The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 76303ee8d54bff6d9a6d55997acd88a6c2ba63cf Gitweb: https://git.kernel.org/tip/76303ee8d54bff6d9a6d55997acd88a6c2ba63cf Author: Jann Horn jannh@google.com AuthorDate: Wed, 02 Jul 2025 10:32:04 +02:00 Committer: Dave Hansen dave.hansen@linux.intel.com CommitterDate: Wed, 09 Jul 2025 07:46:36 -07:00

x86/mm: Disable hugetlb page table sharing on 32-bit

Only select ARCH_WANT_HUGE_PMD_SHARE on 64-bit x86. Page table sharing requires at least three levels because it involves shared references to PMD tables; 32-bit x86 has either two-level paging (without PAE) or three-level paging (with PAE), but even with three-level paging, having a dedicated PGD entry for hugetlb is only barely possible (because the PGD only has four entries), and it seems unlikely anyone's actually using PMD sharing on 32-bit.

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.)

Closes: https://lore.kernel.org/r/srhpjxlqfna67blvma5frmy3aa@altlinux.org Fixes: cfe28c5d63d8 ("x86: mm: Remove x86 version of huge_pmd_share.") Reported-by: Vitaly Chikunov vt@altlinux.org Suggested-by: Dave Hansen dave.hansen@intel.com Signed-off-by: Jann Horn jannh@google.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Acked-by: Oscar Salvador osalvador@suse.de Acked-by: David Hildenbrand david@redhat.com Tested-by: Vitaly Chikunov vt@altlinux.org Cc:stable@vger.kernel.org Link: https://lore.kernel.org/all/20250702-x86-2level-hugetlb-v2-1-1a98096edf92%40... --- arch/x86/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 71019b3..4e0fe68 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 X86_64 select ARCH_WANT_LD_ORPHAN_WARN select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP if X86_64 select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP if X86_64