Re: [RFC PATCH 1/4] mm/mempolicy: Expose policy_nodemask() in include/linux/mempolicy.h

[Bijan Tabatabai <bijan311@gmail.com>]

On Tue, Jun 17, 2025 at 1:58 PM SeongJae Park <sj@kernel.org> wrote:
>
> On Mon, 16 Jun 2025 17:16:16 -0500 Bijan Tabatabai <bijan311@gmail.com> wrote:
>
> > Hi Gregory,
> >
> > On Mon, Jun 16, 2025 at 12:43 PM Gregory Price <gourry@gourry.net> wrote:
> > >
> > > On Mon, Jun 16, 2025 at 09:16:55AM -0500, Bijan Tabatabai wrote:
> [...]
> > > I will just say that mempolicy is *extremely* current-task centric - and
> > > very much allocation-time centric (i.e. the internal workings don't
> > > really want to consider migration all that much).  You'll probably find
> > > that this project requires rethinking mempolicy's external interfaces in
> > > general (which is sorely needed anyway).
> > >
> > > I think this path to modifying mempolicy to support DAMON is a bit
> > > ambitious for where mempolicy is at the moment. You may be better off
> > > duplicating the interleave-weight logic and making some helper functions
> > > to get the weight data, and then coming back around to generalize it
> > > later.
>
> Thank you for the nice clarification and opinion, Gregory.
>
> >
> > This may be true, but I think I will be able to avoid a lot of this
> > nastiness with what I need. I am going to try with the mempolicy
> > approach for the next revision, but if I get too much resistance, I
> > will probably switch to this approach.
>
> I have no strong opinion about use of mempolicy for now, as long as mempolicy
> folks are fine.
>
> Nonetheless, I just wanted to mention Gregory's suggestion also sounds fairly
> good to me.  It would avoid unnecessary coupling of the concepts of
> allocation-time interleaving and after-allocation migration.  Also it feels
> even more aligned with a potential future extension of this project that we
> discussed[1]: letting users set multiple target nodes for
> DAMOS_MIGRATE_{HOT,COLD} with arbitrary weights.
>
> [1] https://lore.kernel.org/20250613171237.44776-1-sj@kernel.org

Given this discussion, as well as Joshua's comments earlier [1], it
sounds like while people aren't exactly opposed to using mempolicy for
this, the building consensus is that it would be best not to. I will
move the interleave logic to DAMON for the next revision. However, I
still think it makes sense to use the global weights (probably via
get_il_weight) for now to avoid allocating pages a certain way and
then migrating them soon after.

I'll try to send the next version of the patch set by the end of the week.

Thanks everyone for their feedback,
Bijan

[1] https://lore.kernel.org/linux-mm/20250613152517.225529-1-joshua.hahnjy@gmail.com/
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