linux-kernel - Re: [PATCH] x86, mm: Avoid step_size overflow and add comments

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Date:	Wed, 21 Aug 2013 09:34:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Pekka Enberg <penberg@...nel.org>,
	Jacob Shin <jacob.shin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, mm: Avoid step_size overflow and add comments

* Yinghai Lu <yinghai@...nel.org> wrote:

> Current code

"Current code has two problems:"

> 1. shift step_size directly, and this could have overflow problem
>    after we shift step_size several times.

The most important piece of information is not explained: 
under what circumstances can this happen currently, what 
are the symptoms if this happens, roughly what kind of 
systems are affected, etc?

I mean the kind of information you'd put into a BIOS errata 
description or into any software changelog. You wouldn't 
primarily write about the mechanics of the change, but 
about the _actual impact_ of the change, to end users, 
right?

> 2. use MACRO to have shift to set to 5, but there is not
>    explanation about selection.
> 
> We can fix them with:
> 1. Add get_new_step_size(), and check overflow by comparing new_size
>    with old step_size.
> 2. add comment in it about why we are using 5.
> 
> That will make code more readable while not increasing size of
> init_mem_mapping().

Well, it gets not just more readable at a small size 
increase, but also the above unexplained overflow bug will 
be fixed as well, right?

Thanks,

	Ingo
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