Re: [PATCH] nvme: zns: limit max_zone_append by max_segments

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jul 31, 2023 at 08:46:32PM +0900, Shin'ichiro Kawasaki wrote:
> To avoid the unexpected BIO split, reflect the max_segments limit to the
> max_zone_append limit. In the worst case, the safe max_zone_append size
> is max_segments multiplied by PAGE_SIZE. Compare it with the
> max_zone_append size obtained from ZNS devices, and set the smaller
> value as the max_zone_append limit.

This is true only for NVMe and a hand full of drivers that
set the virt_boundary.  For others the maximum size is completely
unrelated to the maximum number of segments.

Can you reword the commit log a bit to make that more clear?

> diff --git a/drivers/nvme/host/zns.c b/drivers/nvme/host/zns.c
> index ec8557810c21..9ee77626c235 100644
> --- a/drivers/nvme/host/zns.c
> +++ b/drivers/nvme/host/zns.c
> @@ -10,9 +10,11 @@
>  int nvme_revalidate_zones(struct nvme_ns *ns)
>  {
>  	struct request_queue *q = ns->queue;
> +	unsigned int max_sectors = queue_max_segments(q) << PAGE_SECTORS_SHIFT;
>  
>  	blk_queue_chunk_sectors(q, ns->zsze);
> -	blk_queue_max_zone_append_sectors(q, ns->ctrl->max_zone_append);
> +	max_sectors = min(max_sectors, ns->ctrl->max_zone_append);
> +	blk_queue_max_zone_append_sectors(q, max_sectors);

And while this looks correct, it also feels pretty ugly.  Shouldn't a

	blk_queue_max_zone_append_sectors(q,
		queue_max_sectors(q), ns->ctrl->max_zone_append);

do the same thing in a somewhat more obvious way given that max_segments
is already taken into account for the queue_max_sectors calculation?