[lkml]   [2020]   [Sep]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
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

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;

 \ /
  Last update: 2020-09-30 19:31    [W:0.078 / U:4.664 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site