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

[peterz]

On Mon, Sep 14, 2020 at 12:28:41PM -0300, Arnaldo Carvalho de Melo wrote:

> > >   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).

I suspect it is so that sizeof(reserve+buildid) is a multiple of 8. But
yes, that's a wee bit daft, since the next field is a variable length
character array.

> > >     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.

Well, the 'funny' thing is that if you do use buildid, then
{maj,min,ino,ino_generation} are indeed superfluous, but are combined
also large enough to contain buildid.

> If we want to ditch useless stuff, then trow away pid, tid too, as we
> can select those via sample_type.

Correct.

So something like:

struct {
  struct perf_event_header header;

  u64                      addr;
  u64                      len;
  u64                      pgoff;
  union {
    struct {
      u32                  maj;
      u32                  min;
      u64                  ino;
      u64                  ino_generation;
    };
    u8                     buildid[20];
  };
  u32                      prot, flags;
  char			   filename[];
  struct sample_id         sample_id;
};

Using one of the MISC bits to resolve the union. Might actually bring
benefit to everyone. Us normal people get to have a smaller MMAP record,
while the buildid folks can have it too.

Even more extreme would be using 2 MISC bits and allowing the union to
be 0 sized for anon.

That said; I have the nagging feeling there were unresolved issues with
mmap2, but I can't seem to find any relevant emails on it :/ My
google-fu is weak today.