Re: [PATCH v2 0/5] add static PMD zero page support

[David Hildenbrand <david@redhat.com>]

On 15.07.25 17:25, Pankaj Raghav (Samsung) wrote:
> On Tue, Jul 15, 2025 at 04:16:44PM +0200, David Hildenbrand wrote:
>> On 15.07.25 16:12, Lorenzo Stoakes wrote:
>>> On Tue, Jul 15, 2025 at 04:06:29PM +0200, David Hildenbrand wrote:
>>>> I think at some point we discussed "when does the PMD-sized zeropage make
>>>> *any* sense on these weird arch configs" (512MiB on arm64 64bit)
>>>>
>>>> No idea who wants to waste half a gig on that at runtime either.
>>>
>>> Yeah this is a problem we _really_ need to solve. But obviously somewhat out of
>>> scope here.
>>>
>>>>
>>>> But yeah, we should let the arch code opt in whether it wants it or not (in
>>>> particular, maybe only on arm64 with CONFIG_PAGE_SIZE_4K)
>>>
>>> I don't think this should be an ARCH_HAS_xxx.
>>>
>>> Because that's saying 'this architecture has X', this isn't architecture
>>> scope.
>>>
>>> I suppose PMDs may vary in terms of how huge they are regardless of page
>>> table size actually.
>>>
>>> So maybe the best solution is a semantic one - just rename this to
>>> ARCH_WANT_STATIC_PMD_ZERO_PAGE
>>>
>>> And then put the page size selector in the arch code.
>>>
>>> For example in arm64 we have:
>>>
>>> 	select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36)
>>>
>>> So doing something similar here like:
>>>
>>> 	select ARCH_WANT_STATIC_PMD_ZERO_PAGE if ARM64_4K_PAGES
>>>
>>> Would do thie job and sort everything out.
>>
>> Yes.
> 
> Actually I had something similar in one of my earlier versions[1] where we
> can opt in from arch specific Kconfig with *WANT* instead *HAS*.
> 
> For starters, I will enable this only from x86. We can probably extend
> this once we get the base patches up.

Makes sense.

-- 
Cheers,

David / dhildenb