netdev - Re: Block device throttling [Re: Distributed storage.]

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Message-ID: <20070813081441.GA30089@2ka.mipt.ru>
Date:	Mon, 13 Aug 2007 12:14:41 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	Jens Axboe <jens.axboe@...cle.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Block device throttling [Re: Distributed storage.]

On Sun, Aug 12, 2007 at 11:44:00PM -0700, Daniel Phillips (phillips@...nq.net) wrote:
> On Sunday 12 August 2007 22:36, I wrote:
> > Note!  There are two more issues I forgot to mention earlier.
> 
> Oops, and there is also:
> 
> 3) The bio throttle, which is supposed to prevent deadlock, can itself 
> deadlock.  Let me see if I can remember how it goes.
> 
>   * generic_make_request puts a bio in flight
>   * the bio gets past the throttle and initiates network IO
>   * net calls sk_alloc->alloc_pages->shrink_caches
>   * shrink_caches submits a bio recursively to our block device
>   * this bio blocks on the throttle
>   * net may never get the memory it needs, and we are wedged

If system is in such condition, it is already broken - throttle limit
must be lowered (next time) not to allow such situation.

-- 
	Evgeniy Polyakov
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