Re: [PATCH V2] block: optimize for small BS IO

[Christoph Hellwig <hch@infradead.org>]

On Tue, Oct 29, 2019 at 03:06:21PM +0800, Ming Lei wrote:
> __blk_queue_split() may be a bit heavy for small BS(such as 512B, or

Maybe spell out block size.  BS has another much less nice connotation.

> bch_bio_map() should be the only one which doesn't use bio_add_page(),
> so force to mark bio built via bch_bio_map() as MULTI_PAGE.

We really need to fix that up.  I had patches back in the day which
Kent didn't particularly like for non-technical reason, that might serve
as a starting point.

> @@ -789,6 +794,10 @@ void __bio_add_page(struct bio *bio, struct page *page,
>  	bio->bi_iter.bi_size += len;
>  	bio->bi_vcnt++;
>  
> +	if (!bio_flagged(bio, BIO_MULTI_PAGE) && (bio->bi_vcnt >= 2 ||
> +				(bio->bi_vcnt == 1 && len > PAGE_SIZE)))
> +		bio_set_flag(bio, BIO_MULTI_PAGE);

This looks pretty ugly and does more (and more confusing) checks than
actually needed Maybe we need a little bio_is_multi_page helper to clean
this up a bit:

/*
 * Check if the bio contains more than a page and thus needs special
 * treatment in the bio splitting code.
 */
static inline bool bio_is_multi_page(struct bio *bio)
{
	return bio->bi_vcnt > 1 || bio->bi_io_vec[0].bv_len > PAGE_SIZE;
}

and then this becomes:

	if (!bio_flagged(bio, BIO_MULTI_PAGE) && bio_is_multi_page(bio))

Then again these checks are so cheap that we can just use the
bio_is_multi_page helper directly and skip the flag entirely.