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Subject: Re: [PATCH 12/12] io_uring: support true async buffered reads, if
 file provides it
To:     Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20200523185755.8494-1-axboe@kernel.dk>
 <20200523185755.8494-13-axboe@kernel.dk>
From:   Pavel Begunkov <asml.silence@gmail.com>
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Message-ID: <9c2cc031-e4ce-4c5f-5e14-21ea48c327f6@gmail.com>
Date:   Tue, 26 May 2020 10:38:40 +0300
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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@kernel.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().

> +	io_commit_cqring(ctx);
> +	spin_unlock_irq(&ctx->completion_lock);
> +
> +	io_cqring_ev_posted(ctx);
> +	req_set_fail_links(req);
> +	io_double_put_req(req);
> +}


-- 
Pavel Begunkov