[PATCH] perf/events: Replace READ_ONCE() with standard pgtable accessors

[PATCH] perf/events: Replace READ_ONCE() with standard pgtable accessors

[Anshuman Khandual]

Replace raw READ_ONCE() dereferences of pgtable entries with corresponding
standard page table accessors pxdp_get() in perf_get_pgtable_size(). These
accessors default to READ_ONCE() on platforms that don't override them. So
there is no functional change on such platforms.

However arm64 platform is being extended to support 128 bit page tables via
a new architecture feature i.e FEAT_D128 in which case READ_ONCE() will not
provide required single copy atomic access for 128 bit page table entries.
Although pxdp_get() accessors can later be overridden on arm64 platform to
extend required single copy atomicity support on 128 bit entries.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-perf-users@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This patch applies both on v7.0-rc1 and mm-unstable.

Part of the D128 series but independent. Hence could be considered on its own.

https://lore.kernel.org/all/20260224051153.3150613-5-anshuman.khandual@arm.com/

Collected Peter's tag from an off list conversation.

 kernel/events/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ac70d68217b6..4ee151cd2c6d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8422,7 +8422,7 @@ static u64 perf_get_pgtable_size(struct mm_struct *mm, unsigned long addr)
 	pte_t *ptep, pte;
 
 	pgdp = pgd_offset(mm, addr);
-	pgd = READ_ONCE(*pgdp);
+	pgd = pgdp_get(pgdp);
 	if (pgd_none(pgd))
 		return 0;
 
@@ -8430,7 +8430,7 @@ static u64 perf_get_pgtable_size(struct mm_struct *mm, unsigned long addr)
 		return pgd_leaf_size(pgd);
 
 	p4dp = p4d_offset_lockless(pgdp, pgd, addr);
-	p4d = READ_ONCE(*p4dp);
+	p4d = p4dp_get(p4dp);
 	if (!p4d_present(p4d))
 		return 0;
 
@@ -8438,7 +8438,7 @@ static u64 perf_get_pgtable_size(struct mm_struct *mm, unsigned long addr)
 		return p4d_leaf_size(p4d);
 
 	pudp = pud_offset_lockless(p4dp, p4d, addr);
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (!pud_present(pud))
 		return 0;
 
-- 
2.30.2

Re: [PATCH] perf/events: Replace READ_ONCE() with standard pgtable accessors

[David Hildenbrand (Arm)]

On 2/27/26 07:27, Anshuman Khandual wrote:
> Replace raw READ_ONCE() dereferences of pgtable entries with corresponding
> standard page table accessors pxdp_get() in perf_get_pgtable_size(). These
> accessors default to READ_ONCE() on platforms that don't override them. So
> there is no functional change on such platforms.
> 
> However arm64 platform is being extended to support 128 bit page tables via
> a new architecture feature i.e FEAT_D128 in which case READ_ONCE() will not
> provide required single copy atomic access for 128 bit page table entries.
> Although pxdp_get() accessors can later be overridden on arm64 platform to
> extend required single copy atomicity support on 128 bit entries.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: linux-perf-users@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David