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

peterz
2020-09-14T16:37:17Z

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.