linux-kernel - Re: [PATCH -mm] vmscan: make mapped executable pages the first class citizen

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Date:	Fri, 8 May 2009 21:04:18 -0700
From:	Elladan <elladan@...imo.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Elladan <elladan@...imo.com>, Christoph Lameter <cl@...ux.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tytso@....edu" <tytso@....edu>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH -mm] vmscan: make mapped executable pages the first
	class citizen

On Fri, May 08, 2009 at 12:04:27PM -0400, Rik van Riel wrote:
> Elladan wrote:
>
>>> Nobody (except you) is proposing that we completely disable
>>> the eviction of executable pages.  I believe that your idea
>>> could easily lead to a denial of service attack, with a user
>>> creating a very large executable file and mmaping it.
>>>
>>> Giving executable pages some priority over other file cache
>>> pages is nowhere near as dangerous wrt. unexpected side effects
>>> and should work just as well.
>>
>> I don't think this sort of DOS is relevant for a single user or trusted user
>> system.  
>
> Which not all systems are, meaning that the mechanism
> Christoph proposes can never be enabled by default and
> would have to be tweaked by the user.
>
> I prefer code that should work just as well 99% of the
> time, but can be enabled by default for everybody.
> That way people automatically get the benefit.

I read Christopher's proposal as essentially, "have a desktop switch which
won't evict executable pages unless they're using more than some huge
percentage of RAM" (presumably, he wants anonymous pages to get special
treatment too) -- this would essentially be similar to mlocking all your
executables, only with a safety net if you go above x% and without affecting
non-executable file maps.

Given that, the DOS possibility you proposed seemed to just be one where a user
could push a lot of unprotected pages out quickly and make the system run slow.

I don't see how that's any different than just asking malloc() for a lot of ram
and then touching it a lot to make it appear very hot to the VM.  Any user can
trivially do that already, and some apps (eg. a jvm) happily do that for you.
The pathology is the same, and if anything an executable mmap is harder.

-E
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