Messages in this thread | | | Date | Tue, 22 Jan 2019 12:46:55 +0100 | From | Roman Penyaev <> | Subject | Re: [RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace |
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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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