Re: [PATCH v3 2/7] mm: allow shared folios to be promoted to a fast tier

[Gregory Price]

On Thu, Sep 24, 2026 at 01:59:15PM +0200, David Hildenbrand (Arm) wrote:
> I wonder if a better name would just make the comment above (about memory
> tiering) implicit and make more sense of the sysctl_numa_balancing_mode &
> NUMA_BALANCING_MEMORY_TIERING) check.
> 
> 	folio_promotable()
> 
> 	folio_is_promotable()
> 
> 	folio_numa_promotable()
>
> etc.
> 
> because that seems to be what we really test with both things combined.
>

maybe:  folio_numab_promotable() ?

> > -		if ((vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
> > +		if ((vma->vm_flags & VM_EXEC) &&
> > +		    folio_maybe_mapped_shared(folio) &&
> > +		    (!folio_in_lowtier(folio) || !node_is_toptier(node)))
> >  			return -EACCES;
> >  
> >  		/*
> 
> Sashiko has some comment about anonymous shared COW folios. I think it has a
> point, but didn't look too closely.
>

its tl;dr:

`skipping this check means those folios could end up freely migrating east-west
 between slow tiers`

Which cannot happen (we only migrate to top tier, numab will never
migrate *to* a slow tier).

basically false-positive

~Gregory