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
