linux-kernel - Re: Re: [RFC] [PATCH 8/9] memcg: remove page

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Date:	Thu, 11 Sep 2008 23:38:09 +0900 (JST)
From:	kamezawa.hiroyu@...fujitsu.com
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	balbir@...ux.vnet.ibm.com, xemul@...nvz.org, hugh@...itas.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, menage@...gle.com
Subject: Re: Re: [RFC] [PATCH 8/9] memcg: remove page_cgroup pointer from memmap

>On Thursday 11 September 2008 21:22, KAMEZAWA Hiroyuki wrote:
>> Remove page_cgroup pointer from struct page.
>>
>> This patch removes page_cgroup pointer from struct page and make it be able
>> to get from pfn. Then, relationship of them is
>>
>> Before this:
>>   pfn <-> struct page <-> struct page_cgroup.
>> After this:
>>   struct page <-> pfn -> struct page_cgroup -> struct page.
>
>So...
>
>pfn -> *hash* -> struct page_cgroup, right?
>
right.

>While I don't think there is anything wrong with the approach, I
>don't understand exactly where you guys are hoping to end up with
>this?
>
No. but this is simple. I'd like to use linear mapping like
sparsemem-vmemmap and HUGTLB kernel pages at the end.
But it needs much more work and should be done in the future.
(And it seems to have to depend on SPARSEMEM.)
This is just a generic one.

>I thought everyone was happy with preallocated page_cgroups because
>of their good performance and simplicity, but this seems to be
>going the other way again.

I don't think so. Purpose of this is adding an interface to get 
page_cgroup from pfn.

>
>I'd worry that the hash and lookaside buffers and everything makes
>performance more fragile, adds code and data and icache to fastpaths.
>
I worry, too.

Thanks,
-Kame
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