linux-kernel - Re: [RFC v1 2/2] mm: SLUB Freelist randomization

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Date:	Fri, 20 May 2016 11:15:58 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Thomas Garnier <thgarnie@...gle.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Pranith Kumar <bobby.prani@...il.com>,
	David Howells <dhowells@...hat.com>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Petr Mladek <pmladek@...e.com>,
	Kees Cook <keescook@...omium.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Greg Thelen <gthelen@...gle.com>,
	kernel-hardening@...ts.openwall.com
Subject: Re: [RFC v1 2/2] mm: SLUB Freelist randomization

2016-05-20 5:20 GMT+09:00 Thomas Garnier <thgarnie@...gle.com>:
> I ran the test given by Joonsoo and it gave me these minimum cycles
> per size across 20 usage:

I can't understand what you did here. Maybe, it's due to my poor Engling.
Please explain more. You did single thread test? Why minimum cycles
rather than average?

> size,before,after
> 8,63.00,64.50 (102.38%)
> 16,64.50,65.00 (100.78%)
> 32,65.00,65.00 (100.00%)
> 64,66.00,65.00 (98.48%)
> 128,66.00,65.00 (98.48%)
> 256,64.00,64.00 (100.00%)
> 512,65.00,66.00 (101.54%)
> 1024,68.00,64.00 (94.12%)
> 2048,66.00,65.00 (98.48%)
> 4096,66.00,66.00 (100.00%)

It looks like performance of all size classes are the same?

> I assume the difference is bigger if you don't have RDRAND support.

What does RDRAND means? Kconfig? How can I check if I have RDRAND?

> Christoph, Joonsoo: Do you think it would be valuable to add a CONFIG
> to disable additional randomization per new page? It will remove
> additional entropy but increase performance for machines without arch
> specific randomization instructions.

I don't think that it deserve another CONFIG. If performance is a matter,
I think that removing additional entropy is better until it is proved that
entropy is a problem.

Thanks.