Re: [PATCH/RFC] Shared page tables

From: Martin Bligh
Date: Fri Jan 27 2006 - 13:22:35 EST


Andi Kleen wrote:
On Tuesday 24 January 2006 08:06, Arjan van de Ven wrote:

The randomization is not for cache coloring, but for security purposes
(except for the old very small stack randomization that was used
to avoid conflicts on HyperThreaded CPUs). I would be surprised if the
mmap made much difference because it's page aligned and at least
on x86 the L2 and larger caches are usually PI.

randomization to a large degree is more important between machines than
within the same machine (except for setuid stuff but lets call that a
special category for now). Imo prelink is one of the better bets to get
"all code for a binary/lib on the same 2 mb page",


Probably yes.


all distros ship prelink nowadays anyway


SUSE doesn't use it.


(it's too much of a win that nobody can afford to not ship it ;)


KDE and some other people disagree on that.


and within prelink the balance between randomization for security and 2Mb sharing can be struck best. In fact it needs know
about the 2Mb thing anyway to place it there properly and for all
binaries... the kernel just can't do that.


Well, we first have to figure out if the shared page tables
are really worth all the ugly code, nasty locking and other problems (inefficient TLB flush etc.) I personally would prefer
to make large pages work better before going down that path.

That needs defragmentation, etc, etc. etc. It also requires changes to userspace apps. Large pages are crippled right now, and it looks like they're going to stay that way. We need some sort of solution, and this is pretty clean and transparent, by comparison.

m.
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