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

[David Hildenbrand <david@redhat.com>]

On 15.07.25 16:02, Lorenzo Stoakes wrote:
> On Wed, Jul 09, 2025 at 10:03:51AM +0200, Pankaj Raghav wrote:
>> Hi Zi,
>>
>>>> Add a config option STATIC_PMD_ZERO_PAGE that will always allocate the huge_zero_folio via
>>>> memblock, and it will never be freed.
>>>
>>> Do the above users want a PMD sized zero page or a 2MB zero page? Because on systems with non
>>> 4KB base page size, e.g., ARM64 with 64KB base page, PMD size is different. ARM64 with 64KB base
>>> page has 512MB PMD sized pages. Having STATIC_PMD_ZERO_PAGE means losing half GB memory. I am
>>> not sure if it is acceptable.
>>>
>>
>> That is a good point. My intial RFC patches allocated 2M instead of a PMD sized
>> page.
>>
>> But later David wanted to reuse the memory we allocate here with huge_zero_folio. So
>> if this config is enabled, we simply just use the same pointer for huge_zero_folio.
>>
>> Since that happened, I decided to go with PMD sized page.
>>
>> This config is still opt in and I would expect the users with 64k page size systems to not enable
>> this.
>>
>> But to make sure we don't enable this for those architecture, I could do a per-arch opt in with
>> something like this[1] that I did in my previous patch:
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 340e5468980e..c3a9d136ec0a 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -153,6 +153,7 @@ config X86
>>   	select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP	if X86_64
>>   	select ARCH_WANT_HUGETLB_VMEMMAP_PREINIT if X86_64
>>   	select ARCH_WANTS_THP_SWAP		if X86_64
>> +	select ARCH_HAS_STATIC_PMD_ZERO_PAGE	if X86_64
>>   	select ARCH_HAS_PARANOID_L1D_FLUSH
>>   	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
>>   	select BUILDTIME_TABLE_SORT
>>
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 781be3240e21..fd1c51995029 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -826,6 +826,19 @@ config ARCH_WANTS_THP_SWAP
>>   config MM_ID
>>   	def_bool n
>>
>> +config ARCH_HAS_STATIC_PMD_ZERO_PAGE
>> +	def_bool n
> 
> Hm is this correct? arm64 supports mutliple page tables sizes, so while the
> architecture might 'support' it, it will vary based on page size, so actually we
> don't care about arch at all?
> 
>> +
>> +config STATIC_PMD_ZERO_PAGE
>> +	bool "Allocate a PMD page for zeroing"
>> +	depends on ARCH_HAS_STATIC_PMD_ZERO_PAGE
> 
> Maybe need to just make this depend on !CONFIG_PAGE_SIZE_xx?

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.

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)

-- 
Cheers,

David / dhildenb