linux-kernel - Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE

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Date:	Mon, 18 Mar 2013 11:59:14 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Lin Feng <linfeng@...fujitsu.com>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, penberg@...nel.org,
	jacob.shin@....com
Subject: Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro

On 03/18/2013 11:53 AM, Yinghai Lu wrote:
> On Mon, Mar 18, 2013 at 3:21 AM, Lin Feng <linfeng@...fujitsu.com> wrote:
>> For x86 PUD_SHIFT is 30 and PMD_SHIFT is 21, so the consequence of
>> (PUD_SHIFT-PMD_SHIFT)/2 is 4. Update the comments to the code.
>>
>> Cc: Yinghai Lu <yinghai@...nel.org>
>> Signed-off-by: Lin Feng <linfeng@...fujitsu.com>
>> ---
>>  arch/x86/mm/init.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index 59b7fc4..637a95b 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -389,7 +389,7 @@ static unsigned long __init init_range_memory_mapping(
>>         return mapped_ram_size;
>>  }
>>
>> -/* (PUD_SHIFT-PMD_SHIFT)/2 */
>> +/* (PUD_SHIFT-PMD_SHIFT)/2+1 */
>>  #define STEP_SIZE_SHIFT 5
>>  void __init init_mem_mapping(void)
>>  {
> 
> 9/2=4.5, so it becomes 5.
> 

No, it doesn't.  This is C, not elementary school  Now I'm really bothered.

The comment doesn't say *why* (PUD_SHIFT-PMD_SHIFT)/2 or any other
variant is correct, furthermore I suspect that the +1 is misplaced.
However, what is really needed is:

1. Someone needs to explain what the logic should be and why, and
2. replace the macro with a symbolic macro, not with a constant and a
   comment explaining, incorrectly, how that value was derived.

	-hpa

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