Re: [RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace

[Roman Penyaev]

On 2019-01-21 22:34, Linus Torvalds wrote:
> So I'm not entirely convinced, but I guess actual numbers and users
> might convince me otherwise.
>
> However, a quick comment:
>
> On Tue, Jan 22, 2019 at 9:15 AM Roman Penyaev <rpenyaev@xxxxxxx> wrote:
> > +struct epoll_uitem {
> > +       __poll_t ready_events;
> > +       struct epoll_event event;
> > +};
>
> This really ends up being a horrible data structure.
>
> struct epoll_event is declared as
>
>     struct epoll_event {
>             __poll_t events;
>             __u64 data;
>     } EPOLL_PACKED;
>
> and __poll_t is "unsigned". So on pretty much all 64-bit architectures
> except for x86-64 (which sets that packed attribute), you have a
> packing hole there in between the events and the data, and "struct
> epoll_event" has 8-byte alignment.
>
> Now, in "struct epoll_uitem", you end up having *another* packing hold
> in between "ready_events" and "struct epoll_event".
>
> So this data structure that has 16 bytes of actual data, ends up being
> 24 bytes in size.
>
> Again, x86-64 happens to be the exception to this, but that's a random
> small implementation detail, not a design thing.
>
> I think "struct epoll_event" was badly designed to begin with to have
> this issue, but it shouldn't then be an excuse to make things even
> worse with this array of "struct epoll_uitem" things.
>
> Hmm?

Ha! Yes, you are right.  Eyes see "packed" and brain responds
"ok, this is 12 bytes, + 4 for ready_events = 16, perfect".
I have not paid any attention to how actually this EPOLL_PACKED is
defined.  Not nice at all.  I will unfold the structure like this:

/*
 * Item, shared with userspace.  Unfortunately we can't embed 
epoll_event
 * structure, because it is badly aligned on all 64-bit archs, except
 * x86-64 (see EPOLL_PACKED).  sizeof(epoll_uitem) == 16
 */
struct epoll_uitem {
	__poll_t ready_events;
	__poll_t events;
	__u64 data;
};

Also BUILD_BUG_ON(sizeof(epoll_uitem) != 16) somewhere in alloc won't
hurt.

--
Roman