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Date:   Wed, 30 Sep 2020 19:30:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Stephane Eranian <eranian@google.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>
Subject: Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
Message-ID: <20200930173042.GD2628@hirez.programming.kicks-ass.net>
References: <20200921152653.3924-1-kan.liang@linux.intel.com>
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 <4e974520-6d0f-68af-7eb8-fa52d95ba77b@linux.intel.com>
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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;