Re: [PATCH] perf: fix panic by disable ftrace on fault.c

[Dave Hansen]

On 9/12/21 8:30 PM, 王贇 wrote:
> According to the trace we know the story is like this, the NMI
> triggered perf IRQ throttling and call perf_log_throttle(),
> which triggered the swevent overflow, and the overflow process
> do perf_callchain_user() which triggered a user PF, and the PF
> process triggered perf ftrace which finally lead into a suspected
> stack overflow.
> 
> This patch disable ftrace on fault.c, which help to avoid the panic.
...
> +# Disable ftrace to avoid stack overflow.
> +CFLAGS_REMOVE_fault.o = $(CC_FLAGS_FTRACE)

Was this observed on a mainline kernel?

How reproducible is this?

I suspect we're going into do_user_addr_fault(), then falling in here:

>         if (unlikely(faulthandler_disabled() || !mm)) {
>                 bad_area_nosemaphore(regs, error_code, address);
>                 return;
>         }

Then something double faults in perf_swevent_get_recursion_context().
But, you snipped all of the register dump out so I can't quite see
what's going on and what might have caused *that* fault.  But, in my
kernel perf_swevent_get_recursion_context+0x0/0x70 is:

	   mov    $0x27d00,%rdx

which is rather unlikely to fault.

Either way, we don't want to keep ftrace out of fault.c.  This patch is
just a hack, and doesn't really try to fix the underlying problem.  This
situation *should* be handled today.  There's code there to handle it.

Something else really funky is going on.