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 [off-list ref] wrote:

On Tue,  7 Oct 2025 00:34:17 +0000
Runping Lai [off-list ref] wrote:

quoted

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

quoted

Signed-off-by: Runping Lai <redacted>
Reported-by: Wattson CI <redacted>
---
 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);