/dev/mem: nowait zero/null ops

Make read_iter_zero() to honor IOCB_NOWAIT, so /dev/zero can be
advertised as FMODE_NOWAIT. It's useful for io_uring, which needs it to
apply certain optimisations when doing I/O against the device.

Set FMODE_NOWAIT for /dev/null as well, it never waits and therefore
trivially meets the criteria.
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f11090f97ddc2b2ce49ea1211258658ddfbc5563.1631127867.git.asml.silence@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/mem.c

@@ -495,6 +495,10 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
 		written += n;
 		if (signal_pending(current))
 			return written ? written : -ERESTARTSYS;
+		if (!need_resched())
+			continue;
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			return written ? written : -EAGAIN;
 		cond_resched();
 	}
 	return written;
@@ -696,11 +700,11 @@ static const struct memdev {
 #ifdef CONFIG_DEVMEM
 	 [DEVMEM_MINOR] = { "mem", 0, &mem_fops, FMODE_UNSIGNED_OFFSET },
 #endif
-	 [3] = { "null", 0666, &null_fops, 0 },
+	 [3] = { "null", 0666, &null_fops, FMODE_NOWAIT },
 #ifdef CONFIG_DEVPORT
 	 [4] = { "port", 0, &port_fops, 0 },
 #endif
-	 [5] = { "zero", 0666, &zero_fops, 0 },
+	 [5] = { "zero", 0666, &zero_fops, FMODE_NOWAIT },
 	 [7] = { "full", 0666, &full_fops, 0 },
 	 [8] = { "random", 0666, &random_fops, 0 },
 	 [9] = { "urandom", 0666, &urandom_fops, 0 },