Re: [PATCH v2 2/3] block: Split and submit bios in LBA order

[Christoph Hellwig <hch@lst.de>]

On Mon, Mar 20, 2023 at 04:49:04PM -0700, Bart Van Assche wrote:
> Submit the bio fragment with the lowest LBA first. This approach prevents
> write errors when submitting large bios to host-managed zoned block devices.
> This patch only modifies the behavior of drivers that call
> bio_split_to_limits() directly. This includes DRBD, pktcdvd, dm, md and
> the NVMe multipath code.

Umm, doesn't it also change how blk-mq splits, which is the prime
reason why you're looking into that?

> +		if (current->bio_list) {
> +			/*
> +			 * The caller will submit the first half ('split')
> +			 * before the second half ('bio').
> +			 */
> +			bio_chain(split, bio);
> +			submit_bio_noacct(bio);
> +			return split;
> +		}
> +		/*
> +		 * Submit the first half ('split') let the caller submit the
> +		 * second half ('bio').
> +		 */
> +		*nr_segs = bio_chain_nr_segments(bio, lim);
> +		bio_chain(split, bio);
> +		submit_bio_noacct(split);

I'm really confused on why you want to change the behavior here
for the case where run in a stacking context vs not, and neither
the comments nor the commit log help me trying to figure out why.