Re: [PATCH 2/5] io_uring: add support for IORING_OP_URING_CMD

Stefan Metzmacher
2021-02-22T20:04:58Z

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Hi Jens,

>> I've been thinking along the same lines, because having a sparse sqe layout
>> for the uring cmd is a pain. I do think 'personality' is a bit too specific
>> to be part of the shared space, that should probably belong in the pdu
>> instead if the user needs it. One thing they all have in common is that they'd
>> need a sub-command, so why not make that u16 that?
>>
>> There's also the option of simply saying that the uring_cmd sqe is just
>> a different type, ala:
>>
>> struct io_uring_cmd_sqe {
>> __u8 opcode; /* IO_OP_URING_CMD */
>> __u8 flags;
>> __u16 target_op;
>> __s32 fd;
>> __u64 user_data;
>> strut io_uring_cmd_pdu cmd_pdu;
>> };
>>
>> which is essentially the same as your suggestion in terms of layout
>> (because that is the one that makes the most sense), we just don't try
>> and shoe-horn it into the existing sqe. As long as we overlap
>> opcode/flags, then init is fine. And past init, sqe is already consumed.
>>
>> Haven't tried and wire that up yet, and it may just be that the simple
>> layout change you did is just easier to deal with. The important part
>> here is the layout, and I certainly think we should do that. There's
>> effectively 54 bytes of data there, if you include the target op and fd
>> as part of that space. 48 fully usable for whatever.
>
> OK, folded in some of your stuff, and pushed out a new branch. Find it
> here:
>
> https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-fops.v3
>
> I did notice while doing so that you put the issue flags in the cmd,
> I've made them external again. Just seems cleaner to me, otherwise
> you'd have to modify the command for reissue rather than just
> pass in the flags directly.

I think the first two commits need more verbose comments, which clearly
document the uring_cmd() API.

Event before uring_cmd(), it's really not clear to me why we have
'enum io_uring_cmd_flags', as 'enum'.
As it seems to be use it as 'flags' (IO_URING_F_NONBLOCK|IO_URING_F_COMPLETE_DEFER).

With uring_cmd() it's not clear what the backend is supposed to do with these flags.

I'd assume that uring_cmd() would per definition never block and go async itself,
by returning -EIOCBQUEUED. And a single &req->uring_cmd is only ever passed once to
uring_cmd() without any retry.

It's also not clear if IOSQE_ASYNC should have any impact.

I think we also need a way to pass IORING_OP_ASYNC_CANCEL down.

> I also retained struct file * in the cmd - that's a requirement for
> the layout of io_kiocb, so might as well keep it in there and not
> pass in the file. Plus that one won't ever change...

Ah, ok.

> Since we just need that one branch in req init, I do think that your
> suggestion of just modifying io_uring_sqe is the way to go. So that's
> what the above branch does.

Thanks! I think it's much easier to handle the personality logic in
the core only.

For fixed files or fixed buffers I think helper functions like this:

struct file *io_uring_cmd_get_file(struct io_uring_cmd *cmd, int fd, bool fixed);

And similar functions for io_buffer_select or io_import_fixed.

> I tested the block side, and it works for getting the bs of the
> device. That's all the testing that has been done so far :-)

I've added EXPORT_SYMBOL(io_uring_cmd_done); and split your net patch,
similar to the two block patches. So we can better isolate the core
from the first consumers.

See https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-fops.v3

> Comments welcome! Would like to move this one forward and hopefully
> target 5.13 for it.

Great!

metze

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