linux-kernel - Re: [PATCH 12/12] io_uring: support true async buffered reads, if file provides it

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Date:   Tue, 26 May 2020 07:47:21 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 12/12] io_uring: support true async buffered reads, if
 file provides it

On 5/26/20 1:38 AM, Pavel Begunkov wrote:
> On 23/05/2020 21:57, Jens Axboe wrote:
>> If the file is flagged with FMODE_BUF_RASYNC, then we don't have to punt
>> the buffered read to an io-wq worker. Instead we can rely on page
>> unlocking callbacks to support retry based async IO. This is a lot more
>> efficient than doing async thread offload.
>>
>> The retry is done similarly to how we handle poll based retry. From
>> the unlock callback, we simply queue the retry to a task_work based
>> handler.
>>
>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>> ---
>>  fs/io_uring.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 99 insertions(+)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index e95481c552ff..dd532d2634c2 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -498,6 +498,8 @@ struct io_async_rw {
>>  	struct iovec			*iov;
>>  	ssize_t				nr_segs;
>>  	ssize_t				size;
>> +	struct wait_page_queue		wpq;
>> +	struct callback_head		task_work;
>>  };
>>  
>>  struct io_async_ctx {
>> @@ -2568,6 +2570,99 @@ static int io_read_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe,
>>  	return 0;
>>  }
>>  
>> +static void io_async_buf_cancel(struct callback_head *cb)
>> +{
>> +	struct io_async_rw *rw;
>> +	struct io_ring_ctx *ctx;
>> +	struct io_kiocb *req;
>> +
>> +	rw = container_of(cb, struct io_async_rw, task_work);
>> +	req = rw->wpq.wait.private;
>> +	ctx = req->ctx;
>> +
>> +	spin_lock_irq(&ctx->completion_lock);
>> +	io_cqring_fill_event(req, -ECANCELED);
> 
> It seems like it should go through kiocb_done()/io_complete_rw_common().
> My concern is missing io_put_kbuf().

Yeah, I noticed that too after sending it out. If you look at the
current one that I updated yesterday, it does add that (and also
renames the iter read helper):

https://git.kernel.dk/cgit/linux-block/commit/?h=async-buffered.5&id=6f4e3a4066d0db3e3478e58cc250afb16d8d4d91

-- 
Jens Axboe