linux-kernel - Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE

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Date:   Wed, 30 Sep 2020 07:48:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
        kirill.shutemov@...ux.intel.com,
        Michael Ellerman <mpe@...erman.id.au>,
        benh@...nel.crashing.org, Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

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.