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

["Liang, Kan" <kan.liang@linux.intel.com>]

On 9/30/2020 6:45 PM, Stephane Eranian wrote:
> On Wed, Sep 30, 2020 at 10:30 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> 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.
>>
> I think Peter is right. This code is called in the context of NMI, so
> if active_mm is set to NULL inside
> a locked section, this is not enough to protect the perf_events code
> from seeing it.
> 
>> 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;
>>
>>

I will send a V9 with the changes Peter suggests.

Thanks,
Kan