linux-kernel - Re: [RFC][PATCH] Fix area->nr_free-- went (-1) issue in buddy system

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Date:	Tue, 19 Dec 2006 17:34:39 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Aubrey <aubreylee@...il.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] Fix area->nr_free-- went (-1) issue in buddy system

Hi Aubery!

Aubrey wrote:
> Hi Nick,
> 
> Thanks for your reply again, ;-).
> 
> On 12/19/06, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
>>
>> This should not happen because the pages are checked to ensure they are
>> from the same zone before merging.
> 
> 
> How? page_is_buddy() only check if the buddy has the buddy flag and
> has the same order.
> Where can I find the same zone is checked?

Ah OK, you're using 2.6.16? Later kernels have a check for this. I
guess you could backport it?

> 
>>
>> What kind of system do you have? What is the dmesg and the .config?
> 
> 
> I'm using the blackfin uclinux. dmesg and .config is attached.
> 
>> It could be that the zones are not properly aligned and 
>> CONFIG_HOLES_IN_ZONE
>> is not set.
> 
> 
> I changed the code in paging_init(), see below:
> -----------------------------------------
> #if 0
>                zones_size[ZONE_DMA] = (end_mem - PAGE_OFFSET) >> 
> PAGE_SHIFT;
>                zones_size[ZONE_NORMAL] = 0;
> #else
>                zones_size[ZONE_DMA] = (end_mem/2 - PAGE_OFFSET) >> 
> PAGE_SHIFT;
>                zones_size[ZONE_NORMAL] = (end_mem/2 - PAGE_OFFSET) >>
> PAGE_SHIFT;
> #endif
> -----------------------------------------
> This is only what I did the change. I also suspect the zones are not
> properly aligned, But how to align it? I think our system doesn't need
> CONFIG_HOLES_IN_ZONE.

That's right. I guess you can either align your zone sizes (must be
aligned to MAX_ORDER size), or add the zone check in page_is_buddy.

Hope that works.

Nick

-- 
SUSE Labs, Novell Inc.
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