Re: [PATCH] mm: mempolicy: fix policy_nodemask() for MPOL_PREFERRED_MANY case

[Feng Tang <feng.tang@intel.com>]

On Mon, Aug 01, 2022 at 05:06:14PM +0800, Michal Hocko wrote:
> On Mon 01-08-22 16:42:07, Muchun Song wrote:
> > policy_nodemask() is supposed to be returned a nodemask representing a mempolicy
> > for filtering nodes for page allocation, which is a hard restriction (see the user
> > of allowed_mems_nr() in hugetlb.c).  However, MPOL_PREFERRED_MANY is a preferred
> > mode not a hard restriction.  Now it breaks the user of HugeTLB.  Remove it from
> > policy_nodemask() to fix it, which will not affect current users of policy_nodemask()
> > since all of the users already have handled the case of MPOL_PREFERRED_MANY before
> > calling it.  BTW, it is found by code inspection.
> 
> I am not sure this is the right fix. It is quite true that
> policy_nodemask is a tricky function to use. It pretends to have a
> higher level logic but all existing users are expected to be policy
> aware and they special case allocation for each policy. That would mean
> that hugetlb should do the same.

Yes, when I worked on the MPOL_PREFERRED_MANY patches, I was also
confused about policy_nodemask(), as it is never a 'strict' one as
the old code is:

	if (unlikely(mode == MPOL_BIND) &&
		apply_policy_zone(policy, gfp_zone(gfp)) &&
		cpuset_nodemask_valid_mems_allowed(&policy->nodes))
		return &policy->nodes;

	return NULL

Even when the MPOL_BIND's nodes is not allowed by cpuset, it will 
still return NULL (equals all nodes).

From the semantics of allowed_mems_nr(), I think it does get changed
a little by b27abaccf8e8. And to enforce the 'strict' semantic for
'allowed', we may need a more strict nodemask API for it.

> I haven't checked the actual behavior implications for hugetlb here. Is
> MPOL_PREFERRED_MANY even supported for hugetlb? Does this change make it
> work? From a quick look this just ignores MPOL_PREFERRED_MANY
> completely.

IIRC, the hugetlb will hornor MPOL_PREFERRED_MANY. And I can double
check and report back if otherwise.

> > Fixes: b27abaccf8e8 ("mm/mempolicy: add MPOL_PREFERRED_MANY for multiple preferred nodes")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/mempolicy.c | 3 ---
> >  1 file changed, 3 deletions(-)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 6c27acb6cd63..4deec7e598c6 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1845,9 +1845,6 @@ nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
> >  		cpuset_nodemask_valid_mems_allowed(&policy->nodes))
> >  		return &policy->nodes;
> >  
> > -	if (mode == MPOL_PREFERRED_MANY)
> > -		return &policy->nodes;

I think it will make MPOL_PREFERRED_MANY not usable.

Thanks,
Feng

> > -
> >  	return NULL;
> >  }
> >  
> > -- 
> > 2.11.0
> 
> -- 
> Michal Hocko
> SUSE Labs