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

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Date:	Sun, 10 May 2009 19:06:34 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"hannes@...xchg.org" <hannes@...xchg.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

>> I don't think this is desirable, like Andrew already said, there's tons
>> of ways to defeat any of this and we've so far always priorized mappings
>> over !mappings. Limiting this to only PROT_EXEC mappings is already less
>> than it used to be.
>
> Yeah. One thing I realized in readahead is that *anything* can happen.
> When it comes to caching, app/user behaviors are *far more* unpredictable.
> We can make the heuristics as large as 1000LOC (and leave users and
> ourselves lost in the mist) or as simple as 100LOC (and make it happy
> to hacking or even abuse).

umm. I think it isn't good example.
Please see recent_scan/rotate stastics. it use only less 100LOC.

Plus, I don't think stastics is wrong.
if the page can claim "I'm high priority", it's risky. bad userland
program might exploit this rule.
but if the page claim "I think PROT_EXEC is important, maybe", it
isn't risky. if user-program want to exploit the rule, kernel ignore
the claim.
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