Re: [PATCH] kmemcheck: SMP support

From: Jeremy Fitzhardinge
Date: Fri May 23 2008 - 12:14:32 EST

Vegard Nossum wrote:
On Fri, May 23, 2008 at 5:06 PM, Ingo Molnar <mingo@xxxxxxx> wrote:
Vegard, wanna have a look at introducing per CPU kernel pagetables? I
tried that once in the past and it wasnt too horrible. (the patches are
gone though) We could do it before bringing other CPUs online, i.e. much
of the really yucky boot time pagetable juggling phase would be over
already. Hm?


It really doesn't matter how easy it was for you.

You're one of the x86 maintainers.

And I think you're forgetting how hard these things are for a newbie.
I don't even know which one comes first of pmds and puds.

Per-cpu page tables sounds about on the same scale of as, say,
rewriting the VM or some other major subsystem. Epic!

No, I don't think it would really be all that bad, if you just make the kernel parts of the pagetable percpu; userspace might be a bit trickier. But kernel mappings change sufficiently rarely that keeping them all in sync isn't a huge problem.

If your requirement is that you want to be able to set page permissions on kernel mappings on a per-cpu basis, then it might be easiest to do it on-demand. Start off with a single shared pagetable, and when you want to make a per-cpu page protection change, clone the pagetable from the root down to the page you're changing and then do your update.

There's certainly enough hooking places in which you could implement it without much effect on the core kernel code.

I'm glad to hear from you, though.

Pekka suggested that per-cpu page tables might help NUMA systems too.
Does that sound right to you? Would anybody else benefit from having
per-CPU page tables? If not, I have a hard time believing it will ever
get merged.

(Oh. mmio-trace might. But that's also a hacking tool and doesn't really count.)

I'd need to think about it a bit, but its possible that 64-bit Xen might be able to take advantage of it. Yeah, it could be useful.

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