+static void *blkdev_copy_alloc_buf(sector_t req_size, sector_t *alloc_size,
+		gfp_t gfp_mask)
+{
+	int min_size = PAGE_SIZE;
+	void *buf;
+
+	while (req_size >= min_size) {
+		buf = kvmalloc(req_size, gfp_mask);
+		if (buf) {
+			*alloc_size = req_size;
+			return buf;
+		}
+		/* retry half the requested size */
+		req_size >>= 1;
+	}
+
+	return NULL;

Is there any good reason for using vmalloc instead of a bunch
of distcontiguous pages?

+		ctx = kzalloc(sizeof(struct copy_ctx), gfp_mask);
+		if (!ctx)
+			goto err_ctx;

I'd suspect it would be better to just allocte a single buffer and
only have a single outstanding copy.  That will reduce the bandwith
you can theoretically get, but copies tend to be background operations
anyway.  It will reduce the required memory, and thus the chance for
this operation to fail on a loaded system.  It will also dramatically
reduce the effect on memory managment.

+		read_bio = bio_map_kern(in, buf, buf_len, gfp_mask);
+		if (IS_ERR(read_bio))
+			goto err_read_bio;
+
+		write_bio = bio_map_kern(out, buf, buf_len, gfp_mask);
+		if (IS_ERR(write_bio))
+			goto err_write_bio;

bio_map_kern is only for passthrough I/Os.  You need to use
a bio_add_page loop here.

This is misnamed as it's only used by the fallback code, and also
should be private to blk-lib.c.