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

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Thu, Dec 10 2015, Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, Dec 10, 2015 at 11:59:44AM +0900, Joonsoo Kim wrote:
>> 
>>   [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()?
>
> Yes this can be solved in perf and trace-cmd via the parse-events.c file. And
> as soon as that happens, whatever method we decide upon becomes a userspace
> ABI. So don't think you can change it later.

So somewhat off-topic, but this reminds me of a question I've been
meaning to ask: What makes it safe to stash the pointer values in
vbin_printf and only dereference them later in bstr_printf? For plain
pointer printing (%p) it's of course not a problem, but quite a few of
the %p extensions do dereference the pointer in one way or another (at
least %p[dD], %p[mM], %p[iI], %ph, %pE, %pC, %pNF, %pU, %pa and probably
soon %pg).

Rasmus