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

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Date:	Sun, 10 May 2009 19:15:02 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tytso@....edu" <tytso@....edu>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"elladan@...imo.com" <elladan@...imo.com>,
	"npiggin@...e.de" <npiggin@...e.de>,
	"cl@...ux-foundation.org" <cl@...ux-foundation.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>
Subject: Re: [PATCH -mm] vmscan: make mapped executable pages the first class 
	citizen

>> >> >> The patch seems reasonable but the changelog and the (non-existent)
>> >> >> design documentation could do with a touch-up.
>> >> >
>> >> > Is it right that I as a user can do things like mmap my database
>> >> > PROT_EXEC to get better database numbers by making other
>> >> > stuff swap first ?
>> >> >
>> >> > You seem to be giving everyone a "nice my process up" hack.
>> >>
>> >> How about this?
>> >
>> > Why it deserves more tricks? PROT_EXEC pages are rare.
>> > If user space is to abuse PROT_EXEC, let them be for it ;-)
>>
>> yes, typicall rare.
>> tha problem is, user program _can_ use PROT_EXEC for get higher priority
>> ahthough non-executable memory.
>
> - abuses should be rare
> - large scale abuses will be even more rare,
> - the resulted vmscan overheads are the *expected* side effect
> - the side effects are still safe

Who expect?
The fact is, application developer decide to use PROT_EXEC, but side-effect
cause end-user, not application developer.

In general, side-effect attack mistaked guy, it's no problem. They can
do it their own risk.
but We know application developer and administrator are often different person.


> So if that's what they want, let them have it to their heart's content.
>
> You know it's normal for many users/apps to care only about the result.
> When they want something but cannot get it from the smarter version of
> PROT_EXEC heuristics, they will go on to devise more complicated tricks.
>
> In the end both sides loose.
>
> If the abused case is important enough, then let's introduce a feature
> to explicitly prioritize the pages. But let's leave the PROT_EXEC case
> simple.

No.
explicit priotize mechanism don't solve problem anyway. application
developer don't know end-user environment.
they can't mark proper page priority.
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