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 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.

Thanks,
Miklos