Re: [PATCH V4] mm: fix kernel crash in khugepaged thread

[yalin wang <yalin.wang2010@gmail.com>]

> On Nov 17, 2015, at 10:43, Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> On Tue, 17 Nov 2015 10:21:47 +0800
> yalin wang <yalin.wang2010@gmail.com> wrote:
> 
> 
>> i have not tried ,
>> just a question,
>> if you print a %s , but don’t call trace_define_field() do define this string in
>> __entry ,  how does user space perf tool to get this string info and print it ?
>> i am curious ..
>> i can try this when i have time.  and report to you .
> 
> Because the print_fmt has nothing to do with the fields. You can have
> as your print_fmt as:
> 
> 	TP_printk("Message = %s", "hello dolly!")
> 
> And both userspace and the kernel with process that correctly (if I got
> string processing working in userspace, which I believe I do). The
> string is processed, it's not dependent on TP_STRUCT__entry() unless it
> references a field there. Which can also be used too:
> 
> 	TP_printk("Message = %s", __entry->musical ? "Hello dolly!" :
> 			"Death Trap!")
> 
> userspace will see in the entry:
> 
> print_fmt: "Message = %s", REC->musical ? "Hello dolly!" : "Death Trap!"
> 
> as long as the field "musical" exists, all is well.
> 
> -- Steve
Aha,  i see.
Thanks very much for your explanation.
Better print fat is :   
TP_printk("mm=%p, scan_pfn=%s, writable=%d, referenced=%d, none_or_zero=%d, status=%s, unmapped=%d",
               __entry->mm,
		__entry->pfn == (-1UL) ? "(null)" :  itoa(buff,  __entry->pin, 10), …..)

is this possible ?

Thanks