferred_mbind, unsigned long, start, unsigned long, len,
>> 		unsigned long, preferred_node, const unsigned long __user *, nmask,
>> 		unsigned long, maxnode)
>> {
>> 	return kernel_mbind(start, len, MPOL_PREFERRED_STRICT, preferred_node,
>> 			    nmask, maxnode, 0);
>> }
> 
> Semantic? How does it interact with MPOL_PREFERRED_MANY, MPOL_BIND and
> other others?
> 

This allows to specify a new memory policy for the va range. We are 
forced to use a new syscall because of the limitation of the current 
mbind(2) syscall. We could make a generic sys_mbind2(), but i was not 
sure whether we need to make it that complex. mbind() is already a 6 
argument syscall.

> Besides that it would be really great to finish the discussion about the
> usecase before suggesting a new userspace API.
> 

Application would like to hint a preferred node for allocating memory 
backing a va range and at the same time wants to avoid fallback to some 
set of nodes (in the use case I am interested don't fall back to slow 
memory nodes).


-aneesh