linux-kernel - Re: [PATCH] arch/arm64/mm/fault: Implement exceptions tracepoints

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Message-ID: <535066e3-bd75-49df-b043-e5b0a74dc68d@arm.com>
Date: Mon, 13 Oct 2025 10:15:48 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Balbir Singh <balbirs@...dia.com>, linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Paul Walmsley <pjw@...nel.org>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>
Subject: Re: [PATCH] arch/arm64/mm/fault: Implement exceptions tracepoints

On 13/10/25 9:25 AM, Balbir Singh wrote:
> x86 and riscv provide trace points for page-faults (user and kernel
> tracepoints). Some scripts [1],[2] rely on these trace points. The
> tracepoint is useful for tracking faults and their reasons.

Agreed.
> 
> Adding the tracepoints is simple and straight-forward. For arm64
> use esr as error code and untagged memory address as addr.

Providing the entire esr register value makes sense.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Paul Walmsley <pjw@...nel.org>
> Cc: Palmer Dabbelt <palmer@...belt.com>
> Cc: Albert Ou <aou@...s.berkeley.edu>
> Cc: Alexandre Ghiti <alex@...ti.fr>
> 
> [1] https://www.brendangregg.com/FlameGraphs/memoryflamegraphs.html
> [2] https://taras.glek.net/posts/ebpf-mmap-page-fault-tracing/
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
> 
> Tested at my end with a kernel-compile and running a user space
> program to check that the trace points are indeed reported.
> 
>  arch/arm64/mm/fault.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index d816ff44faff..9d7b86e92434 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -44,6 +44,9 @@
>  #include <asm/tlbflush.h>
>  #include <asm/traps.h>
>  
> +#define CREATE_TRACE_POINTS
> +#include <trace/events/exceptions.h>
> +
>  struct fault_info {
>  	int	(*fn)(unsigned long far, unsigned long esr,
>  		      struct pt_regs *regs);
> @@ -572,8 +575,12 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>  	if (faulthandler_disabled() || !mm)
>  		goto no_context;
>  
> -	if (user_mode(regs))
> +	if (user_mode(regs)) {
>  		mm_flags |= FAULT_FLAG_USER;
> +		trace_page_fault_user(addr, regs, esr);
> +	} else {
> +		trace_page_fault_kernel(addr, regs, esr);
> +	}
>  
>  	/*
>  	 * vm_flags tells us what bits we must have in vma->vm_flags

LGTM

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>