Re: [PATCH v5 2/3] fs: openat2: Extend open_how to allow userspace-selected fds

[Miklos Szeredi <miklos@szeredi.hu>]

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