linux-kernel - Re: [PATCH 7/7] block: fix deadlock through percpu allocation in blk-cgroup

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Message-ID: <CAOS58YOH10ecpV3z1mR=uUnz0+hqdkBOF0q4HkXSSC-LiQjU+w@mail.gmail.com>
Date:	Fri, 23 Dec 2011 14:54:03 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	akpm@...ux-foundation.org, avi@...hat.com, nate@...nel.net,
	cl@...ux-foundation.org, oleg@...hat.com, axboe@...nel.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] block: fix deadlock through percpu allocation in blk-cgroup

Hello,

On Thu, Dec 22, 2011 at 5:00 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> So this pool is global and shared by all devices (both by CFQ and
> throttling logic). I am not sure if there are any advantages of having
> this pool per queue. In one of the mails you mentioned that dm is having
> one pool across many devices which defeats the purpose of having mempool.
>
> **********************************************************************
> It seemed the problem was dm sharing the same pool from multiple devices
> (which defeats the purpose of using mempool BTW)
> *********************************************************************
>
> Not sure why devices sharing a mempool is a bad idea. Having a global pool
> definitely simplifies the implementation a lot in this case.

Ooh, sharing breaks the forward progress guarantee by mempool. blkcg
isn't using mempool for that. It's just using it as allocation buffer
so it isn't relevant.

Thanks.

-- 
tejun
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