linux-kernel - Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in deferred user space stacktrace

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Message-ID: <CAHk-=wj7rL47QetC+e70y7pgyH4v7Q2vcSZatRsCk+Z6urA3hw@mail.gmail.com>
Date: Fri, 29 Aug 2025 15:40:07 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>, Steven Rostedt <rostedt@...nel.org>, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, 
	Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, 
	Beau Belgrave <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Florian Weimer <fweimer@...hat.com>, 
	Sam James <sam@...too.org>, Kees Cook <kees@...nel.org>, "Carlos O'Donell" <codonell@...hat.com>
Subject: Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in
 deferred user space stacktrace

On Fri, 29 Aug 2025 at 14:18, Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > Just do the parsing at parse time. End of story.
>
> What does "parsing at parse time" mean?

In user space. When parsing the trace events.

Not in kernel space, when generating the events.

Arnaldo already said it was workable.

> When I get a user space stack trace, I get the virtual addresses of each of
> the user space functions. This is saved into an user stack trace event in
> the ring buffer that usually gets mapped right to a file for post
> processing.
>
> I still do the:
>
>  user_stack_trace() {
>    foreach addr each stack frame
>       vma = vma_lookup(mm, addr);
>       callchain[i++] = (addr - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);
>
> Are you saying that this shouldn't be done either?

I'm saying that's ALL that should be done. And then that *ONE* single thing:

     callchain_filehash[i++] = hash(vma->vm_file);

BUT NOTHING ELSE.

None of that trace_file_map() garbage.

None of that add_into_hash() garbage.

NOTHING like that. You don't look at the hash. You don't "register"
it. You don't touch it in any way. You literally just use it as a
value, and user space will figure it out later. At event parsing time.

At most, you could have some trivial optimization to avoid hashing the
same pointer twice, ie have some single-entry cache of "it's still the
same file pointer, I'll just use the same hash I calculated last
time".

And I mean *single*-level, because siphash is fast enough that doing
anything *more* than that is going to be slower than just
re-calculating the hash.

In fact, you should probably do that optimization at the whole
vma_lookup() level, and try to not look up the same vma multiple times
when a common situation is probably that you'll have multiple stack
frames all with entries pointing to the same executable (or library)
mapping. Because "vma_lookup()" is likely about as expensive as the
hashing is.

           Linus