The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.4.y git checkout FETCH_HEAD git cherry-pick -x 76303ee8d54bff6d9a6d55997acd88a6c2ba63cf # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025071423-imbecile-slander-d8e9@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 76303ee8d54bff6d9a6d55997acd88a6c2ba63cf Mon Sep 17 00:00:00 2001 From: Jann Horn jannh@google.com Date: Wed, 2 Jul 2025 10:32:04 +0200 Subject: [PATCH] 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...

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 71019b3b54ea..4e0fe688cc83 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