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

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Message-ID: <CABPqkBTk6H0Wku8gGJeBD2naFU=WOqp42VSjFBhk_1Wy6bRzXQ@mail.gmail.com>
Date:   Wed, 30 Sep 2020 10:17:41 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        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 Wed, Sep 30, 2020 at 7:48 AM Dave Hansen <dave.hansen@...el.com> 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.

I tried that on a backport of the patch to an older kernel. Maybe the
behavior of active_mm has change compared to tip.git.
I will try again with tip.git.