linux-kernel - Re: [RFC] on general object IDs again

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Date:	Wed, 11 Jan 2012 22:22:29 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	Cyrill Gorcunov <gorcunov@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kyle Moffett <kyle@...fetthome.net>, Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrey Vagin <avagin@...nvz.org>
Subject: Re: [RFC] on general object IDs again

On 01/11/2012 10:19 PM, KOSAKI Motohiro wrote:
>> Hi Kosaki,
>>
>> the idea on user-space is something like
>>
>>  - collect all pids to dump
>>  - collect IDs for every pid
>>  - sort the IDs obtained
>>  - find the same IDs (which will be kind of find intersections in a sets of IDs) and
>>   set up CLONE_ flags on restore procedure as appropriate (for example if
>>   GEN_OBJ_ID_VM IDs for two or more tasks are the same we need to use CLONE_VM
>>   at restore time, and so on).
> 
> Then, you only need to compare. not any other calculation. i.e. only
> need id uniqueness.
> And any resource are referenced from tasks. so, can you reuse pid for
> this? example,
> two taska share one mm.
> 
> task-a(pid: 100)
>               |-----------------mm
> task-b(pid: 200)
> 
> 
> gen_obj_id(task-b, GEN_OBJ_ID_VM) return 100. (youngest pid of referenced tasks)

We can, but determining the youngest pid for an mm struct is O(N) algo.
Having N tasks with N mm_structs getting the sharing picture becomes O(N^2).

> .
> 

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