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

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Date:	Mon, 18 Mar 2013 12:13:06 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Mon, Mar 18, 2013 at 11:59 AM, H. Peter Anvin <hpa@...or.com> wrote:
> 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.

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
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