dm: fix bio splitting and its bio completion order for regular IO

dm_queue_split() is removed because __split_and_process_bio() _must_
handle splitting bios to ensure proper bio submission and completion
ordering as a bio is split.

Otherwise, multiple recursive calls to ->submit_bio will cause multiple
split bios to be allocated from the same ->bio_split mempool at the same
time. This would result in deadlock in low memory conditions because no
progress could be made (only one bio is available in ->bio_split
mempool).

This fix has been verified to still fix the loss of performance, due
to excess splitting, that commit 120c9257f5f1 provided.

Fixes: 120c9257f5f1 ("Revert "dm: always call blk_queue_split() in dm_process_bio()"")
Cc: [e-mail] # 5.0+, requires custom backport due to 5.9 changes
Reported-by: Ming Lei <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm.c

···
 	return ret;
 }
 
-static void dm_queue_split(struct mapped_device *md, struct dm_target *ti, struct bio **bio)
-{
-	unsigned len, sector_count;
-
-	sector_count = bio_sectors(*bio);
-	len = min_t(sector_t, max_io_len((*bio)->bi_iter.bi_sector, ti), sector_count);
-
-	if (sector_count > len) {
-		struct bio *split = bio_split(*bio, len, GFP_NOIO, &md->queue->bio_split);
-
-		bio_chain(split, *bio);
-		trace_block_split(md->queue, split, (*bio)->bi_iter.bi_sector);
-		submit_bio_noacct(*bio);
-		*bio = split;
-	}
-}
-
 static blk_qc_t dm_process_bio(struct mapped_device *md,
 			       struct dm_table *map, struct bio *bio)
 {
···
 	if (current->bio_list) {
 		if (is_abnormal_io(bio))
 			blk_queue_split(&bio);
-		else
-			dm_queue_split(md, ti, &bio);
+		/* regular IO is split by __split_and_process_bio */
 	}
 
 	if (dm_get_md_type(md) == DM_TYPE_NVME_BIO_BASED)
 		return __process_bio(md, map, bio, ti);
-	else
-		return __split_and_process_bio(md, map, bio);
+	return __split_and_process_bio(md, map, bio);
 }
 
 static blk_qc_t dm_submit_bio(struct bio *bio)