Re: [PATCH V2 1/4] arm64/mm: Use pmdp_get() for PMD accesses

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/09/2026 13:22, Anshuman Khandual wrote:
> On Wed, Sep 23, 2026 at 12:31:42PM +0100, Ryan Roberts wrote:
>> On 23/09/2026 12:01, Anshuman Khandual wrote:
>>> On Wed, Sep 23, 2026 at 09:32:17AM +0100, Ryan Roberts wrote:
>>>> On 22/09/2026 07:16, Anshuman Khandual wrote:
>>>>> Replace READ_ONCE() with pmdp_get() for PMD accesses in preparation for
>>>>> supporting both D64 and D128 translation table formats.
>>>>>
>>>>> READ_ONCE() cannot currently be used for 128-bit page table entries on
>>>>> arm64 because it does not provide the required 128-bit single-copy
>>>>> atomicity, causing builds to fail for accesses wider than 64 bits.
>>>>>
>>>>> Although LDP/STP provide the required atomicity when FEAT_LSE is
>>>>> available (as required by FEAT_D128), extending READ_ONCE() to support
>>>>> 128-bit accesses is undesirable. READ_ONCE() is a general-purpose API,
>>>>> so doing so could encourage other 128-bit users that would either fail
>>>>> to build in configurations without D128 support or, if D128 becomes a
>>>>> runtime option, silently permit tearing on systems without the required
>>>>> hardware support.
>>>>>
>>>>> Instead, standardize PMD accesses on the existing page-table helpers.
>>>>> These can be overridden on arm64 to provide 128-bit single-copy
>>>>> atomicity when required. No functional change intended.
>>>>
>>>> I notice you have an unconverted READ_ONCE(*pmdp) in
>>>> pmdp_test_and_clear_young(). Is that intentional?
>>>
>>> Converting the above READ_ONCE() instances as pmdp_get() cuases a build
>>> failure as pmpd_get() which is defined in generic pgtable header is not
>>> visible inside platform specific pgtable header, even with a forward
>>> declaration.

I was wondering why the forward declaration doesn't work, but looks like some c
files include asm/pgtable.h directly, which explains it. Let's assume there are
good reasons and unpicking it so that only linux/pgtable.h includes
asm/pgtable.h is not practical.

>>>
>>> In D128 V2 series these conversions were performed later in the series,
>>> when platform specific pxdp_get() overrides were added via ptval_get(). 
>>>
>>> But to avoid this, we could just move pmdp_test_and_clear_young() inside
>>> arch/arm64/mm/mmu.c file instead. Folding in the following change builds.
>>
>> I'm not sure if/how this might affect performance? Probably not significant
>> since they are for higher levels, so not called as frequently as pte helpers.
> 
> Right, should not have much performance impact.
> 
>>
>> Another option would be to implement arm64-specific pxdp_get() helpers in
>> asm/pgtable.h to override the generic versions?
> 
> For now those will still be based on READ_ONCE() just like the default ones.
> Although trying to drop similar redundancies from other platforms :)
> 
> https://lore.kernel.org/linux-mm/20260923043226.331880-1-anshuman.khandual@arm.com/
> 
> But given D128 is going to define pxdp_get() anyway it might make sense to
> define them now and withdraw the above series :) Seems like s390 platform
> also has similar situation.

Given the desired end state is that arm64 defines it's own pxdp_get() helpers, I
think the best approach is just to define them now (as READ_ONCE()).

Thanks,
Ryan