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mm: skip ptlock_init() for kernel PMDs

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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 Wilcox (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>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Kevin Brodsky 2025-04-08 10:52:17 +01:00 committed by Andrew Morton
parent 10a2e444e4
commit 8240d8d3c5
1 changed files with 1 additions and 1 deletions
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2
include/linux/mm.h
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@ -3257,7 +3257,7 @@ static inline spinlock_t *pmd_lock(struct mm_struct *mm, pmd_t *pmd)
static inline bool pagetable_pmd_ctor(struct mm_struct *mm, static inline bool pagetable_pmd_ctor(struct mm_struct *mm,
struct ptdesc *ptdesc) struct ptdesc *ptdesc)
{ {
if (!pmd_ptlock_init(ptdesc)) if (mm != &init_mm && !pmd_ptlock_init(ptdesc))
return false; return false;
ptdesc_pmd_pts_init(ptdesc); ptdesc_pmd_pts_init(ptdesc);
__pagetable_ctor(ptdesc); __pagetable_ctor(ptdesc);