Re: [PATCHv2 3/3] block: relax direct io memory alignment

[Keith Busch <kbusch@kernel.org>]

On Wed, May 18, 2022 at 07:08:11PM -0700, Eric Biggers wrote:
> On Wed, May 18, 2022 at 07:59:36PM -0600, Keith Busch wrote:
> > I'm aware that spanning pages can cause bad splits on the bi_max_vecs
> > condition, but I believe it's well handled here. Unless I'm terribly confused,
> > which is certainly possible, I think you may have missed this part of the
> > patch:
> > 
> > @@ -1223,6 +1224,8 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> >  	pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
> > 
> >  	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> > +	if (size > 0)
> > +		size = ALIGN_DOWN(size, queue_logical_block_size(q));
> >  	if (unlikely(size <= 0))
> >  		return size ? size : -EFAULT;
> > 
> 
> That makes the total length of each "batch" of pages be a multiple of the
> logical block size, but individual logical blocks within that batch can still be
> divided into multiple bvecs in the loop just below it:

I understand that, but the existing code conservatively assumes all pages are
physically discontiguous and wouldn't have requested more pages if it didn't
have enough bvecs for each of them:

	unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;

So with the segment alignment guarantee, and ensured available bvec space, the
created bio will always be a logical block size multiple.

If we need to split it later due to some other constraint, we'll only split on
a logical block size, even if its in the middle of a bvec.

> 	for (left = size, i = 0; left > 0; left -= len, i++) {
> 		struct page *page = pages[i];
> 
> 		len = min_t(size_t, PAGE_SIZE - offset, left);
> 
> 		if (__bio_try_merge_page(bio, page, len, offset, &same_page)) {
> 			if (same_page)
> 				put_page(page);
> 		} else {
> 			if (WARN_ON_ONCE(bio_full(bio, len))) {
> 				bio_put_pages(pages + i, left, offset);
> 				return -EINVAL;
> 			}
> 			__bio_add_page(bio, page, len, offset);
> 		}
> 		offset = 0;
> 	}
> 
> - Eric