dm writecache: have ssd writeback wait if the kcopyd workqueue is busy

Make dm-writecache wait if the kcopyd workqueue is busy (as will
happen if waiting for page allocation or inside submit_bio).

This change improves performance of "mkfs.ext2" by approximately 20%
on one testbed.

Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-kcopyd.c

···
 	kfree(kc);
 }
 EXPORT_SYMBOL(dm_kcopyd_client_destroy);
+
+void dm_kcopyd_client_flush(struct dm_kcopyd_client *kc)
+{
+	flush_workqueue(kc->kcopyd_wq);
+}
+EXPORT_SYMBOL(dm_kcopyd_client_flush);

drivers/md/dm-writecache.c

···
 	struct writeback_list wbl;
 	unsigned long n_walked;
 
+	if (!WC_MODE_PMEM(wc)) {
+		/* Wait for any active kcopyd work on behalf of ssd writeback */
+		dm_kcopyd_client_flush(wc->dm_kcopyd);
+	}
+
 	wc_lock(wc);
 restart:
 	if (writecache_has_error(wc)) {

include/linux/dm-kcopyd.h

···
 struct dm_kcopyd_client;
 struct dm_kcopyd_client *dm_kcopyd_client_create(struct dm_kcopyd_throttle *throttle);
 void dm_kcopyd_client_destroy(struct dm_kcopyd_client *kc);
+void dm_kcopyd_client_flush(struct dm_kcopyd_client *kc);
 
 /*
  * Submit a copy job to kcopyd.  This is built on top of the