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: <4acaa46f-f856-4116-917e-08e7c1f3156a@redhat.com>
Date: Fri, 13 Jun 2025 15:53:58 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: 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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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).

Thanks!

-- 
Cheers,

David / dhildenb