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From:   Stephane Eranian <eranian@google.com>
Date:   Wed, 30 Sep 2020 15:45:13 -0700
Message-ID: <CABPqkBTxpSY--BdL+-AF_Zug8QAO97O-q5mAPNXzD6Tygg+Wag@mail.gmail.com>
Subject: Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        "Liang, Kan" <kan.liang@linux.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>
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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;
>
>