On Mon, Jun 30, 2025 at 10:39 PM Dave Hansen dave.hansen@intel.com wrote:
On 6/30/25 12:07, Jann Horn wrote:
--- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -147,7 +147,7 @@ config X86 select ARCH_WANTS_DYNAMIC_TASK_STRUCT select ARCH_WANTS_NO_INSTR select ARCH_WANT_GENERAL_HUGETLB
select ARCH_WANT_HUGE_PMD_SHARE
select ARCH_WANT_HUGE_PMD_SHARE if PGTABLE_LEVELS > 2 select ARCH_WANT_LD_ORPHAN_WARN select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP if X86_64 select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP if X86_64Does pmd sharing really even work on 32-bit? Just practically, you only ever have 3GB of address space and thus 3 possible PGDs that can be used for sharing (with the 3:1 split configured). You presumably need *some* address space for the binary to even execve(). The vdso and friends go somewhere and we normally don't let anything get mapped at 0x0.
I think that leaves _maybe_ one slot.
Barring something some specific and compelling actual use case, this should probably just be:
select ARCH_WANT_HUGE_PMD_SHARE if X86_64
Yeah, makes sense. I was also thinking that it would be more reasonable to restrict this to 64-bit only, but figured it would be less risky to make this more specific change.
But now that I think about it, it's not like stuff is actually going to break from this change, worst case the kernel memory usage goes up a bunch in a very unlikely configuration... so yeah, I guess I'll resend this later with "if X86-64".