linux-kernel - Re: [RFC PATCH 1/4] mm/mempolicy: Expose policy

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Message-ID: <CAMvvPS5U8exSvy0fknfhv8ym_dKgMVa7cfMOqn0fGyd+NSjSuQ@mail.gmail.com>
Date: Fri, 13 Jun 2025 11:33:18 -0500
From: Bijan Tabatabai <bijan311@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, sj@...nel.org, akpm@...ux-foundation.org, 
	corbet@....net, ziy@...dia.com, matthew.brost@...el.com, 
	joshua.hahnjy@...il.com, rakie.kim@...com, byungchul@...com, 
	gourry@...rry.net, ying.huang@...ux.alibaba.com, apopple@...dia.com, 
	bijantabatab@...ron.com, venkataravis@...ron.com, emirakhur@...ron.com, 
	ajayjoshi@...ron.com, vtavarespetr@...ron.com, damon@...ts.linux.dev
Subject: Re: [RFC PATCH 1/4] mm/mempolicy: Expose policy_nodemask() in include/linux/mempolicy.h

On Fri, Jun 13, 2025 at 8:45 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 12.06.25 20:13, Bijan Tabatabai wrote:
> > From: Bijan Tabatabai <bijantabatab@...ron.com>
> >
> > This patch is to allow DAMON to call policy_nodemask() so it can
> > determine where to place a page for interleaving.
> >
> > Signed-off-by: Bijan Tabatabai <bijantabatab@...ron.com>
> > ---
> >   include/linux/mempolicy.h | 9 +++++++++
> >   mm/mempolicy.c            | 4 +---
> >   2 files changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
> > index 0fe96f3ab3ef..e96bf493ff7a 100644
> > --- a/include/linux/mempolicy.h
> > +++ b/include/linux/mempolicy.h
> > @@ -133,6 +133,8 @@ struct mempolicy *__get_vma_policy(struct vm_area_struct *vma,
> >   struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
> >               unsigned long addr, int order, pgoff_t *ilx);
> >   bool vma_policy_mof(struct vm_area_struct *vma);
> > +nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *pol,
> > +             pgoff_t ilx, int *nid);
> >
> >   extern void numa_default_policy(void);
> >   extern void numa_policy_init(void);
> > @@ -232,6 +234,13 @@ static inline struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
> >       return NULL;
> >   }
> >
> > +static inline nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *pol,
> > +                             pgoff_t ilx, int *nid)
> > +{
> > +     *nid = NUMA_NO_NODE;
> > +     return NULL;
> > +}
> > +
> >   static inline int
> >   vma_dup_policy(struct vm_area_struct *src, struct vm_area_struct *dst)
> >   {
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 3b1dfd08338b..54f539497e20 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -596,8 +596,6 @@ static const struct mempolicy_operations mpol_ops[MPOL_MAX] = {
> >
> >   static bool migrate_folio_add(struct folio *folio, struct list_head *foliolist,
> >                               unsigned long flags);
> > -static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *pol,
> > -                             pgoff_t ilx, int *nid);
> >
> >   static bool strictly_unmovable(unsigned long flags)
> >   {
> > @@ -2195,7 +2193,7 @@ static unsigned int interleave_nid(struct mempolicy *pol, pgoff_t ilx)
> >    * Return a nodemask representing a mempolicy for filtering nodes for
> >    * page allocation, together with preferred node id (or the input node id).
> >    */
> > -static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *pol,
> > +nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *pol,
> >                                  pgoff_t ilx, int *nid)
> >   {
> >       nodemask_t *nodemask = NULL;
>
> You actually only care about the nid for your use case.
>
> Maybe we should add
>
> get_vma_policy_node() that internally does a get_vma_policy() to then
> give you only the node back.
>
> If get_vma_policy() is not the right thing (see my reply to patch #2),
> of course a get_task_policy_node() could be added.
>
> --
> Cheers,
>
> David / dhildenb

Hi David,

I did not use get_vma_policy or mpol_misplaced, which I believe is the
closest function that exists for what I want in this patch, because
those functions
seem to assume they are called inside of the task that the folio/vma
is mapped to.
More specifically, mpol_misplaced assumes it is being called within a
page fault.
This doesn't work for us, because we call it inside of a kdamond process.

I would be open to adding a new function that takes in a folio, vma,
address, and
task_struct and returns the nid the folio should be placed on. It could possibly
be implemented as a function internal to mpol_misplaced because the two would
be very similar.

How would you propose we handle MPOL_BIND and MPOL_PREFFERED_MANY
in this function? mpol_misplaced chooses a nid based on the node and
cpu the fault
occurred on, which we wouldn't have in a kdamond context. The two options I see
are either:
1. return the nid of the first node in the policy's nodemask
2. return NUMA_NO_NODE
I think I would lean towards the first.

Thanks,
Bijan