Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables

[Joerg Roedel <joro@8bytes.org>]

On Fri, Jul 20, 2018 at 10:06:54AM -0700, Andy Lutomirski wrote:
> > On Jul 20, 2018, at 6:22 AM, Joerg Roedel <joro@8bytes.org> wrote:
> >
> > From: Joerg Roedel <jroedel@suse.de>
> >
> > The ring-buffer is accessed in the NMI handler, so we better
> > avoid faulting on it. Sync the vmalloc range with all
> > page-tables in system to make sure everyone has it mapped.
> >
> > This fixes a WARN_ON_ONCE() that can be triggered with PTI
> > enabled on x86-32:
> >
> >    WARNING: CPU: 4 PID: 0 at arch/x86/mm/fault.c:320 vmalloc_fault+0x220/0x230
> >
> > This triggers because with PTI enabled on an PAE kernel the
> > PMDs are no longer shared between the page-tables, so the
> > vmalloc changes do not propagate automatically.
> 
> It seems like it would be much more robust to fix the vmalloc_fault()
> code instead.

The question is whether the NMI path is nesting-safe, then we can remove
the WARN_ON_ONCE(in_nmi()) in the vmalloc_fault path. It should be
nesting-safe on x86-32 because of the way the stack-switch happens
there. If its also nesting-safe on x86-64 the warning there can be
removed.

Or did you think of something else to fix there?


Thanks,

	Joerg