Re: mm: opaque hardware page-table entry handles

[Alexander Gordeev <agordeev@linux.ibm.com>]

On Wed, Jul 15, 2026 at 05:15:36PM +0100, Muhammad Usama Anjum wrote:
> Hi,
> 
> [Moved some already involved people to To. So they can help with the plan
> details mentioned below.]
> 
> On 07/07/2026 2:17 pm, David Hildenbrand (Arm) wrote:
> > [...]
> > 
> >>>
> >>> typedef struct {
> >>> 	pte_t __pte;
> >>> } hw_pte_t;
> >>>
> >>> And then simply use
> >>>
> >>> hw_pte_t *hptep;
> >> Make sense. So you have suggested to just hide put pte_t inside a structure
> >> instead of complex structure of pointer. I've tried to implement and it reduces
> >> churn enormously.
> > 
> > Right. And for most architectures we can probable leave both types be the same
> > under the hood.
> > 
> > So we'd only have to convert common code first, and can then e.g., look into
> > making architectures that care about the difference (e.g., arm64) actually have
> > it be two separate types.
> It makes a lot of sense.
> 
> Let's even divide the series into more parts as there are several places where
> conversion is controversial. (Xi Yan had mentioned one example earlier in this
> thread.) Most of those controversial conversions are pmd related. I propose
> that we convert pte_t first, then pmd_t and others. It'll keep the number of
> patches manageable and easier to review. 
> 
> I think wider agreement for this approach will be very helpful before I post the
> actual code.
> 
> > 
> > Just an idea to further reduce the churn and limit it only to core code (because
> > I saw some very ugly stuff in some arch code that would make such a conversion
> > harder).
> > 
> > [...]
> > 
> >>> Why do we need this and what would we use it for?
> >> The idea was that there should be two different functions to read value. Let's
> >> leave this out of the first initial series. It is complicating the original
> >> proposal.
> > 
> > Right, let's leave that out for now. I'm currently working with Levi on an
> > approach that tries to avoid the overhead due to READ_ONCE with folded page
> > tables. [1]
> So you are referring to page folding improvements. I also stumbled upon those
> while doing a dirty implementation.
> 
> > 
> > We're still struggling with some bits, but looks like we can make it fly and
> > have it be fairly robust.
> > 
> > With that, maybe there is no reason left to have separate pXXp_get() vs.
> > pXXp_get_once(). TBD :)
> Yeah, I was reviewing that series earlier today. I've not looked deeply, but it
> seems there are still a lot of cases where (mostly) pmd is getting dereferenced
> directly. To complete the conversion, direct dereferences need to be converted
> into an API. I've been thinking if there should be a dereference macro or we
> must always use pmdp_get() even though it ensures ordering. It may add excessive 
> ordering in some functions if pmdp_get() is getting called multiple times. But
> storing its output in a tmp variable would solve this. 
> 
> Do you agree with converting all direct dereferences into pXXp_get()?

For the clarity (e.g. on the PTE level) is it goint to be converted to?

pte_t ptep_get(hw_pte_t *ptep);
pte_t set_pte(hw_pte_t *ptep, pte_t pte);

While variables on stack are still may be dereferenced directly via pte_t*?

What about unlinked/temporary page tables in memory?

> > [1] https://lore.kernel.org/all/08ecabe9-0664-4aea-82fb-f9cb1739f762@kernel.org/
> > 
> > [...]
> > 
> >>>
> >>> I'm still not sure about the _once() really, and if we need that right now. We
> >>> survived without is so far, why do we need it now?
> >> The idea is to convert all current pXXp_get() to pXXp_get_once() and convert raw
> >> dereference to pXXp_get(). Let's keep this idea separate for the other work. Let's
> >> discuss it later sometime again later.
> > 

> > Sounds good.
> 
> I've tried to do conversions already. Converting by call hierarchy wise is very
> difficult and error prune.
> 
> Converting component by component (such as page walk, huge page) is also difficult
> as code is tangled. Some helper which is getting used in one component is also used
> in another component.
> 
> I see the following way forward:
> * Add the new type
> * Identify which functions need explicitly pointer to a stack variable. These
>   must not be converted. These variables must be renamed to a common name. For
>   pte_t pointers on stack some functions already use ptentp, which is unique name
>   if we look at generic code. So ptentp would be used for all such variables.
> * One commit per controversial change would be done at this point. We need new
>   separate function for stack types. Also we can do more renaming to a name which
>   will not be converted.
> * Run Coccinelle script directory-by-directory which would ignore converting any
>   ptentp (and similarly for other types). Coccinelle doesn't converts in some case
>   (pte_t *a. *b) which can be done by hand at this point.
> * Update any remaining functions

What is the approach to STRICT_MM_TYPECHECKS?

We would like to keep it, and I guess some other architectures too.

> -- 
> Thanks,
> Usama

Thanks!