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

[David Hildenbrand (Red Hat)]

On 11/26/25 16:08, Lorenzo Stoakes wrote:
> 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?

I think at leaf because I think the previous ones should essentially be 
only used by stubs.

But I haven't fully digested how this is all working. Or supposed to work.

I'm trying to chew through the arch/arm/include/asm/pgtable-2level.h 
example to see if I can make sense of it,

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

Oh no. :)

> > > > 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 :)

Well, not yet ... :)

--
Cheers

David