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

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

On Wed, Nov 05, 2025 at 11:27:18AM +1100, Balbir Singh wrote:
> On 11/4/25 04:26, Catalin Marinas wrote:
> > 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/
> 
> Thanks for the pointer, since it's been five to six months since the
> discussion, I don't see the kprobe handler being merged with the trace
> point.

I was hinting that whoever needs these tracepoints can do the work ;).

> The real issue is that while we fix the issue some scripts are broken
> by default on arm64, see [1] and [2] above and a simple search for
> exceptions:page_fault will show up many more. It's just hard to be
> have all of those break and fix them as and when needed.

Does Steve's proposal in the earlier discussion help with fixing these
scripts:

https://lore.kernel.org/all/20250519120837.794f6738@batman.local.home/

-- 
Catalin