Re: [PATCH v3 2/3] mm/huge_memory: don't mark refcounted folios special in vmf_insert_folio_pmd()

[Lorenzo Stoakes]

On Fri, Jun 13, 2025 at 03:53:58PM +0200, David Hildenbrand wrote:
> On 13.06.25 15:49, Oscar Salvador wrote:
> > On Fri, Jun 13, 2025 at 11:27:01AM +0200, David Hildenbrand wrote:
> > > Marking PMDs that map a "normal" refcounted folios as special is
> > > against our rules documented for vm_normal_page(): normal (refcounted)
> > > folios shall never have the page table mapping marked as special.
> > >
> > > Fortunately, there are not that many pmd_special() check that can be
> > > mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
> > > would get this wrong right now are rather harmless: e.g., none so far
> > > bases decisions whether to grab a folio reference on that decision.
> > >
> > > Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
> > > implications as it seems.
> > >
> > > Getting this right will get more important as we use
> > > folio_normal_page_pmd() in more places.
> > >
> > > Fix it by teaching insert_pfn_pmd() to properly handle folios and
> > > pfns -- moving refcount/mapcount/etc handling in there, renaming it to
> > > insert_pmd(), and distinguishing between both cases using a new simple
> > > "struct folio_or_pfn" structure.
> > >
> > > Use folio_mk_pmd() to create a pmd for a folio cleanly.
> > >
> > > Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
> > > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > > Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > Tested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >
> > Altough we have it quite well explained here in the changelog, maybe
> > having a little comment in insert_pmd() noting why pmds mapping normal
> > folios cannot be marked special would be nice.
>
> Well, I don't think we should be replicating that all over the place. The
> big comment above vm_normal_page() is currently our source of truth (which I
> will teak soon further).

Suggestion:

"Kinda self-explanatory (special means don't touch) unless you use museum piece
hardware OR IF YOU ARE XEN!"

;)

>
> Thanks!
>
> --
> Cheers,
>
> David / dhildenb
>