Re: [PATCH V8 5/7] io_uring: support leased group buffer with REQ_F_GROUP_KBUF

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/30/24 02:04, Ming Lei wrote:
> On Wed, Oct 30, 2024 at 01:25:33AM +0000, Pavel Begunkov wrote:
>> On 10/30/24 00:45, Ming Lei wrote:
>>> On Tue, Oct 29, 2024 at 04:47:59PM +0000, Pavel Begunkov wrote:
>>>> On 10/25/24 13:22, Ming Lei wrote:
>>>> ...
>>>>> diff --git a/io_uring/rw.c b/io_uring/rw.c
>>>>> index 4bc0d762627d..5a2025d48804 100644
>>>>> --- a/io_uring/rw.c
>>>>> +++ b/io_uring/rw.c
>>>>> @@ -245,7 +245,8 @@ static int io_prep_rw_setup(struct io_kiocb *req, int ddir, bool do_import)
>>>>>     	if (io_rw_alloc_async(req))
>>>>>     		return -ENOMEM;
>>>>> -	if (!do_import || io_do_buffer_select(req))
>>>>> +	if (!do_import || io_do_buffer_select(req) ||
>>>>> +	    io_use_leased_grp_kbuf(req))
>>>>>     		return 0;
>>>>>     	rw = req->async_data;
>>>>> @@ -489,6 +490,11 @@ static bool __io_complete_rw_common(struct io_kiocb *req, long res)
>>>>>     		}
>>>>>     		req_set_fail(req);
>>>>>     		req->cqe.res = res;
>>>>> +		if (io_use_leased_grp_kbuf(req)) {
>>>>
>>>> That's what I'm talking about, we're pushing more and
>>>> into the generic paths (or patching every single hot opcode
>>>> there is). You said it's fine for ublk the way it was, i.e.
>>>> without tracking, so let's then pretend it's a ublk specific
>>>> feature, kill that addition and settle at that if that's the
>>>> way to go.
>>>
>>> As I mentioned before, it isn't ublk specific, zeroing is required
>>> because the buffer is kernel buffer, that is all. Any other approach
>>> needs this kind of handling too. The coming fuse zc need it.
>>>
>>> And it can't be done in driver side, because driver has no idea how
>>> to consume the kernel buffer.
>>>
>>> Also it is only required in case of short read/recv, and it isn't
>>> hot path, not mention it is just one check on request flag.
>>
>> I agree, it's not hot, it's a failure path, and the recv side
>> is of medium hotness, but the main concern is that the feature
>> is too actively leaking into other requests.
>   
> The point is that if you'd like to support kernel buffer. If yes, this
> kind of change can't be avoided.

There is no guarantee with the patchset that there will be any IO done
with that buffer, e.g. place a nop into the group, and even then you
have offsets and length, so it's not clear what the zeroying is supposed
to achieve. Either the buffer comes fully "initialised", i.e. free of
kernel private data, or we need to track what parts of the buffer were
used.

-- 
Pavel Begunkov