On 8/15/21 11:48 AM, Pavel Begunkov wrote: > On 8/15/21 4:31 AM, Josh Triplett wrote: >> On Sat, Aug 14, 2021 at 01:50:24PM +0100, Pavel Begunkov wrote: >>> On 8/13/21 8:00 PM, Josh Triplett wrote: >>>> 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. >>> >>> io_uring is really thin, and so I absolutely don't want any extra >>> overhead in the generic path, IOW anything affecting >>> reads/writes/sends/recvs. >> >> There are already several checks for valid flags in io_init_req. For >> instance: > > Yes, it's horrible and I don't want to make it any worse. > >> if ((sqe_flags & IOSQE_BUFFER_SELECT) && >> !io_op_defs[req->opcode].buffer_select) >> return -EOPNOTSUPP; >> It'd be trivial to make io_op_defs have a "valid flags" byte, and one >> bitwise op tells you if any invalid flags were passed. *Zero* additional >> overhead for other operations. > > Good point > >> Alternatively, since there are so few operations that open a file >> descriptor, you could just add a separate opcode for those few >> operations. That still seems preferable to overloading a 16-bit index >> field for this. > > I don't think so > >> With this new mechanism, I think we're going to want to support more >> than 65535 fixed-file entries. I can easily imagine wanting to handle >> hundreds of thousands of files or sockets this way. > > May be. What I'm curious about is that the feature doesn't really > change anything in this regard, but seems I haven't heard people > asking for larger tables. > >>> The other reason is that there are only 2 bits left in sqe->flags, >>> and we may use them for something better, considering that it's >>> only open/accept and not much as this. >> >> pipe, dup3, socket, socketpair, pidfds (via either pidfd_open or a >> ring-based spawn mechanism), epoll_create, inotify, fanotify, signalfd, >> timerfd, eventfd, memfd_create, userfaultfd, open_tree, fsopen, fsmount, >> memfd_secret. > > We could argue for many of those whether they should be in io_uring, > and whether there are many benefits having them async and so. It would > have another story if all the ecosystem was io_uring centric, but > that's speculations. > >> Of those, I personally would *love* to have at least pipe, socket, >> pidfd, memfd_create, and fsopen/fsmount/open_tree, plus some manner of >> dup-like operation for moving things between the fixed-file table and >> file descriptors. >> >> I think this is valuable and versatile enough to merit a flag. It would >> also be entirely reasonable to create separate operations for these. But >> either way, I don't think this should just be determined by whether a >> 16-bit index is non-zero. >> >>> I agree that it feels error-prone, but at least it can be wrapped >>> nicely enough in liburing, e.g. >>> >>> void io_uring_prep_openat_direct(struct io_uring_sqe *sqe, int dfd, >>> const char *path, int flags, >>> mode_t mode, int slot_idx); >> >> That wrapper wouldn't be able to handle more than a 16-bit slot index >> though. > > It would. Note, the index is "int" there, so if doesn't fit > into u16, we can fail it. And do conversion if required. > >>>> 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. >>> >>> There is no nofile in SQEs, though >>> >>> req->open.nofile = rlimit(RLIMIT_NOFILE); >> >> nofile isn't needed for opening into the fixed-file table, so it could >> be omitted in that case, and another field unioned with it. > > There is no problem to place it internally. Moreover, it's at the > moment uniformly placed inside io_kiocb, but with nofile we'd need > to find the place on per-op basis. > > Not like any matters, it's just bike shedding. > >> allow passing a 32-bit fixed-file index into open and accept without >> growing the size of their structures. I think, with this new capability, >> we're going to want a large number of fixed files available. >> >> In the SQE, you could overlap it with the splice_fd_in field, which >> isn't needed by any calls other than splice. > > But it doesn't mean it won't be used, as happened with pretty every > other field in SQE. So, it rather depends on what packing is wanted. > And reusing almost never used ->buf_index (and potentially ->ioprio), > sounds reasonable. Aliasing with ->splice_fd_in looks better indeed (apart from it inherently not being checked, but meh?), But I still don't think it's a good option to use sqe->flags, and so still needs some way to switch between modes. Can be sqe->rw_flags as once was done with SPLICE_F_FD_IN_FIXED, but it's IMHO an ugly hackish way. I still lean to the 0 vs >0 encoding . -- Pavel Begunkov
