Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

["Huang, Ying" <ying.huang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> writes:

> On Thu 27-10-22 14:47:22, Huang, Ying wrote:
>> Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> writes:
> [...]
>> > I can imagine workloads which wouldn't like to get their memory demoted
>> > for some reason but wouldn't it be more practical to tell that
>> > explicitly (e.g. via prctl) rather than configuring cpusets/memory
>> > policies explicitly?
>> 
>> If my understanding were correct, prctl() configures the process or
>> thread.
>
> Not necessarily. There are properties which are per adddress space like
> PR_[GS]ET_THP_DISABLE. This could be very similar.
>
>> How can we get process/thread configuration at demotion time?
>
> As already pointed out in previous emails. You could hook into
> folio_check_references path, more specifically folio_referenced_one
> where you have all that you need already - all vmas mapping the page and
> then it is trivial to get the corresponding vm_mm. If at least one of
> them has the flag set then the demotion is not allowed (essentially the
> same model as VM_LOCKED).

Got it!  Thanks for detailed explanation.

One bit may be not sufficient.  For example, if we want to avoid or
control cross-socket demotion and still allow demoting to slow memory
nodes in local socket, we need to specify a node mask to exclude some
NUMA nodes from demotion targets.

From overhead point of view, this appears similar as that of VMA/task
memory policy?  We can make mm->owner available for memory tiers
(CONFIG_NUMA && CONFIG_MIGRATION).  The advantage is that we don't need
to introduce new ABI.  I guess users may prefer to use `numactl` than a
new ABI?

Best Regards,
Huang, Ying