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

[Oscar Salvador <osalvador@suse.de>]

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@nvidia.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Tested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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.

But just saying :-)

Reviewed-by: Oscar salvador <osalvador@suse.de>

 

-- 
Oscar Salvador
SUSE Labs