Re: [PATCH v2 2/5] block: Support atomic writes limits for stacked devices

[Christoph Hellwig <hch@lst.de>]

On Wed, Oct 30, 2024 at 09:49:09AM +0000, John Garry wrote:
> Allow stacked devices to support atomic writes by aggregating the minimum
> capability of all bottom devices.
> 
> Flag BLK_FEAT_ATOMIC_WRITES_STACKED is set for stacked devices which
> have been enabled to support atomic writes.
> 
> Some things to note on the implementation:
> - For simplicity, all bottom devices must have same atomic write boundary
>   value (if any)
> - The atomic write boundary must be a power-of-2 already, but this
>   restriction could be relaxed. Furthermore, it is now required that the
>   chunk sectors for a top device must be aligned with this boundary.
> - If a bottom device atomic write unit min/max are not aligned with the
>   top device chunk sectors, the top device atomic write unit min/max are
>   reduced to a value which works for the chunk sectors.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  block/blk-settings.c   | 89 ++++++++++++++++++++++++++++++++++++++++++
>  include/linux/blkdev.h |  4 ++
>  2 files changed, 93 insertions(+)
> 
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 1642e65a6521..6a900ef86e5a 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -496,6 +496,93 @@ static unsigned int blk_round_down_sectors(unsigned int sectors, unsigned int lb
>  	return sectors;
>  }
>  
> +static void blk_stack_atomic_writes_limits(struct queue_limits *t, struct queue_limits *b)

Avoid the overly long line here.

> +	if (t->atomic_write_hw_max) {

Maybe split this branch and the code for when it is not set into
separate helpers to keep the function to a size where it can be
easily understood?

> +	/* Check first bottom device limits */
> +	if (!b->atomic_write_hw_boundary)
> +		goto check_unit;
> +	/*
> +	 * Ensure atomic write boundary is aligned with chunk sectors. Stacked
> +	 * devices store chunk sectors in t->io_min.
> +	 */
> +	if (b->atomic_write_hw_boundary > t->io_min &&
> +	    b->atomic_write_hw_boundary % t->io_min)
> +		goto unsupported;
> +	else if (t->io_min > b->atomic_write_hw_boundary &&

No need for the else here.

> +		 t->io_min % b->atomic_write_hw_boundary)
> +		goto unsupported;
> +
> +	t->atomic_write_hw_boundary = b->atomic_write_hw_boundary;
> +
> +check_unit:

Maybe instead of the check_unit goto just move the checks between the
goto above and this into a branch?

Otherwise this looks conceptually fine to me.