linux-kernel - Re: [PATCH v1] Revert "tracing: Fix tracing_marker may trigger page fault during preempt

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Message-ID: <CABgk4RQwGsn4CdP0K+_7A0j7RVOiHNfoF1ESk17wEuzCea16pA@mail.gmail.com>
Date: Tue, 7 Oct 2025 11:19:34 -0700
From: Runping Lai <runpinglai@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Wattson CI <wattson-external@...gle.com>, kernel-team@...roid.com, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	Luo Gengkun <luogengkun@...weicloud.com>, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v1] Revert "tracing: Fix tracing_marker may trigger page
 fault during preempt_disable"

On Mon, Oct 6, 2025 at 7:08 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue,  7 Oct 2025 00:34:17 +0000
> Runping Lai <runpinglai@...gle.com> wrote:
>
> > This reverts commit 3d62ab32df065e4a7797204a918f6489ddb8a237.
> >
> > It's observed on Pixel 6 that this commit causes a severe functional
> > regression: all user-space writes to trace_marker now fail. The write
> > does not goes through at all. The error is observed in the shell as
> > 'printf: write: Bad address'. This breaks a primary ftrace interface
> > for user-space debugging and profiling. In kernel trace file, it's
> > logged as 'tracing_mark_write: <faulted>'. After reverting this commit,
> > functionality is restored.
>
> This is very interesting. The copy is being done in an atomic context. If
> the fault had to do anything other than update a page table, it is likely
> not to do anything and return a fault.
>
> What preemption model is Pixel 6 running in? CONFIG_PREEMPT_NONE?

Hey Steve,

On Pixel6, CONFIG_PREEMPT is set. And CONFIG_PREEMPT_NONE
is not set. I'll paste the full PREEMPT configs:

~/aosp_kernel > common/scripts/extract-ikconfig
out/slider/dist/vmlinux | less | grep PREEMPT
CONFIG_PREEMPT_BUILD=y
CONFIG_ARCH_HAS_PREEMPT_LAZY=y
# CONFIG_PREEMPT_NONE is not set
# CONFIG_PREEMPT_VOLUNTARY is not set
CONFIG_PREEMPT=y
# CONFIG_PREEMPT_LAZY is not set
# CONFIG_PREEMPT_RT is not set
CONFIG_PREEMPT_COUNT=y
CONFIG_PREEMPTION=y
# CONFIG_PREEMPT_DYNAMIC is not set
CONFIG_PREEMPT_RCU=y
CONFIG_HAVE_PREEMPT_DYNAMIC=y
CONFIG_HAVE_PREEMPT_DYNAMIC_KEY=y
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_DEBUG_PREEMPT is not set
CONFIG_PREEMPTIRQ_TRACEPOINTS=y
# CONFIG_PREEMPT_TRACER is not set
# CONFIG_PREEMPTIRQ_DELAY_TEST is not set

>
> The original code is buggy, but if this is causing a regression, then we
> likely need to do something else, like copy in a pre-allocated buffer?

Sounds like a good plan. Before the long term fix, can we please
revert this commit?

Best,
Runping

>
> -- Steve
>
>
> >
> > Signed-off-by: Runping Lai <runpinglai@...gle.com>
> > Reported-by: Wattson CI <wattson-external@...gle.com>
> > ---
> >  kernel/trace/trace.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 156e7e0bf559..bb9a6284a629 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -7213,7 +7213,7 @@ static ssize_t write_marker_to_buffer(struct trace_array *tr, const char __user
> >       entry = ring_buffer_event_data(event);
> >       entry->ip = ip;
> >
> > -     len = copy_from_user_nofault(&entry->buf, ubuf, cnt);
> > +     len = __copy_from_user_inatomic(&entry->buf, ubuf, cnt);
> >       if (len) {
> >               memcpy(&entry->buf, FAULTED_STR, FAULTED_SIZE);
> >               cnt = FAULTED_SIZE;
> > @@ -7310,7 +7310,7 @@ static ssize_t write_raw_marker_to_buffer(struct trace_array *tr,
> >
> >       entry = ring_buffer_event_data(event);
> >
> > -     len = copy_from_user_nofault(&entry->id, ubuf, cnt);
> > +     len = __copy_from_user_inatomic(&entry->id, ubuf, cnt);
> >       if (len) {
> >               entry->id = -1;
> >               memcpy(&entry->buf, FAULTED_STR, FAULTED_SIZE);
>