Re: [PATCH 1/2] mm, printk: introduce new format string for flags

[Rasmus Villemoes]

On Thu, Dec 03 2015, yalin wang <yalin.wang2010@xxxxxxxxx> wrote:

>> On Dec 2, 2015, at 13:04, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>> 
>> On 12/02/2015 06:40 PM, yalin wang wrote:
>> 
>> (please trim your reply next time, no need to quote whole patch here)
>> 
>>> i am thinking why not make %pg* to be more generic ?
>>> not restricted to only GFP / vma flags / page flags .
>>> so could we change format like this ?
>>> define a flag spec struct to include flag and trace_print_flags and some other option :
>>> typedef struct { 
>>> unsigned long flag;
>>> structtrace_print_flags *flags;
>>> unsigned long option; } flag_sec;
>>> flag_sec my_flag;
>>> in printk we only pass like this :
>>> printk(â%pg\nâ, &my_flag) ;
>>> then it can print any flags defined by user .
>>> more useful for other drivers to use .
>> 
>> I don't know, it sounds quite complicated

Agreed, I think this would be premature generalization. There's also
some value in having the individual %pgX specifiers, as that allows
individual tweaks such as the mask_out for page flags.

 given that we had no flags printing
>> for years and now there's just three kinds of them. The extra struct flag_sec is
>> IMHO nuissance. No other printk format needs such thing AFAIK? For example, if I
>> were to print page flags from several places, each would have to define the
>> struct flag_sec instance, or some header would have to provide it?
> this can be avoided by provide a macro in header file .
> we can add a new struct to declare trace_print_flags :
> for example:
> #define DECLARE_FLAG_PRINTK_FMT(name, flags_array)   flag_spec name = { .flags = flags_array};
> #define FLAG_PRINTK_FMT(name, flag) ({  name.flag = flag;  &name})
>
> in source code :
> DECLARE_FLAG_PRINTK_FMT(my_flag, vmaflags_names);
> printk(â%pg\nâ, FLAG_PRINTK_FMT(my_flag, vma->flag));
>

Compared to printk("%pgv\n", &vma->flag), I know which I'd prefer to read.

> i am not if DECLARE_FLAG_PRINTK_FMT and FLAG_PRINTK_FMT macro 
> can be defined into one macro ?
> maybe need some trick here .
>
> is it possible ?

Technically, I think the answer is yes, at least in C99 (and I suppose
gcc would accept it in gnu89 mode as well).

printk("%pg\n", &(struct flag_printer){.flags = my_flags, .names = vmaflags_names});

Not tested, and I still don't think it would be particularly readable
even when macroized

printk("%pg\n", PRINTF_VMAFLAGS(my_flags));

Rasmus
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