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
