linux-kernel - Re: [PATCH] memory_hotplug: always initialize pageblock bitmap.

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Date:	Fri, 9 May 2008 08:45:12 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Whitcroft <apw@...dowen.org>,
	Dave Hansen <haveblue@...ibm.com>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] memory_hotplug: always initialize pageblock bitmap.

On Fri, May 09, 2008 at 03:39:10PM +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 9 May 2008 08:06:09 +0200
> Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> 
> > From: Heiko Carstens <heiko.carstens@...ibm.com>
> > 
> > Trying to online a new memory section that was added via memory hotplug
> > sometimes results in crashes when the new pages are added via
> > __free_page. Reason for that is that the pageblock bitmap isn't
> > initialized and hence contains random stuff.
> 
> Hmm, curious. In my understanding, memmap_init_zone() initializes it.
> 
>  __add_pages()
> 	-> __add_section()
> 		-> sparse-add_one_section() // allocate usemap
> 		-> __add_zone()
> 			-> memmap_init_zone() // reset pageblock's bitmap 
> 
> Can't memmap_init_zone() does proper initialization ?

Well, it just _sets_ some bits. But nobody has initialized the bitmap
before to zero. It doesn't reset the pageblock's bitmap as your
comment would indicate.

> ........................
> Ah, ok. I see. grow_zone_span() is not called at __add_zone(), then,
> memmap_init_zone() doesn't initialize usemap because memmap is not in zone's
> range.
> 
> Recently, I added a check "zone's start_pfn < pfn < zone's end"
> to memmap_init_zone()'s usemap initialization for !SPARSEMEM case bug FIX.
> (and I think the fix itself is sane.)
> 
> How about calling grow_pgdat_span()/grow_zone_span() from __add_zone() ?

Dunno.. just fixed a few bugs to get it working.. somehow.. ;)
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