[PATCH 14/17] io_uring: add polling support for uring-cmd

[Kanchan Joshi]

On Tue, Mar 15, 2022 at 09:57:45AM +0100, Christoph Hellwig wrote:
>On Mon, Mar 14, 2022 at 03:46:08PM +0530, Kanchan Joshi wrote:
>> But, after you did bio based polling, we need just the bio to poll.
>> iocb is a big structure (48 bytes), and if we try to place it in
>> struct io_uring_cmd, we will just blow up the cacheline in io_uring
>> (first one in io_kiocb).
>> So we just store that bio pointer in io_uring_cmd on submission
>> (please see patch 15).
>> And here on completion we pick that bio, and put that into this local
>> iocb, simply because  ->iopoll needs it.
>> Do you see I am still missing anything here?
>
>Yes.  The VFS layer interface for polling is the kiocb.  Don't break
>it.  The bio is just an implementation detail.

So how about adding ->async_cmd_poll in file_operations (since this
corresponds to ->async_cmd)?
It will take struct io_uring_cmd pointer as parameter.
Both ->iopoll and ->async_cmd_poll will differ in what they accept (kiocb
vs io_uring_cmd). The provider may use bio_poll, or whatever else is the
implementation detail.

for read/write, submission interface took kiocb, and completion
interface (->iopoll) also operated on the same.
for uring/async-cmd, submission interface took io_uring_cmd, but
completion used kiocb based ->iopoll. The new ->async_cmd_poll should
settle this.