linux-kernel - Re: [PATCH v5] RO/NX protection for loadable kernel modules

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Date:	Mon, 13 Jul 2009 09:59:04 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Siarhei Liakh <sliakh.lkml@...il.com>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	James Morris <jmorris@...ei.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@....de>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-cris-kernel@...s.com
Subject: Re: [PATCH v5] RO/NX protection for loadable kernel modules

 > > (I like the idea of trying kmalloc and falling back, simply because it reduces 
 > > TLB pressure,
 > 
 > I implemented this for 32bit in 2.4, but I always had second thoughts
 > if that was really reducing TLB pressure.

Certainly for non-x86 it can be very worthwhile.  A long time ago I
worked on an embedded product that used PowerPC 440, which has only 64
(software-loaded) TLB entries.  On PPC 440, Linux has a pinned TLB entry
for the kernel mapping, and modifying how the module loader allocated
space to load modules into that mapping vs. one that had dynamic TLB
entries was worth a factor of 2 in performance -- ie the TLB miss
handling for .text was literally taking half the CPU time of the module
code!

 - R.
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