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

Yinghai Lu
2013-03-18T19:13:07Z

On Mon, Mar 18, 2013 at 11:59 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> 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.

yes, we should find out free_mem_size instead to decide next step size.

But that will come out page table size estimation problem again.

Yinghai