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

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Oct 13, 2025 at 02:55:32PM +1100, 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.
> 
> Adding the tracepoints is simple and straight-forward. For arm64
> use esr as error code and untagged memory address as addr.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Paul Walmsley <pjw@kernel.org>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Albert Ou <aou@eecs.berkeley.edu>
> Cc: Alexandre Ghiti <alex@ghiti.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@nvidia.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);
> +	}

This has come up before and rejected:

https://lore.kernel.org/all/aG0aIKzxApp9j7X0@willie-the-truck/

-- 
Catalin