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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 22 Jan 2019 10:34:06 +1300
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Subject: Re: [RFC PATCH v2 02/13] epoll: introduce user structures for polling
 from userspace
To:     Roman Penyaev <rpenyaev@suse.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Davidlohr Bueso <dbueso@suse.de>,
        Jason Baron <jbaron@akamai.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
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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?

              Linus