linux-kernel - Re: [PATCH v4 34/39] perf: Skip user unwind if !current->mm

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Message-ID: <20250122142910.GW7145@noisy.programming.kicks-ass.net>
Date: Wed, 22 Jan 2025 15:29:10 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 34/39] perf: Skip user unwind if !current->mm

On Tue, Jan 21, 2025 at 06:31:26PM -0800, Josh Poimboeuf wrote:
> If the task doesn't have any memory, there's no stack to unwind.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  kernel/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 99f0f28feeb5..a886bb83f4d0 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7792,7 +7792,7 @@ struct perf_callchain_entry *
>  perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  {
>  	bool kernel = !event->attr.exclude_callchain_kernel;
> -	bool user   = !event->attr.exclude_callchain_user;
> +	bool user   = !event->attr.exclude_callchain_user && current->mm;

What about things like io_uring helpers, don't they keep ->mm but are
never in userspace?