linux-kernel - Re: [PATCH 02/26] perf: Introduce mmap3 version of mmap event

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Message-ID: <20200914193907.GO1714160@krava>
Date:   Mon, 14 Sep 2020 21:39:07 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Song Liu <songliubraving@...com>,
        "Frank Ch. Eigler" <fche@...hat.com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH 02/26] perf: Introduce mmap3 version of mmap event

On Mon, Sep 14, 2020 at 12:28:41PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Sep 14, 2020 at 02:38:27PM +0900, Namhyung Kim escreveu:
> > On Mon, Sep 14, 2020 at 6:03 AM Jiri Olsa <jolsa@...nel.org> wrote:
> > > Add new version of mmap event. The MMAP3 record is an
> > > augmented version of MMAP2, it adds build id value to
> > > identify the exact binary object behind memory map:
> 
> > >   struct {
> > >     struct perf_event_header header;
> 
> > >     u32                      pid, tid;
> > >     u64                      addr;
> > >     u64                      len;
> > >     u64                      pgoff;
> > >     u32                      maj;
> > >     u32                      min;
> > >     u64                      ino;
> > >     u64                      ino_generation;
> > >     u32                      prot, flags;
> > >     u32                      reserved;
> 
> What for this reserved? its all nicely aligned already, u64 followed by
> two u32 (prot, flags).
> 
> > >     u8                       buildid[20];
>  
> > Do we need maj, min, ino, ino_generation for mmap3 event?
> > I think they are to compare binaries, then we can do it with
> > build-id (and I think it'd be better)..
> 
> Humm, I thought MMAP2 would be a superset of MMAP and MMAP3 would be a
> superset of MMAP2.
> 
> If we want to ditch useless stuff, then trow away pid, tid too, as we
> can select those via sample_type.
> 
> Having said that, at this point I don't even know if adding new
> PERF_RECORD_ that are an update for a preexisting one is the right way
> to proceed.
> 
> Perhaps we should attach a BPF program to point where a mmap/munmap is
> being done (perf_event_mmap()) and allow userspace to ask for whatever
> it wants?  With a kprobes there right now we can implement this MMAP3
> easily, no?

hmm, I'm always woried about solutions based on kprobes,
because once the function is moved/removed you're screwed
and need to keep up with function name changes and be
backward compatible..

jirka