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

On Fri, Aug 13, 2021 at 05:43:09PM +0100, Pavel Begunkov wrote:

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.

Thank you for working to get this over the finish line!

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.

Rather than using sqe->file_index - 1, which feels like an error-prone
interface, I think it makes sense to use a dedicated flag for this, like
IOSQE_OPEN_FIXED. That flag could work for any open-like operation,
including open, accept, and in the future many other operations such as
memfd_create. (Imagine using a single ring submission to open a memfd,
write a buffer into it, seal it, send it over a UNIX socket, and then
close it.)

The only downside is that you'll need to reject that flag in all
non-open operations. One way to unify that code might be to add a flag
in io_op_def for open-like operations, and then check in common code for
the case of non-open-like operations passing IOSQE_OPEN_FIXED.

Also, rather than using a 16-bit index for the fixed file table and
potentially requiring expansion into a different field in the future,
what about overlapping it with the nofile field in the open and accept
requests? If they're not opening a normal file descriptor, they don't
need nofile. And in the original sqe, you can then overlap it with a
32-bit field like splice_fd_in.

EEXIST seems like the wrong error-code to use if the index is already in
use; open can already return EEXIST if you pass O_EXCL. How about EBADF,
or better yet EBADSLT which is unlikely to be returned for any other
reason?

- Josh Triplett