Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 30, 2020 at 07:48:48AM -0700, Dave Hansen wrote:
> On 9/30/20 7:42 AM, Liang, Kan wrote:
> >> When I tested on my kernel, it panicked because I suspect
> >> current->active_mm could be NULL. Adding a check for NULL avoided the
> >> problem. But I suspect this is not the correct solution.
> > 
> > I guess the NULL active_mm should be a rare case. If so, I think it's
> > not bad to add a check and return 0 page size.
> 
> I think it would be best to understand why ->active_mm is NULL instead
> of just papering over the problem.  If it is papered over, and this is
> common, you might end up effectively turning off your shiny new feature
> inadvertently.

context_switch() can set prev->active_mm to NULL when it transfers it to
@next. It does this before @current is updated. So an NMI that comes in
between this active_mm swizzling and updating @current will see
!active_mm.

In general though; I think using ->active_mm is a mistake though. That
code should be doing something like:


	mm = current->mm;
	if (!mm)
		mm = &init_mm;