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

[Jiri Olsa <jolsa@redhat.com>]

On Mon, Sep 14, 2020 at 06:35:34PM +0200, peterz@infradead.org wrote:
> 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.

yay! nice

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

can we? I think you could disable sample_id then
you won't have pid/tid in mmap event

> 
> 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.

I like that idea, I'll check on it

thanks,
jirka