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

[Josh Triplett]

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