+ mm-skip-ptlock_init-for-kernel-pmds.patch added to mm-new branch

+ mm-skip-ptlock_init-for-kernel-pmds.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: skip ptlock_init() for kernel PMDs
has been added to the -mm mm-new branch.  Its filename is
     mm-skip-ptlock_init-for-kernel-pmds.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-skip-ptlock_init-for-kernel-pmds.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Kevin Brodsky <kevin.brodsky@arm.com>
Subject: mm: skip ptlock_init() for kernel PMDs
Date: Tue, 8 Apr 2025 10:52:17 +0100

Split page table locks are not used for pgtables associated to init_mm, at
any level.  pte_alloc_kernel() does not call ptlock_init() as a result. 
There is however no separate alloc/free functions for kernel PMDs, and
pmd_ptlock_init() is called unconditionally.  When ALLOC_SPLIT_PTLOCKS is
true (e.g.  32-bit architectures or if CONFIG_PREEMPT_RT is selected),
this results in unnecessary dynamic memory allocation every time a kernel
PMD is allocated.

Now that pagetable_pmd_ctor() is passed the associated mm, we can easily
remove this overhead by skipping pmd_ptlock_init() if the pgtable is
associated to init_mm.  No special-casing is needed on the dtor path, as
ptlock_free() is already called unconditionally for all levels. 
(ptlock_free() is a no-op unless a ptlock was allocated for the given
PTP.)

Link: https://lkml.kernel.org/r/20250408095222.860601-8-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Linus Waleij <linus.walleij@linaro.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: <x86@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/mm.h~mm-skip-ptlock_init-for-kernel-pmds
+++ a/include/linux/mm.h
@@ -3240,7 +3240,7 @@ static inline spinlock_t *pmd_lock(struc
 static inline bool pagetable_pmd_ctor(struct mm_struct *mm,
 				      struct ptdesc *ptdesc)
 {
-	if (!pmd_ptlock_init(ptdesc))
+	if (mm != &init_mm && !pmd_ptlock_init(ptdesc))
 		return false;
 	ptdesc_pmd_pts_init(ptdesc);
 	__pagetable_ctor(ptdesc);
_

Patches currently in -mm which might be from kevin.brodsky@arm.com are

mm-pass-mm-down-to-pagetable_ptepmd_ctor.patch
x86-pgtable-always-use-pte_free_kernel.patch
mm-call-ctor-dtor-for-kernel-ptes.patch
m68k-mm-call-ctor-dtor-for-kernel-ptes.patch
powerpc-mm-call-ctor-dtor-for-kernel-ptes.patch
sparc64-mm-call-ctor-dtor-for-kernel-ptes.patch
mm-skip-ptlock_init-for-kernel-pmds.patch
arm64-mm-use-enum-to-identify-pgtable-level-instead-of-_shift.patch
arm64-mm-always-call-pte-pmd-ctor-in-__create_pgd_mapping.patch
riscv-mm-clarify-ctor-mm-argument-in-alloc_ptepmd_late.patch
arm64-mm-call-pud-p4d-ctor-in-__create_pgd_mapping.patch
riscv-mm-call-pud-p4d-ctor-in-special-kernel-pgtable-alloc.patch