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

[SeongJae Park <sj@kernel.org>]

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:
[...]
> > Hate to interject here,

Please don't hesitate :)

[...]
> > 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


Thanks,
SJ

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