Re: [PATCH] perf, x86: Fall back to vmalloc for BTS buffer allocation

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 10, 2014 at 06:35:56PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 10, 2014 at 04:40:12PM +0200, Andi Kleen wrote:
> > > We did this once, and that blew up big time.
> > > 
> > > Even now, vmalloc_fault() has a very explicit:
> > > 
> > >   WARN_ON_ONCE(in_nmi());
> > > 
> > > So this isn't going to happen until you fix that.
> > 
> > Good point.
> > 
> > We just need to call vmalloc_sync_all at allocation time,
> > no need to change vmalloc_fault.
> 
> Somehow that wasn't considered adequate, but lemme try and dig out that
> thread. It should be somewhere...
> 
> https://lkml.org/lkml/2010/7/14/465

Note that since then we have actually fixed the 'cannot fault from NMI
context' thing, so we should be able to actually take those faults. I
suspect we can simply remove those WARNs from the vmalloc fault path,
but it would need double checking to see if there's no other reasons.

Also:

  https://lkml.org/lkml/2014/4/10/581
  https://patchwork.kernel.org/patch/3048111/

I'm not entirely sure what the final conclusion is, except to note that
it didn't happen.