Re: [PATCH v2 1/3] mm, printk: introduce new format string for flags

Joonsoo Kim
2015-12-10T02:59:36Z

Ccing, Steven to ask trace-cmd problem.

On Fri, Dec 04, 2015 at 04:16:33PM +0100, Vlastimil Babka wrote:
> In mm we use several kinds of flags bitfields that are sometimes printed for
> debugging purposes, or exported to userspace via sysfs. To make them easier to
> interpret independently on kernel version and config, we want to dump also the
> symbolic flag names. So far this has been done with repeated calls to
> pr_cont(), which is unreliable on SMP, and not usable for e.g. sysfs export.
>
> To get a more reliable and universal solution, this patch extends printk()
> format string for pointers to handle the page flags (%pgp), gfp_flags (%pgg)
> and vma flags (%pgv). Existing users of dump_flag_names() are converted and
> simplified.
>
> It would be possible to pass flags by value instead of pointer, but the %p
> format string for pointers already has extensions for various kernel
> structures, so it's a good fit, and the extra indirection in a non-critical
> path is negligible.

I'd like to use %pgp in tracepoint output. It works well when I do
'cat /sys/kernel/debug/tracing/trace' but not works well when I do
'./trace-cmd report'. It prints following error log.

[page_ref:page_ref_unfreeze] bad op token &
[page_ref:page_ref_set] bad op token &
[page_ref:page_ref_mod_unless] bad op token &
[page_ref:page_ref_mod_and_test] bad op token &
[page_ref:page_ref_mod_and_return] bad op token &
[page_ref:page_ref_mod] bad op token &
[page_ref:page_ref_freeze] bad op token &

Following is the format I used.

TP_printk("pfn=0x%lx flags=%pgp count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
__entry->pfn, &__entry->flags, __entry->count,
__entry->mapcount, __entry->mapping, __entry->mt,
__entry->val, __entry->ret)

Could it be solved by 'trace-cmd' itself?
Or it's better to pass flags by value?
Or should I use something like show_gfp_flags()?

Thanks.