Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro

H. Peter Anvin
2013-03-18T18:59:29Z

On 03/18/2013 11:53 AM, Yinghai Lu wrote:
> On Mon, Mar 18, 2013 at 3:21 AM, Lin Feng <linfeng@cn.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@kernel.org>
>> Signed-off-by: Lin Feng <linfeng@cn.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