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

[Jiri Olsa]

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@xxxxxxxxxx> 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