FAILED: patch "[PATCH] x86/mm: Disable hugetlb page table sharing on 32-bit" failed to apply to 5.15-stable tree

FAILED: patch "[PATCH] x86/mm: Disable hugetlb page table sharing on 32-bit" failed to apply to 5.15-stable tree

[gregkh]

The patch below does not apply to the 5.15-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-5.15.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 '2025071421-molar-theme-43d7@gregkh' --subject-prefix 'PATCH 5.15.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%40google.com

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

[PATCH 5.15.y] x86/mm: Disable hugetlb page table sharing on 32-bit

[Jann Horn]

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%40google.com
(cherry picked from commit 76303ee8d54bff6d9a6d55997acd88a6c2ba63cf)
Signed-off-by: Jann Horn <jannh@google.com>
---
 arch/x86/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4eca434fd80b..3b9ba4b227d5 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -118,7 +118,7 @@ config X86
 	select ARCH_WANT_DEFAULT_BPF_JIT	if X86_64
 	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_WANTS_NO_INSTR
-	select ARCH_WANT_HUGE_PMD_SHARE
+	select ARCH_WANT_HUGE_PMD_SHARE		if X86_64
 	select ARCH_WANT_LD_ORPHAN_WARN
 	select ARCH_WANTS_THP_SWAP		if X86_64
 	select ARCH_HAS_PARANOID_L1D_FLUSH
-- 
2.50.0.727.gbf7dc18ff4-goog