linux-kernel - Re: aio: questions with ioctx_alloc() and large num_possible

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Message-ID: <20161005181741.GL23336@kvack.org>
Date:   Wed, 5 Oct 2016 14:17:41 -0400
From:   Benjamin LaHaise <bcrl@...ck.org>
To:     Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: aio: questions with ioctx_alloc() and large num_possible_cpus()

On Wed, Oct 05, 2016 at 02:58:12PM -0300, Mauricio Faria de Oliveira wrote:
> Hi Benjamin,
> 
> On 10/05/2016 02:41 PM, Benjamin LaHaise wrote:
> >I'd suggest increasing the default limit by changing how it is calculated.
> >The current number came about 13 years ago when machines had orders of
> >magnitude less RAM than they do today.
> 
> Thanks for the suggestion.
> 
> Does the default also have implications other than memory usage?
> For example, concurrency/performance of as much aio contexts running,
> or if userspace could try to exploit some point with a larger number?

Anything's possible when a local user can run code.  It's the same problem 
as determining how much memory can be mlock()ed, or how much i/o a process 
should be allowed to do.  Nothing prevents an app from doing a huge amount 
of readahed() calls to make the system prefetch gigabytes of data.  That 
said, local users tend not to DoS themselves.

> Wondering about it because it can be set based on num_possible_cpus(),
> but that might be really large on high-end systems.

Today's high end systems are tomorrow's desktops...  It probably makes 
sense to implement per-user limits rather than the current global limit, 
and maybe even convert them to an rlimit to better fit in with the 
available frameworks for managing these things.

		-ben

> Regards,
> 
> -- 
> Mauricio Faria de Oliveira
> IBM Linux Technology Center

-- 
"Thought is the essence of where you are now."