Re: [WIP PATCH] io_uring: Support opening a file into the fixed-file table

[josh@joshtriplett.org]

Clay Harris wrote:
> On Tue, Jul 14 2020 at 14:08:26 -0700, Josh Triplett quoth thus:
> > The other next step would be to add an IORING_OP_CLOSE_FIXED_FILE
> > (separate from the existing CLOSE op) that removes an entry currently in
> > the fixed file table and calls fput on it. (With some care, that
> > *should* be possible even for an entry that was originally registered
> > from a file descriptor.)
>
> I'm curious why you wouldn't use IOSQE_FIXED_FILE here.  I understand
> your reasoning for OPEN_FIXED_FILE, but using the flag with the existing
> CLOSE OP seems like a perfect fit.  To me, this looks like a suboptimal
> precedent that would result in defining a new opcode for every request
> which could accept either a fixed file or process descriptor.

We absolutely could use IORING_OP_CLOSE with IOSQE_FIXED_FILE as the
userspace interface. It would take a bit of refactoring to make that
work, since unlike other cases of IOSQE_FIXED_FILE we don't just want a
struct file in both cases and instead we need the file descriptor (to
close it) in the non-fixed-file case, but I agree that that would make
sense as the userspace interface.