On Wed, Nov 22 2017 at  1:24pm -0500,
Mikulas Patocka [off-list ref] wrote:
 

Another problem is this:

struct bio *b = bio_clone_bioset(bio, GFP_NOIO, md->queue->bio_split);
bio_advance(b, (bio_sectors(b) - ci.sector_count) << 9);
bio_chain(b, bio);

What if it blocks because the bioset is exhausted?

The code basically builds a chain of bios of unlimited length (suppose for 
example a case when we are splitting on every sector boundary, so there 
will be one bio for every sector in the original bio), it could exhaust 
the bioset easily.

It would be better to use mechanism from md-raid that chains all the 
sub-bios to the same master bio and doesn't create long chains of bios:

        if (max_sectors < bio_sectors(bio)) {
                struct bio *split = bio_split(bio, max_sectors,
                                              gfp, conf->bio_split);
                bio_chain(split, bio);
                generic_make_request(bio);
                bio = split;
                r1_bio->master_bio = bio;
                r1_bio->sectors = max_sectors;
        }

I'd be happy to take an incremental patch that improves on this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.16&id=b46d6a08f1ae7bf53e4cde28e0ccdf91567d432e

But short of that I'll have to come back to this.

Thanks,
Mike