Re: [PATCH] x86/mm: Disable hugetlb page table sharing on non-PAE 32-bit

[Vitaly Chikunov <vt@altlinux.org>]

Jann,

On Mon, Jun 30, 2025 at 09:07:34PM +0200, Jann Horn wrote:
> Only select ARCH_WANT_HUGE_PMD_SHARE if hugetlb page table sharing is
> actually possible; page table sharing requires at least three levels,
> because it involves shared references to PMD tables.
> 
> Having ARCH_WANT_HUGE_PMD_SHARE enabled on non-PAE 32-bit X86 (which
> has 2-level paging) became particularly problematic after commit
> 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count"),
> since that changes `struct ptdesc` such that the `pt_mm` (for PGDs) and
> the `pt_share_count` (for PMDs) share the same union storage - and with
> 2-level paging, PMDs are PGDs.
> 
> (For comparison, arm64 also gates ARCH_WANT_HUGE_PMD_SHARE on the
> configuration of page tables such that it is never enabled with 2-level
> paging.)
> 
> Reported-by: Vitaly Chikunov <vt@altlinux.org>
> Closes: https://lore.kernel.org/r/srhpjxlqfna67blvma5frmy3aa@altlinux.org
> Fixes: cfe28c5d63d8 ("x86: mm: Remove x86 version of huge_pmd_share.")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>

Tested on i586 over v6.1.142 (where the problem was surfaced).

Tested-by: Vitaly Chikunov <vt@altlinux.org>

Thanks,

> ---
>  arch/x86/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 71019b3b54ea..917f523b994b 100644
> --- 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_64
> 
> ---
> base-commit: d0b3b7b22dfa1f4b515fd3a295b3fd958f9e81af
> change-id: 20250630-x86-2level-hugetlb-b1d8feb255ce
> 
> -- 
> Jann Horn <jannh@google.com>