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

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Date:   Thu, 1 Oct 2020 16:23:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        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>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

On Thu, Oct 01, 2020 at 06:49:36AM -0700, Dave Hansen wrote:
> On 9/30/20 10:30 AM, Peter Zijlstra wrote:
> > 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 was hoping that using ->active_mm would give us the *actual* copy of
> the page tables that's loaded in CR3 for kernel thraeds.  But, there are
> few if any practical advantages of doing that at the moment.

Some of us hate active_mm with a passion and want to remove it entirely
(/me waves at amluto).

Also, if !current->mm, it had better not be accessing anything outside
of the kernel map, so &init_mm should cover that just fine. And given
the kernel maps are shared between all CR3s, they'll see the exact same
pagetables.