Subject: Re: [PATCH v6 05/13] mm/page_vma_mapped: Add flag to page_vma_mapped_walk::flags to track device private pages

From: Alistair Popple <[email-masked]>
Date: Fri, 20 Mar 2026 15:57:40 +1100

On 2026-03-07 at 02:44 +1100, "David Hildenbrand (Arm)" <[email-masked]> wrote...
> On 2/2/26 12:36, Jordan Niethe wrote:
> > A future change will remove device private pages from the physical
> > address space. This will mean that device private pages no longer have
> > normal PFN and must be handled separately.
> > 
> > Prepare for this by adding a PVMW_DEVICE_PRIVATE flag to
> > page_vma_mapped_walk::flags. This indicates that
> > page_vma_mapped_walk::pfn contains a device private offset rather than a
> > normal pfn.
> > 
> > Once the device private pages are removed from the physical address
> > space this flag will be used to ensure a device private offset is
> > returned.
> > 
> > Reviewed-by: Zi Yan <[email-masked]>
> > Signed-off-by: Jordan Niethe <[email-masked]>
> > Signed-off-by: Alistair Popple <[email-masked]>
> > ---
> > v1:
> >   - Update for HMM huge page support
> > v2:
> >   - Move adding device_private param to check_pmd() until final patch
> > v3:
> >   - Track device private offset in pvmw::flags instead of pvmw::pfn
> > v4:
> >   - No change
> > ---
> >  include/linux/rmap.h | 24 ++++++++++++++++++++++--
> >  mm/page_vma_mapped.c |  4 ++--
> >  mm/rmap.c            |  4 ++--
> >  mm/vmscan.c          |  2 +-
> >  4 files changed, 27 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > index daa92a58585d..1b03297f13dc 100644
> > --- a/include/linux/rmap.h
> > +++ b/include/linux/rmap.h
> > @@ -921,6 +921,8 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
> >  #define PVMW_SYNC		(1 << 0)
> >  /* Look for migration entries rather than present PTEs */
> >  #define PVMW_MIGRATION		(1 << 1)
> > +/* pvmw::pfn is a device private offset */
> > +#define PVMW_DEVICE_PRIVATE	(1 << 2)
> >  
> >  /* Result flags */
> >  
> > @@ -939,14 +941,32 @@ struct page_vma_mapped_walk {
> >  	unsigned int flags;
> >  };
> >  
> > +static inline unsigned long page_vma_walk_flags(const struct folio *folio,
> > +						unsigned long flags)
> > +{
> > +	if (folio_is_device_private(folio))
> > +		return flags | PVMW_DEVICE_PRIVATE;
> > +	return flags;
> > +}
> > +
> > +static inline unsigned long folio_page_vma_walk_pfn(const struct folio *folio)
> > +{
> > +	return folio_pfn(folio);
> > +}
> > +
> > +static inline struct folio *page_vma_walk_pfn_to_folio(struct page_vma_mapped_walk *pvmw)
> > +{
> > +	return pfn_folio(pvmw->pfn);
> > +}
> > +
> >  #define DEFINE_FOLIO_VMA_WALK(name, _folio, _vma, _address, _flags)	\
> >  	struct page_vma_mapped_walk name = {				\
> > -		.pfn = folio_pfn(_folio),				\
> > +		.pfn = folio_page_vma_walk_pfn(_folio),			\
> >  		.nr_pages = folio_nr_pages(_folio),			\
> >  		.pgoff = folio_pgoff(_folio),				\
> >  		.vma = _vma,						\
> >  		.address = _address,					\
> > -		.flags = _flags,					\
> > +		.flags = page_vma_walk_flags(_folio, _flags),		\
> >  	}
> 
> That's all rather horrible ...
> 
> 
> I was asking myself recently, why something that is called
> "page_vma_mapped_walk" consume a pfn. It's just a horrible interface.

I don't disagree, and in fact it used to consume a page until 2aff7a4755be ("mm:
Convert page_vma_mapped_walk to work on PFNs"). If this was a page it would
basically resolve all the hackiness of this patch because we would no longer
have to pass PFN context around. So I wonder if there would be any opposition to
changing this back to taking a page?

> 
> 
> * DEFINE_FOLIO_VMA_WALK() users obviously receive a folio.
> * mm/migrate_device.c just abuses page_vma_mapped_walk() to make
>   set_pmd_migration_entry() work. But we have a folio.
> * page_mapped_in_vma() has a page/folio.
> 
> mapping_wrprotect_range_one() and pfn_mkclean_range() are the real
> issues. They all end up calling page_vma_mkclean_one(), which does not
> operate on pages/folios.
> 
> Ideally, the odd pfn case would use it's own simplified infrastructure.
> 
> 
> So, could we simply add a folio+page pointer in case we have one, and
> use that one if set, leaving leaving the pfn unset?
> 
> Then, the pfn would only be set for the
> mapping_wrprotect_range_one/pfn_mkclean_range case. I don't think
> device-private folios would ever have to mess with that.
> 
> 
> Then, you just always have a folio+page and don't even have to worry
> about the pfn?

That sounds reasonable to me. We were hesitant to add a page back to the
interface given it had been removed previously but lets try implementing this to
see what it looks like.

 - Alistair

> -- 
> Cheers,
> David