Subject: [PATCH v2 0/4] open/accept directly into io_uring fixed file table

From: Pavel Begunkov <[email-masked]>
Date: Fri, 13 Aug 2021 17:43:09 +0100

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);
// error handling
io_uring_register_files_update(uring_idx, (fd = cqe->res));
// optionally
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.

Tested on basic cases, will be sent out as liburing patches later.

A copy paste from 2/2 describing user API and some notes:

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.

Note 1: 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.

Note 2: it's u16, where in theory the limit for fixed file tables might
get increased in the future. If would ever happen so, we'll better
workaround later, e.g. by making ioprio to represent upper bits 16 bits.
The layout for open is tight already enough.

since RFC:
 - added attribution
 - updated descriptions
 - rebased

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                 | 114 +++++++++++++++++++++++++++++-----
 include/linux/socket.h        |   3 +
 include/uapi/linux/io_uring.h |   2 +
 net/socket.c                  |  71 +++++++++++----------
 4 files changed, 139 insertions(+), 51 deletions(-)

-- 
2.32.0