Message-ID: <49B875F7.3030305@cosmosbay.com>
Date: Thu, 12 Mar 2009 03:39:51 +0100
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Jeff Moyer <jmoyer@redhat.com>, Avi Kivity <avi@redhat.com>,
       linux-aio <linux-aio@kvack.org>, zach.brown@oracle.com, bcrl@kvack.org,
       linux-kernel@vger.kernel.org, Davide Libenzi <davidel@xmailserver.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
References: <x49bpsaelwa.fsf@segfault.boston.devel.redhat.com>	<49B54143.1010607@redhat.com> <x49sklmd1fo.fsf@segfault.boston.devel.redhat.com> <49B57CB0.5020300@cosmosbay.com>
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Eric Dumazet a ?crit :
> Jeff Moyer a ?crit :
>> Avi Kivity <avi@redhat.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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