Re: [PATCH v4 1/2] block: add folio awareness instead of looping through pages

[Christoph Hellwig <hch@lst.de>]

> @@ -1301,15 +1301,49 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  
>  	for (left = size, i = 0; left > 0; left -= len, i++) {
>  		struct page *page = pages[i];
> +		struct folio *folio = page_folio(page);
> +
> +		/* Calculate the offset of page in folio */
> +		folio_offset = (folio_page_idx(folio, page) << PAGE_SHIFT) +
> +				offset;
> +
> +		len = min_t(size_t, (folio_size(folio) - folio_offset), left);
> +
> +		num_pages = DIV_ROUND_UP(offset + len, PAGE_SIZE);
> +
> +		if (num_pages > 1) {

I still hate having this logic in the block layer.  Even if it is just
a dumb wrapper with the same logic as here I'd much prefer to have
a iov_iter_extract_folios, which can then shift down into a
pin_user_folios_fast and into the MM slowly rather than adding this
logic to the caller.