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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Thu, 23 Apr 2020 11:46:59 +0200
Message-ID: <CAJfpegv-zRp3a3JcmCO4JpXB=f32=TK=+jP1o-peqqxffT5ERw@mail.gmail.com>
Subject: Re: [PATCH v5 2/3] fs: openat2: Extend open_how to allow
 userspace-selected fds
To:     Josh Triplett <josh@joshtriplett.org>
Cc:     Michael Kerrisk <mtk.manpages@gmail.com>, io-uring@vger.kernel.org,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>, Jens Axboe <axboe@kernel.dk>,
        Aleksa Sarai <cyphar@cyphar.com>,
        linux-man <linux-man@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Thu, Apr 23, 2020 at 11:20 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Thu, Apr 23, 2020 at 9:57 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Thu, Apr 23, 2020 at 9:45 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > > > I would prefer to not introduce that limitation in the first place, and
> > > > instead open normal file descriptors.
> > > >
> > > > > The point of O_SPECIFIC_FD is to be able to perform short
> > > > > sequences of open/dosomething/close without having to block and having
> > > > > to issue separate syscalls.
> > > >
> > > > "close" is not a required component. It's entirely possible to use
> > > > io_uring to open a file descriptor, do various things with it, and then
> > > > leave it open for subsequent usage via either other io_uring chains or
> > > > standalone syscalls.
> > >
> > > If this use case arraises, we could add an op to dup/move a private
> > > descriptor to a public one.  io_uring can return values, right?
> > >
> > > Still not convinced...
> >
> > Oh, and we haven't even touched on the biggest advantage of a private
> > fd table: not having to dirty a cacheline on fdget/fdput due to the
> > possibility of concurrent close() in a MT application.
> >
> > I believe this is a sticking point in some big enterprise apps and it
> > may even be a driving force for io_uring.
>
> https://lwn.net/Articles/787473/
>
> And an interesting (very old) article referenced from above, that
> gives yet a new angle on fd allocation issues:
>
> https://lwn.net/Articles/236843/
>
> A private fd space would be perfect for libraries such as glibc.

Ah, io_uring already implements a fixed private fd table via
io_uring_register(IORING_REGISTER_FILES,...), we just need a way to
wire up open, socket, accept, etc. to fill a slot in that table
instead of, or in addition to allocating a slot in the fd_table.

Thanks,
Miklos