Re: [PATCH 1/1] Implement shared page tables

["Martin J. Bligh" <mbligh@mbligh.org>]

--Hugh Dickins <hugh@veritas.com> wrote (on Wednesday, August 31, 2005 14:42:38 +0100):

> On Wed, 31 Aug 2005, Arjan van de Ven wrote:
>> On Wed, 2005-08-31 at 12:44 +0100, Hugh Dickins wrote:
>> > I was going to say, doesn't randomize_va_space take away the rest of
>> > the point?  But no, it appears "randomize_va_space", as it currently
>> > appears in mainline anyway, is somewhat an exaggeration: it just shifts
>> > the stack a little, with no effect on the rest of the va space.
>> 
>> it also randomizes mmaps
> 
> Ah, via PF_RANDOMIZE, yes, thanks: so long as certain conditions are
> fulfilled - and my RLIM_INFINITY RLIMIT_STACK has been preventing it.
> 
> And mmaps include shmats: so unless the process specifies non-NULL
> shmaddr to attach at, it'll choose a randomized address for that too
> (subject to those various conditions).
> 
> Which is indeed a further disincentive against shared page tables.

Or shared pagetables a disincentive to randomizing the mmap space ;-)
They're incompatible, but you could be left to choose one or the other
via config option.

3% on "a certain industry-standard database benchmark" (cough) is huge,
and we expect the benefit for PPC64 will be larger as we can share the
underlying hardware PTEs without TLB flushing as well.

M.

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