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

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Message-ID: <37973e21-e8f4-4603-b93d-4e0b1b2499fa@lucifer.local>
Date: Wed, 26 Nov 2025 15:08:43 +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 03:56:13PM +0100, David Hildenbrand (Red Hat) wrote:
> On 11/26/25 15:52, Lorenzo Stoakes wrote:
> >
> > Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up
> > requiring a READ_ONCE() further up the stack?
>
> See my other reply, I think the pmdp_get() is required because all pud_*
> functions are just simple stubs.

OK, thought you were saying we should push further down the stack? Or up
depending on how you view these things :P as in READ_ONCE at leaf?

Anyway. I am now designating you the expert at this ;)

>
> >
> > >
> > > 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...
>
> Looking at include/asm-generic/pgtable-nopmd.h I am not sure we are talking
> about implicit assumptions here. It's kind-of the design that the pud_t
> values are dummies, so why shoul the pudp_get() give you any guarantees.
>
> At least that's my current understanding, which might be very flawed :)

I mean I'm waving my hands around like I'm working on an aircraft carrier here
so if you're _sure_ it's _absolutely_ safe then fine :)

In other words - well volunteered, sir ;)

>
> --
> Cheers
>
> David
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

Cheers, Lorenzo

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