linux-kernel - Re: [PATCH v3 06/22] mm: Always use page table accessor functions

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Message-ID: <ab39d448-aa1c-4c93-8e2b-5d97a9c76af5@lucifer.local>
Date: Wed, 26 Nov 2025 14:52:15 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Ryan Roberts <ryan.roberts@....com>, Wei Yang <richard.weiyang@...il.com>,
        Samuel Holland <samuel.holland@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
        linux-riscv@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        devicetree@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
        Michal Hocko <mhocko@...e.com>, Conor Dooley <conor@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Alexandre Ghiti <alex@...ti.fr>,
        Emil Renner Berthing <kernel@...il.dk>,
        Rob Herring <robh+dt@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Nicolas Palix <nicolas.palix@...g.fr>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions

On Wed, Nov 26, 2025 at 03:46:40PM +0100, David Hildenbrand (Red Hat) wrote:
> On 11/26/25 15:22, Ryan Roberts wrote:
> > On 26/11/2025 13:47, Wei Yang wrote:
> > > On Wed, Nov 26, 2025 at 01:03:42PM +0000, Ryan Roberts wrote:
> > > > On 26/11/2025 12:35, David Hildenbrand (Red Hat) wrote:
> > > [...]
> > > > > > > > > Hi,
> > > > > > > > >
> > > > > > > > > I've just come across this patch and wanted to mention that we could also
> > > > > > > > > benefit from this improved absraction for some features we are looking at for
> > > > > > > > > arm64. As you mention, Anshuman had a go but hit some roadblocks.
> > > > > > > > >
> > > > > > > > > The main issue is that the compiler was unable to optimize away the
> > > > > > > > > READ_ONCE()s
> > > > > > > > > for the case where certain levels of the pgtable are folded. But it can
> > > > > > > > > optimize
> > > > > > > > > the plain C dereferences. There were complaints the the generated code for arm
> > > > > > > > > (32) and powerpc was significantly impacted due to having many more
> > > > > > > > > (redundant)
> > > > > > > > > loads.
> > > > > > > > >
> > > > > > > >
> > > > > > > > We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort
> > > > > > > > this out internally?
> > > > > > > >
> > > > > > >
> > > > > > > Just stumbled over the reply from Christope:
> > > > > > >
> > > > > > > https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org
> > > > > > >
> > > > > > > And wonder if we could handle that somehow directly in the pgdp_get() etc.
> > > >
> > > > I certainly don't like the suggestion of doing the is_folded() test outside the
> > > > helper, but if we can push that logic down into pXdp_get() that would be pretty
> > > > neat. Anshuman and I did briefly play with the idea of doing a C dereference if
> > > > the level is folded and a READ_ONCE() otherwise, all inside each pXdp_get()
> > > > helper. Although we never proved it to be correct. I struggle with the model for
> > > > folding. Do you want to optimize out all-but-the-highest level's access or
> > > > all-but-the-lowest level's access? Makes my head hurt...
> > > >
> > > >
> > >
> > > You mean sth like:
> > >
> > > static inline pmd_t pmdp_get(pmd_t *pmdp)
> > > {
> > > #ifdef __PAGETABLE_PMD_FOLDED
> > > 	return *pmdp;
> > > #else
> > > 	return READ_ONCE(*pmdp);
> > > #endif
> > > }
> >
> > Yes. But I'm not convinced it's correct.
>
> Yeah, I'm also still trying to understand how it could work.
>
> >
> > I *think* (but please correct me if I'm wrong) if the PMD is folded, the PUD and
> > P4D must also be folded, and you effectively have a 2 level pgtable consisting
> > of the PGD table and the PTE table. p4dp_get(), pudp_get() and pmdp_get() are
> > all effectively duplicating the load of the pgd entry? So assuming pgdp_get()
> > was already called and used READ_ONCE(), you might hope the compiler will just
> > drop the other loads and just use the value returned by READ_ONCE(). But I doubt
> > there is any guarantee of that and you might be in a situation where pgdp_get()
> > never even got called (perhaps you already have the pmd pointer).
> With __PAGETABLE_PMD_FOLDED we treat the PUD to be fake-present, like
>
> static inline int pud_present(pud_t pud)	{ return 1; }
>
> And obtaining the pmd_t* is essentially  cast of the pud_t*
>
> static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address)
> {
> 	return (pmd_t *)pud;
> }
>
> So in that case we might want to have the READ_ONCE() remove from the
> pudp_get(), not the pmdp_get()?

Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up
requiring a READ_ONCE() further up the stack?

>
> IOW, push the READ_ONCE() down to the lowest level so the previous ones
> (that will get essentially ignore?) will get folded into the last
> READ_ONCE()?
>
> But my head still hurts and I am focusing on something else concurrently :)

Even if we could make this work, I don't love that there's some implicit
assumption there that could easily break later on.

I'd rather we kept it as stupid/obvious as possible...

>
> --
> Cheers
>
> David