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Date: Fri, 6 Jul 2012 16:30:20 +0800
From: Jiang Liu <jiang.liu@huawei.com>
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To: Christoph Lameter <cl@linux.com>
CC: Jiang Liu <liuj97@gmail.com>, Pekka Enberg <penberg@kernel.org>,
        Matt Mackall <mpm@selenic.com>, Mel Gorman <mgorman@suse.de>,
        Yinghai Lu <yinghai@kernel.org>, Tony Luck <tony.luck@intel.com>,
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        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
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        Keping Chen <chenkeping@huawei.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 2/4] mm: make consistent use of PG_slab flag
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On 2012-7-6 1:37, Christoph Lameter wrote:
>> Hi Chris,
>> 	I think there's a little difference with SLUB and SLOB for compound page.
>> For SLOB, it relies on the page allocator to allocate compound page to fulfill
>> request bigger than one page. For SLUB, it relies on the page allocator if the
>> request is bigger than two pages. So SLUB may allocate a 2-pages compound page
>> to host SLUB managed objects.
>> 	My proposal may be summarized as below:
>> 	1) PG_slab flag marks a memory object is allocated from slab allocator.
>> 	2) PG_slabobject marks a (compound) page hosts SLUB/SLOB managed objects.
>> 	3) Only set PG_slab/PG_slabobject on the head page of compound pages.
>> 	4) For SLAB, PG_slabobject is redundant and so not used.
>>
>> 	A summary of proposed usage of PG_slab(S) and PG_slabobject(O) with
>> SLAB/SLUB/SLOB allocators as below:
>> pagesize	SLAB			SLUB			SLOB
>> 1page		S			S,O			S,O
>> 2page		S			S,O			S
>>> =4page		S			S			S
> 
> There is no point of recognizing such objects because those will be
> kmalloc objects and they can only be freed in a subsystem specific way.
> There is no standard way to even figure out which subsystem allocated
> them. So for all practical purposes those are unrecoverable.

Hi Chris,
	This patch is not for hotplug, but is to fix some issues in current
kernel, such as:
	1) make show_mem() on ARM and unicore32 report consistent information
no matter which slab allocator is used.
	2) make /proc/kpagecount and /proc/kpageflags return accurate information.
	3) Get rid of risks in mm/memory_failure.c and arch/ia64/kernel/mca_drv.c
	Thanks!