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: <85b998c1-b9b7-4a8f-b2f0-f5c6d4ada9c8@redhat.com>
Date: Fri, 13 Jun 2025 18:06:07 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 13.06.25 16:00, Lorenzo Stoakes wrote:
> 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!"
> 
> ;)

I looked into the XEN stuff and it is *extremely* nasty.

No, it doesn't do a pte_mkspecial(). It updates the PTE using ...

	!!! A HYPERCALL !!!

WTF, why did we ever allow that.

It's documented to require GUP to work because ... QEMU AIO. Otherwise 
we could easily convert it to a proper PFNMAP.

-- 
Cheers,

David / dhildenb