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

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Message-ID: <87zfe83qhn.fsf@DESKTOP-5N7EMDA>
Date: Mon, 16 Jun 2025 18:55:32 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Bijan Tabatabai <bijan311@...il.com>
Cc: David Hildenbrand <david@...hat.com>,  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,  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

Bijan Tabatabai <bijan311@...il.com> writes:

> 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.

You can try numa_node_id() first, then fall back to the first nid in
the nodemask.

---
Best Regards,
Huang, Ying