ZEN: dm-crypt: Disable workqueues for crypto ops

Queueing in dm-crypt for crypto operations reduces performance on modern
systems.  As discussed in an article from Cloudflare, they discovered
that queuing was introduced because the crypto subsystem used to be
synchronous.  Since it's now asynchronous, we get double queueing when
using the subsystem through dm-crypt.  This is obviously undesirable and
reduces throughput and increases latency.

Fixes: zen-kernel/zen-kernel#282
Signed-off-by: Alexandre Frade <[email protected]>

drivers/md/dm-crypt.c

@@ -3310,6 +3310,9 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 			goto bad;
 	}
 
+	set_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags);
+	set_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags);
+
 	ret = crypt_ctr_cipher(ti, argv[0], argv[1]);
 	if (ret < 0)
 		goto bad;