linux-kernel - Re: [PATCH 10/21] block: Add fops atomic write support

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Date:   Mon, 4 Dec 2023 10:30:14 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     John Garry <john.g.garry@...cle.com>
Cc:     axboe@...nel.dk, kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org, ming.lei@...hat.com
Subject: Re: [PATCH 10/21] block: Add fops atomic write support

On Fri, Sep 29, 2023 at 10:27:15AM +0000, John Garry wrote:
> Add support for atomic writes, as follows:
> - Ensure that the IO follows all the atomic writes rules, like must be
>   naturally aligned
> - Set REQ_ATOMIC
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
>  block/fops.c | 42 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/block/fops.c b/block/fops.c
> index acff3d5d22d4..516669ad69e5 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -41,6 +41,29 @@ static bool blkdev_dio_unaligned(struct block_device *bdev, loff_t pos,
>  		!bdev_iter_is_aligned(bdev, iter);
>  }
>  
> +static bool blkdev_atomic_write_valid(struct block_device *bdev, loff_t pos,
> +			      struct iov_iter *iter)
> +{
> +	unsigned int atomic_write_unit_min_bytes =
> +			queue_atomic_write_unit_min_bytes(bdev_get_queue(bdev));
> +	unsigned int atomic_write_unit_max_bytes =
> +			queue_atomic_write_unit_max_bytes(bdev_get_queue(bdev));
> +
> +	if (!atomic_write_unit_min_bytes)
> +		return false;

The above check should have be moved to limit setting code path.

> +	if (pos % atomic_write_unit_min_bytes)
> +		return false;
> +	if (iov_iter_count(iter) % atomic_write_unit_min_bytes)
> +		return false;
> +	if (!is_power_of_2(iov_iter_count(iter)))
> +		return false;
> +	if (iov_iter_count(iter) > atomic_write_unit_max_bytes)
> +		return false;
> +	if (pos % iov_iter_count(iter))
> +		return false;

I am a bit confused about relation between atomic_write_unit_max_bytes and
atomic_write_max_bytes.

Here the max IO length is limited to be <= atomic_write_unit_max_bytes,
so looks userspace can only submit IO with write-atomic-unit naturally
aligned IO(such as, 4k, 8k, 16k, 32k, ...), but these user IOs are
allowed to be merged to big one if naturally alignment is respected and
the merged IO size is <= atomic_write_max_bytes.

Is my understanding right? If yes, I'd suggest to document the point,
and the last two checks could be change to:

	/* naturally aligned */
	if (pos % iov_iter_count(iter))
		return false;

	if (iov_iter_count(iter) > atomic_write_max_bytes)
		return false;

Thanks, 
Ming