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

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Message-ID: <514767A5.4020601@zytor.com>
Date:	Mon, 18 Mar 2013 12:14:45 -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 12:13 PM, Yinghai Lu wrote:
>>
>> 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.
> 

Sorry, that comment is double nonsense for someone who isn't intimately
familiar with the code, and it sounds like it is just plain wrong.

Instead, try to explain why 5 is the correct value in the current code
and how it is (or should be!) derived.

	-hpa



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