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SubjectRe: [RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace
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@suse.de> 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


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