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

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Date:	Thu, 3 Dec 2015 10:38:04 -0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Minchan Kim <minchan@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 1/2] mm, printk: introduce new format string for flags


> On Dec 3, 2015, at 00:03, Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
> 
> On Thu, Dec 03 2015, yalin wang <yalin.wang2010@...il.com> wrote:
> 
>>> On Dec 2, 2015, at 13:04, Vlastimil Babka <vbabka@...e.cz> 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(“%pg\n”, &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
>> 
if we use this generic method, %pgX where X can be used to specify some flag to
mask out some thing .  it will be great .

> 
> 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));
i test on gcc 4.9.3, it can work for this method,
so the final solution like this:
printk.h:
struct flag_fmt_spec {
	unsigned long flag;
	struct trace_print_flags *flags;
	int array_size;
	char delimiter; }

#define FLAG_FORMAT(flag, flag_array, delimiter) (&(struct flag_ft_spec){ .flag = flag, .flags = flag_array, .array_size = ARRAY_SIZE(flag_array), .delimiter = delimiter})
#define VMA_FLAG_FORMAT(flag)  FLAG_FORMAT(flag, vmaflags_names, ‘|')

source code:
printk("%pg\n", VMA_FLAG_FORMAT(my_flags)); 

that’s all, see cpumask_pr_args(masks) macro,
it also use macro and  %*pb  to print cpu mask .
i think this method is not very complex to use .

search source code ,
there is lots of printk to print flag into hex number :
$ grep -n  -r 'printk.*flag.*%x’  .
it will be great if this flag string print is generic.

Thanks









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