linux-kernel - Re: [PATCH] x86, mm: Add comments for step

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Date:	Tue, 20 Aug 2013 11:13:44 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Pekka Enberg <penberg@...nel.org>,
	Jacob Shin <jacob.shin@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, mm: Add comments for step_size shift

On Tue, Aug 20, 2013 at 1:18 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Yinghai Lu <yinghai@...nel.org> wrote:
>
>> As request by hpa, add comments for why we choose 5 for
>> step size shift.
>>
>> Signed-off-by: Yinghai Lu <yinghai@...nel.org>
>> Reviewed-by: Tang Chen <tangchen@...fujitsu.com>
>> Tested-by: Tang Chen <tangchen@...fujitsu.com>
>>
>> ---
>>  arch/x86/mm/init.c |   21 ++++++++++++++++++---
>>  1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> Index: linux-2.6/arch/x86/mm/init.c
>> ===================================================================
>> --- linux-2.6.orig/arch/x86/mm/init.c
>> +++ linux-2.6/arch/x86/mm/init.c
>> @@ -395,8 +395,23 @@ static unsigned long __init init_range_m
>>       return mapped_ram_size;
>>  }
>>
>> -/* (PUD_SHIFT-PMD_SHIFT)/2 */
>> -#define STEP_SIZE_SHIFT 5
>> +static unsigned long __init get_new_step_size(unsigned long step_size)
>> +{
>> +     /*
>> +      * initial mapped size is PMD_SIZE, aka 2M.
>> +      * We can not set step_size to be PUD_SIZE aka 1G yet.
>> +      * In worse case, when 1G is cross the 1G boundary, and
>> +      * PG_LEVEL_2M is not set, we will need 1+1+512 pages (aka 2M + 8k)
>> +      * to map 1G range with PTE. Use 5 as shift for now.
>> +      */
>
> This is much more readable:
>
> +       /*
> +        * initial mapped size is PMD_SIZE (2M).
> +        * We can not set step_size to be PUD_SIZE (1G) yet.
> +        * In the worst case, when we cross the 1G boundary, and
> +        * PG_LEVEL_2M is not set, we will need 1+1+512 pages (2M+8k)
> +        * to map 1G range with PTE. Use 5 as shift for now.
> +        */

ok.

>
>
>> +     unsigned long new_step_size = step_size << 5;
>> +
>> +     if (new_step_size > step_size)
>> +             step_size = new_step_size;
>> +
>> +     return  step_size;
>> +}
>> +
>>  void __init init_mem_mapping(void)
>>  {
>>       unsigned long end, real_end, start, last_start;
>> @@ -445,7 +460,7 @@ void __init init_mem_mapping(void)
>>               min_pfn_mapped = last_start >> PAGE_SHIFT;
>>               /* only increase step_size after big range get mapped */
>>               if (new_mapped_ram_size > mapped_ram_size)
>> -                     step_size <<= STEP_SIZE_SHIFT;
>> +                     step_size = get_new_step_size(step_size);
>>               mapped_ram_size += new_mapped_ram_size;
>>       }
>
> As-is the changelog claims it only adds comments - but it
> obviously does more than that ...
>
> Yinghai, for the 1001st time, please use the customary
> changelog style we use in the kernel:
>
>   " Current code does (A), this has a problem when (B).
>     We can improve this doing (C), because (D)."
>
> I'm also going to suggest something radical: how about you
> keep this sugestion of mine in mind for _all_ future
> patches so I don't have to repeat it for every 3rd patch
> like I had to for the past 4 years, non-stop? Okay?

ok.

Sent updated one with new changelog and comments.

Thanks

Yinghai
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