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

Jiri Olsa
2020-09-14T19:38:45Z

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