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

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 <[email protected]>.

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 '<[email protected]>' --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 <[email protected]>
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.)

index 71019b3b54ea..4e0fe688cc83 100644
@@ -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

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.)

---
 arch/x86/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 4eca434fd80b..3b9ba4b227d5 100644
@@ -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