Re: [patch 1/2] x86_64 page fault NMI-safe

Frederic Weisbecker
2010-07-16T11:21:14Z

On Thu, Jul 15, 2010 at 04:35:18PM +0200, Andi Kleen wrote:
> > But then how did the previous tasks get this new mapping? You said
> > we don't walk through every process page tables for vmalloc.
>
> No because those are always shared for the kernel and have been
> filled in for init_mm.
>
> Also most updates only update the lower tables anyways, top level
> updates are extremly rare. In fact on PAE36 they should only happen
> at most once, if at all, and most likely at early boot anyways
> where you only have a single task.
>
> On x86-64 they will only happen once every 512GB of vmalloc.
> So for most systems also at most once at early boot.
> >
> > I would understand this race if we were to walk on every processes page
> > tables and add the new mapping on them, but we missed one new task that
> > forked or so, because we didn't lock (or just rcu).
>
> The new task will always get a copy of the reference init_mm, which
> was already updated.
>
> -Andi

Ok, got it.

But then, in the example here with perf, I'm allocating 8192 bytes per cpu
and my total memory amount is of 2 GB.

And it always fault at least once on access, after the allocation.
I really doubt it's because we are adding a new top level page table,
considering the amount of memory I have.

It seems to me that the mapping of a newly allocated vmalloc area is
always inserted through the lazy way (update on fault). Or there is
something I'm missing.

Thanks.