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Date:   Wed, 4 Mar 2020 12:09:34 -0800
From:   Josh Triplett <josh@joshtriplett.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     io-uring@vger.kernel.org, jlayton@kernel.org
Subject: Re: [PATCHSET v2 0/6] Support selectable file descriptors
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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@joshtriplett.org>
> >>>
> >>> 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