linux-kernel - Re: [PATCH] block: don't ignore REQ

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Date:   Fri, 20 Nov 2020 12:13:51 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: don't ignore REQ_NOWAIT for direct IO

On 11/20/20 10:10 AM, Pavel Begunkov wrote:
> io_uring's direct nowait requests end up waiting on io_schedule() in
> sbitmap, that's seems to be so because blkdev_direct_IO() fails to
> propagate IOCB_NOWAIT to a bio and hence to blk-mq.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  fs/block_dev.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 9e84b1928b94..e7e860c78d93 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -263,6 +263,8 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
>  		bio.bi_opf = dio_bio_write_op(iocb);
>  		task_io_account_write(ret);
>  	}
> +	if (iocb->ki_flags & IOCB_NOWAIT)
> +		bio.bi_opf |= REQ_NOWAIT;
>  	if (iocb->ki_flags & IOCB_HIPRI)
>  		bio_set_polled(&bio, iocb);

Was thinking this wasn't needed, but I guess that users could do sync && NOWAIT
and get -EAGAIN if using preadv2/pwritev2.

> @@ -416,6 +418,8 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  			bio->bi_opf = dio_bio_write_op(iocb);
>  			task_io_account_write(bio->bi_iter.bi_size);
>  		}
> +		if (iocb->ki_flags & IOCB_NOWAIT)
> +			bio->bi_opf |= REQ_NOWAIT;
>  
>  		dio->size += bio->bi_iter.bi_size;
>  		pos += bio->bi_iter.bi_size;

Looks fine to me, we definitely should not be waiting on tags for IOCB_NOWAIT
IO. Will run some shakedown and test for 5.11.

-- 
Jens Axboe