Re: [RFC V1 04/16] perf/events: Replace READ_ONCE() with standard pgtable accessors

[Mark Rutland]

On Tue, Feb 24, 2026 at 10:41:41AM +0530, Anshuman Khandual wrote:
> Replace READ_ONCE() with standard page table accessors i.e pxdp_get() which
> anyways default into READ_ONCE() in cases where platform do not override.

For each of the patches where you make this sort of change, please
explain in the commit message *why* it is necessary to do this.

IIUC the entire point of this is that in subsequent patches, arm64 will
need to use something other than READ_ONCE() for all pXX levels in order
to support D128 translation tables.

Spelling that out in the commit message makes it much easier for
reviewers to see what's going on, and to focus any discussion/questions,
e.g. *why* won't READ_ONCE() work?

Mark.

> 
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: linux-perf-users@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  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;
>  
> -- 
> 2.43.0
>