linux-kernel - Re: [patch 016/104] epoll: introduce resource usage limits

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Date:	Tue, 27 Jan 2009 21:48:07 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Willy Tarreau <w@....eu>
cc:	Greg KH <gregkh@...e.de>, Bron Gondwana <brong@...tmail.fm>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable@...nel.org, Justin Forbes <jmforbes@...uxtx.org>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	Theodore Ts'o <tytso@....edu>,
	Randy Dunlap <rdunlap@...otime.net>,
	Dave Jones <davej@...hat.com>,
	Chuck Wolber <chuckw@...ntumlinux.com>
Subject: Re: [patch 016/104] epoll: introduce resource usage limits

On Wed, 28 Jan 2009, Willy Tarreau wrote:

> On Tue, Jan 27, 2009 at 09:26:30PM -0800, Greg KH wrote:
> > On Tue, Jan 27, 2009 at 08:10:41PM -0800, Davide Libenzi wrote:
> > > In my servers, I know if they are going to be loaded, and I bump NFILES 
> > > (and a few other things) to the correct place. Since many of those 
> > > limits do not actually pre-allocate any resource, I don't need to wait and 
> > > monitor the values, before taking proper action.
> > 
> > But what about people who want to know what the current usages are, so
> > that they _can_ monitor things and adjust them on the fly if things are
> > about to go boom?
> > 
> > I see no reason why we can't leave the value where it is today, and add
> > the ability to both turn the limits off entirely, and also report our
> > current usage.  That keeps the DOS from happening on "default" systems,
> > and lets admins have an idea if they need to bump up the values on their
> > systems as well.
> > 
> > I don't understand your objection to allowing the usage to be monitored.
> 
> Agreed. If sysadmins get trapped by the upgrade, the fix for an
> hypotethical DoS is a 100%-certain DoS by itself. The general sense
> that "if it's not broken, don't fix it" applies here as well. The
> server's sysadmin should not be bothered by a security upgrade (anyway,
> after a few minutes of havoc in prod, he will revert to previous version
> without trying to understand any further). But the campus sysadmin having
> trouble with local users already spends a lot of time tweaking limits.
> Now we offer them a new limit they can tune, they'll happily use it.
> Anyway, even at 128 they'll probably lower it down a lot. So basically
> we're with a medium value which does not fit any usage.

You know, it's not me that decides what goes of certain trees or not ;) 
I've been pinged about the problem, and a patch was sent with values that 
seemed appropriate for typical epoll usages. Epoll is a multiplexing 
interface, so the thought was that not too many instances were lingering 
around. Probably the default max_instances should have been made lomem 
dependent like max_user_watches in the first place, leading to higher 
max_instances values, with respect of the potential DoS.



- Davide


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