On Wed, Apr 08, 2020 at 10:26:01PM +1000, Aleksa Sarai wrote: > On 2020-04-07, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: > > (Note: numbering this updated version v3, to avoid confusion with Jens' > > v2 that built on my v1. Jens, if you like this approach, please feel > > free to stack your additional patches from the io_uring-fd-select branch > > atop this series. 5.8 material, not intended for the current merge window.) > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > the file descriptor opened by a call like openat2, so that it can use > > the resulting file descriptor in subsequent system calls without waiting > > for the response to the initial openat2 syscall. > > > > The first patch is independent of the other two; it allows reserving > > file descriptors below a certain minimum for userspace-selected fd > > allocation only. > > > > The second patch implements userspace-selected fd allocation for > > openat2, introducing a new O_SPECIFIC_FD flag and an fd field in struct > > open_how. In io_uring, this allows sequences like openat2/read/close > > without waiting for the openat2 to complete. Multiple such sequences can > > overlap, as long as each uses a distinct file descriptor. > > > > The third patch adds userspace-selected fd allocation to pipe2 as well. > > I did this partly as a demonstration of how simple it is to wire up > > O_SPECIFIC_FD support for any fd-allocating system call, and partly in > > the hopes that this may make it more useful to wire up io_uring support > > for pipe2 in the future. > > > > If this gets accepted, I'm happy to also write corresponding manpage > > patches. > > > > v3: > > This new version has an API to atomically increase the minimum fd and > > return the previous minimum, rather than just getting and setting the > > minimum; this makes it easier to allocate a range. (A library that might > > initialize after the program has already opened other file descriptors > > may need to check for existing open fds in the range after reserving it, > > and reserve more fds if needed; this can be done entirely in userspace, > > and we can't really do anything simpler in the kernel due to limitations > > on file-descriptor semantics, so this patch series avoids introducing > > any extra complexity in the kernel.) > > > > This new version also supports a __get_specific_unused_fd_flags call > > which accepts the limit for RLIMIT_NOFILE as an argument, analogous to > > __get_unused_fd_flags, since io_uring needs that to correctly handle > > RLIMIT_NOFILE. > > > > Josh Triplett (3): > > fs: Support setting a minimum fd for "lowest available fd" allocation > > fs: openat2: Extend open_how to allow userspace-selected fds > > fs: pipe2: Support O_SPECIFIC_FD > > Aside from my specific comments and questions, the changes to openat2 > deserve at least one or two selftests. Agreed. I don't expect this to get merged until it has tests and manpage patches. - Josh Triplett
