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Subject[PATCH v3 0/4] open/accept directly into io_uring fixed file table
Add an optional feature to open/accept directly into io_uring's fixed
file table bypassing the normal file table. Same behaviour if as the
snippet below, but in one operation:

sqe = prep_[open,accept](...);
cqe = submit_and_wait(sqe);
io_uring_register_files_update(uring_idx, (fd = cqe->res));
close((fd = cqe->res));

The idea in pretty old, and was brough up and implemented a year ago
by Josh Triplett, though haven't sought the light for some reasons.

The behaviour is controlled by setting sqe->file_index, where 0 implies
the old behaviour. If non-zero value is specified, then it will behave
as described and place the file into a fixed file slot
sqe->file_index - 1. A file table should be already created, the slot
should be valid and empty, otherwise the operation will fail.

we can't use IOSQE_FIXED_FILE to switch between modes, because accept
takes a file, and it already uses the flag with a different meaning.

since RFC:
- added attribution
- updated descriptions
- rebased

since v1:
- EBADF if slot is already used (Josh Triplett)
- alias index with splice_fd_in (Josh Triplett)
- fix a bound check bug

Pavel Begunkov (4):
net: add accept helper not installing fd
io_uring: openat directly into fixed fd table
io_uring: hand code io_accept() fd installing
io_uring: accept directly into fixed file table

fs/io_uring.c | 129 +++++++++++++++++++++++++++++-----
include/linux/socket.h | 3 +
include/uapi/linux/io_uring.h | 5 +-
net/socket.c | 71 ++++++++++---------
4 files changed, 157 insertions(+), 51 deletions(-)


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