linux-kernel - Re: [patch] aio: remove aio-max-nr and instead use the memlock rlimit to limit the number of pages pinned for the aio completion ring

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Message-ID: <49B57CB0.5020300@cosmosbay.com>
Date:	Mon, 09 Mar 2009 21:31:44 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, linux-aio <linux-aio@...ck.org>,
	zach.brown@...cle.com, bcrl@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] aio: remove aio-max-nr and instead use the memlock rlimit
 to limit the number of pages pinned for the aio completion ring

Jeff Moyer a écrit :
> Avi Kivity <avi@...hat.com> writes:
> 
>> Jeff Moyer wrote:
>>> Hi,
>>>
>>> Believe it or not, I get numerous questions from customers about the
>>> suggested tuning value of aio-max-nr.  aio-max-nr limits the total
>>> number of io events that can be reserved, system wide, for aio
>>> completions.  Each time io_setup is called, a ring buffer is allocated
>>> that can hold nr_events I/O completions.  That ring buffer is then
>>> mapped into the process' address space, and the pages are pinned in
>>> memory.  So, the reason for this upper limit (I believe) is to keep a
>>> malicious user from pinning all of kernel memory.  Now, this sounds like
>>> a much better job for the memlock rlimit to me, hence the following
>>> patch.
>>>   
>> Is it not possible to get rid of the pinning entirely?  Pinning
>> interferes with page migration which is important for NUMA, among
>> other issues.
> 
> aio_complete is called from interrupt handlers, so can't block faulting
> in a page.  Zach mentions there is a possibility of handing completions
> off to a kernel thread, with all of the performance worries and extra
> bookkeeping that go along with such a scheme (to help frame my concerns,
> I often get lambasted over .5% performance regressions).

This aio_completion from interrupt handlers keep us from using SLAB_DESTROY_BY_RCU
instead of call_rcu() for "struct file" freeing.

http://lkml.org/lkml/2008/12/17/364

I would love it we could get rid of this mess...


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