Re: [PATCH v2 0/4] open/accept directly into io_uring fixed file table

[Josh Triplett]

On Sat, Aug 14, 2021 at 05:03:44PM -0600, Jens Axboe wrote:
> What's the plan in terms of limiting the amount of direct descriptors
> (for lack of a better word)? That seems like an important aspect that
> should get sorted out upfront.
[...]
> Maybe we have a way to size the direct table, which will consume entries
> from the same pool that the regular file table does? That would then
> work both ways, and could potentially just be done dynamically similarly
> to how we expand the regular file table when we exceed its current size.

I think we'll want a way to size the direct table regardless, so that
it's pre-allocated and doesn't need to be resized when an index is used.
Then, we could do one of two equally easy things, depending on what
policy we want to set:

- Deduct the full size of the fixed-file table from the allowed number
  of files the process can have open. So, if RLIMIT_NOFILE is 1048576,
  and you pre-allocate 1000000 entries in the fixed-file table, you can
  have no more than 48576 file descriptors open. Stricter, but
  potentially problematic: a program *might* expect that it can
  dup2(some_fd, nofile - 1) successfully.

- Use RLIMIT_NOFILE as the maximum size of the fixed-file table. There's
  precedent for this: we already use RLIMIT_NOFILE as the maximum number
  of file descriptors you can have in flight over UNIX sockets.

I personally would favor the latter; it seems simple and
straightforward.