linux-kernel - Re: [PATCH v3 2/3] mm/huge_memory: don't mark refcounted folios special in vmf_insert_folio

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Message-ID: <fec14df9-da28-4717-8e4c-e5997a4c32cb@lucifer.local>
Date: Fri, 13 Jun 2025 15:00:46 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Oscar Salvador <osalvador@...e.de>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v3 2/3] mm/huge_memory: don't mark refcounted folios
 special in vmf_insert_folio_pmd()

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@...dia.com>
> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> > > Tested-by: Dan Williams <dan.j.williams@...el.com>
> > > Signed-off-by: David Hildenbrand <david@...hat.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.
>
> 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
>