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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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?

> <snip>
>
> Let me know your thoughts.
>
> [1] https://lore.kernel.org/linux-mm/20250612105100.59144-4-p.raghav@samsung.com/#Z31mm:Kconfig
> --
> Pankaj