On Wed, Mar 04, 2020 at 01:00:05PM -0700, Jens Axboe wrote: > On 3/4/20 12:56 PM, Josh Triplett wrote: > > On Wed, Mar 04, 2020 at 12:10:08PM -0700, Jens Axboe wrote: > >> On 3/4/20 12:03 PM, Josh Triplett wrote: > >>> On Wed, Mar 04, 2020 at 11:00:10AM -0700, Jens Axboe wrote: > >>>> One of the fabled features with chains has long been the desire to > >>>> support things like: > >>>> > >>>> <open fileX><read from fileX><close fileX> > >>>> > >>>> in a single chain. This currently doesn't work, since the read/close > >>>> depends on what file descriptor we get on open. > >>>> > >>>> The original attempt at solving this provided a means to pass > >>>> descriptors between chains in a link, this version takes a different > >>>> route. Based on Josh's support for O_SPECIFIC_FD, we can instead control > >>>> what fd value we're going to get out of open (or accept). With that in > >>>> place, we don't need to do any magic to make this work. The above chain > >>>> then becomes: > >>>> > >>>> <open fileX with fd Y><read from fd Y><close fd Y> > >>>> > >>>> which is a lot more useful, and allows any sort of weird chains without > >>>> needing to nest "last open" file descriptors. > >>>> > >>>> Updated the test program to use this approach: > >>>> > >>>> https://git.kernel.dk/cgit/liburing/plain/test/orc.c?h=fd-select > >>>> > >>>> which forces the use of fd==89 for the open, and then uses that for the > >>>> read and close. > >>>> > >>>> Outside of this adaptation, fixed a few bugs and cleaned things up. > >>> > >>> I posted one comment about an issue in patch 6. > >>> > >>> Patches 2-5 look great; for those: > >>> Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > >>> > >>> Thanks for picking this up and running with it! > >> > >> Thanks for doing the prep work! I think it turned out that much better > >> for it. > >> > >> Are you going to post your series for general review? I just stole > >> your 1 patch that was needed for me. > > > > Since your patch series depends on mine, please feel free to run with > > the series. Would you mind adding my patch 1 and 3 at the end of your > > series? You need patch 1 to make this more usable for userspace, and > > patch 3 would allow for an OP_PIPE which I'd love to have. > > Let me add patch 1 to the top of the stack, for the pipe part that > probably should be taken in separately. But not a huge deal to me, > as long as we can get it reviewed. That works for me; I don't mind if the pipe support goes in a bit later. And there are many other fd-producing syscalls that need support for userspace-selected FDs, including signalfd4, eventfd2, timerfd_create, epoll_create1, memfd_create, userfaultfd, and the pidfd family. > > Do you plan to submit this during the next merge window? > > Maybe? In terms of timing, I think we're well within the opportunity > to do so, at least. I look forward to seeing it go in. - Josh Triplett
