linux-kernel - Re: [PATCH] Add the values related to buddy system for filtering free pages

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Date:	Fri, 7 Dec 2012 10:08:05 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH] Add the values related to buddy system for filtering
 free pages

On Wed, Nov 21, 2012 at 05:02:47PM +0900, Atsushi Kumagai wrote:
> This patch adds the values related to buddy system to vmcoreinfo data
> so that makedumpfile (dump filtering command) can filter out all free
> pages with the new logic.
> It's faster than the current logic because it can distinguish free page
> by analyzing page structure at the same time as filtering for other
> unnecessary pages (e.g. anonymous page).
> OTOH, the current logic has to trace free_list to distinguish free 
> pages while analyzing page structure to filter out other unnecessary
> pages.
> 
> The new logic uses the fact that buddy page is marked by _mapcount == 
> PAGE_BUDDY_MAPCOUNT_VALUE. The values below are required to distinguish
> it.
> 
> Required values:
>   - OFFSET(page._mapcount)
>   - OFFSET(page.private)
>   - SIZE(pageflags)
>   - NUMBER(PG_slab)
>   - NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE)
>   

As per your explanation, you should just need to export page._mapcount
offset and PAGE_BUDDY_MAPCOUNT_VALUE value so that you can figure out
if a page is free or not.

Why do we need rest of the three fields.

- OFFSET(page.private)
- SIZE(pageflags)
- NUMBER(PG_slab)

Thanks
Vivek
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