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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Date:	Thu, 12 Mar 2009 03:39:51 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Jeff Moyer <jmoyer@...hat.com>, Avi Kivity <avi@...hat.com>,
	linux-aio <linux-aio@...ck.org>, zach.brown@...cle.com,
	bcrl@...ck.org, linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.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

Eric Dumazet a écrit :
> 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 if we could get rid of this mess...

Speaking of that, I tried to take a look at this aio stuff and have one question.

Assuming that __fput() cannot be called from interrupt context.
-> fput() should not be called from interrupt context as well.

How comes we call fput(req->ki_eventfd) from really_put_req()
from interrupt context ?

If user program closes eventfd, then inflight AIO requests can trigger
a bug.


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