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Subject: Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
To:     Stephane Eranian <eranian@google.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>, Andi Kleen <ak@linux.intel.com>,
        kirill.shutemov@linux.intel.com,
        Michael Ellerman <mpe@ellerman.id.au>,
        benh@kernel.crashing.org, Paul Mackerras <paulus@samba.org>
References: <20200921152653.3924-1-kan.liang@linux.intel.com>
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 <CABPqkBTxpSY--BdL+-AF_Zug8QAO97O-q5mAPNXzD6Tygg+Wag@mail.gmail.com>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <c22332d2-3d07-cb05-3516-e90ca441962f@linux.intel.com>
Date:   Thu, 1 Oct 2020 09:30:06 -0400
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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