Re: [PATCH] io_uring: Add IORING_OP_DUP

[Daniele Di Proietto <daniele.di.proietto@gmail.com>]

On Wed, Mar 11, 2026 at 11:30 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 3/10/26 18:42, Daniele Di Proietto wrote:
> > On Tue, Mar 10, 2026 at 4:24 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >> On 3/10/26 9:49 AM, Daniele Di Proietto wrote:
> >>> The new operation is like dup3(). The source file can be a regular file
> >>> descriptor or a direct descriptor. The destination is a regular file
> >>> descriptor.
> >>>
> >>> The direct descriptor variant is useful to move a descriptor to an fd
> >>> and close the existing fd with a single acquisition of the `struct
> >>> files_struct` `file_lock`. Combined with IORING_OP_ACCEPT or
> >>> IORING_OP_OPENAT2 with direct descriptors, it can reduce lock contention
> >>> for multithreaded applications.
> >>
> >> Overall comment - how does this interact with direct descriptors? Feels
> >> like this should support both, rather than just normal file descriptors.
> >
> > As implemented, the operation supports:
> > 1. src: direct, dst: normal (this is the use case I mostly care about)
> > 2. src: normal, dst: normal ()
> >
> > I can extend it to also support
> > 3. src: direct, dst: direct
> > 4, src: normal, dst: direct
> >
> > I can use IOSQE_FIXED_FILE to pick the source and I guess I can use a
> > bit in dup_flags (something like IORING_DUP_DIRECT) to decide whether
> > the destination is a direct descriptor or normal.
> >
> > Does that make sense?
>
> Let's not try to reuse IOSQE_FIXED_FILE for that. We may want to
> operate with io_uring's filetable entries and not just files you get
> from there. Two separate IORING_DUP_* flags should be better. And
> you can extract a helper function out of
>
> io_uring/splice.c::io_splice_get_file()

Done, thanks!

>
> It might also be better to make it a part of IORING_OP_FILES_UPDATE
> instead of wasting another opcode, and liburing can provide a
> prep_dup helper for convenience.

I decided to keep IORING_OP_DUP because IORING_OP_FILES_UPDATE seems
designed for multiple file updates, while this only supports one. Let
me know, thanks!
>
> --
> Pavel Begunkov
>