Re: [PATCH V8 0/8] io_uring: support sqe group and leased group kbuf

[Jens Axboe <axboe@kernel.dk>]

On 10/29/24 9:08 PM, Ming Lei wrote:
> On Tue, Oct 29, 2024 at 08:43:39PM -0600, Jens Axboe wrote:
>> On 10/29/24 8:03 PM, Ming Lei wrote:
>>> On Tue, Oct 29, 2024 at 03:26:37PM -0600, Jens Axboe wrote:
>>>> On 10/29/24 2:06 PM, Jens Axboe wrote:
>>>>> On 10/29/24 1:18 PM, Jens Axboe wrote:
>>>>>> Now, this implementation requires a user buffer, and as far as I'm told,
>>>>>> you currently have kernel buffers on the ublk side. There's absolutely
>>>>>> no reason why kernel buffers cannot work, we'd most likely just need to
>>>>>> add a IORING_RSRC_KBUFFER type to handle that. My question here is how
>>>>>> hard is this requirement? Reason I ask is that it's much simpler to work
>>>>>> with userspace buffers. Yes the current implementation maps them
>>>>>> everytime, we could certainly change that, however I don't see this
>>>>>> being an issue. It's really no different than O_DIRECT, and you only
>>>>>> need to map them once for a read + whatever number of writes you'd need
>>>>>> to do. If a 'tag' is provided for LOCAL_BUF, it'll post a CQE whenever
>>>>>> that buffer is unmapped. This is a notification for the application that
>>>>>> it's done using the buffer. For a pure kernel buffer, we'd either need
>>>>>> to be able to reference it (so that we KNOW it's not going away) and/or
>>>>>> have a callback associated with the buffer.
>>>>>
>>>>> Just to expand on this - if a kernel buffer is absolutely required, for
>>>>> example if you're inheriting pages from the page cache or other
>>>>> locations you cannot control, we would need to add something ala the
>>>>> below:
>>>>
>>>> Here's a more complete one, but utterly untested. But it does the same
>>>> thing, mapping a struct request, but it maps it to an io_rsrc_node which
>>>> in turn has an io_mapped_ubuf in it. Both BUFFER and KBUFFER use the
>>>> same type, only the destruction is different. Then the callback provided
>>>> needs to do something ala:
>>>>
>>>> struct io_mapped_ubuf *imu = node->buf;
>>>>
>>>> if (imu && refcount_dec_and_test(&imu->refs))
>>>> 	kvfree(imu);
>>>>
>>>> when it's done with the imu. Probably an rsrc helper should just be done
>>>> for that, but those are details.
>>>>
>>>> diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
>>>> index 9621ba533b35..050868a4c9f1 100644
>>>> --- a/io_uring/rsrc.c
>>>> +++ b/io_uring/rsrc.c
>>>> @@ -8,6 +8,8 @@
>>>>  #include <linux/nospec.h>
>>>>  #include <linux/hugetlb.h>
>>>>  #include <linux/compat.h>
>>>> +#include <linux/bvec.h>
>>>> +#include <linux/blk-mq.h>
>>>>  #include <linux/io_uring.h>
>>>>  
>>>>  #include <uapi/linux/io_uring.h>
>>>> @@ -474,6 +476,9 @@ void io_free_rsrc_node(struct io_rsrc_node *node)
>>>>  		if (node->buf)
>>>>  			io_buffer_unmap(node->ctx, node);
>>>>  		break;
>>>> +	case IORING_RSRC_KBUFFER:
>>>> +		node->kbuf_fn(node);
>>>> +		break;
>>>
>>> Here 'node' is freed later, and it may not work because ->imu is bound
>>> with node.
>>
>> Not sure why this matters? imu can be bound to any node (and has a
>> separate ref), but the node will remain for as long as the submission
>> runs. It has to, because the last reference is put when submission of
>> all requests in that series ends.
> 
> Fine, how is the imu found from OP? Not see related code to add the
> allocated node into submission_state or ctx->buf_table.

Just didn't do that, see the POC test patch I did for rw for just
grabbing the fixed one in io_submit_state. Really depends on how many
we'd need - if it's just 1 per submit, then whatever I had would work
and the OP just needs to know to look there.

> io_rsrc_node_lookup() needs to find the buffer any way, right?

That's for table lookup, for the POC there's just the one node hence
nothing really to lookup. It's either rsrc_empty_node, or a valid node.

>>> I think the reference should be in `node` which need to be live if any
>>> consumer OP isn't completed.
>>
>> That is how it works... io_req_assign_rsrc_node() will assign a node to
>> a request, which will be there until the request completes.
>>
>>>> +	node->buf = imu;
>>>> +	node->kbuf_fn = kbuf_fn;
>>>> +	return node;
>>>
>>> Also this function needs to register the buffer to table with one
>>> pre-defined buf index, then the following request can use it by
>>> the way of io_prep_rw_fixed().
>>
>> It should not register it with the table, the whole point is to keep
>> this node only per-submission discoverable. If you're grabbing random
>> request pages, then it very much is a bit finicky and needs to be of
>> limited scope.
> 
> There can be more than 1 buffer uses in single submission, can you share
> how OP finds the specific buffer with ->buf_index from submission state?
> This part is missed in your patch.

If we need more than one, then yeah we'd need an index rather than just
a single pointer. Doesn't really change the mechanics, you'd need to
provide an index like with ->buf_index.

It's not missed in the patch, it's really just a POC patch to show how
it can be done, by no means a done solution! But we can certainly get it
there.

>> Each request type would need to support it. For normal read/write, I'd
>> suggest just adding IORING_OP_READ_LOCAL and WRITE_LOCAL to do that.
>>
>>> If OP dependency can be avoided, I think this approach is fine,
>>> otherwise I still suggest sqe group. Not only performance, but
>>> application becomes too complicated.
>>
>> You could avoid the OP dependency with just a flag, if you really wanted
>> to. But I'm not sure it makes a lot of sense. And it's a hell of a lot
> 
> Yes, IO_LINK won't work for submitting multiple IOs concurrently, extra
> syscall makes application too complicated, and IO latency is increased.

It's really not a big deal to prepare-and-submit the dependencies
separately, but at the same time, I don't think it'd be a bad idea to
support eg 2 local buffers per submit. Or whatever we need there.

This is more from a usability point of view, because the rest of the
machinery is so much more expensive than a single extra syscall that the
latter is not goinbg to affect IO latencies at all.

-- 
Jens Axboe