/dev/zero: also implement ->read

Christophe reported a major speedup due to avoiding the iov_iter
overhead, so just add this trivial function.  Note that /dev/zero
already implements both an iter and non-iter writes so this just
makes it more symmetric.
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20200903155922.1111551-1-hch@lst.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/mem.c

@@ -726,6 +726,27 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
 	return written;
 }
 
+static ssize_t read_zero(struct file *file, char __user *buf,
+			 size_t count, loff_t *ppos)
+{
+	size_t cleared = 0;
+
+	while (count) {
+		size_t chunk = min_t(size_t, count, PAGE_SIZE);
+
+		if (clear_user(buf + cleared, chunk))
+			return cleared ? cleared : -EFAULT;
+		cleared += chunk;
+		count -= chunk;
+
+		if (signal_pending(current))
+			return cleared ? cleared : -ERESTARTSYS;
+		cond_resched();
+	}
+
+	return cleared;
+}
+
 static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 {
 #ifndef CONFIG_MMU
@@ -921,6 +942,7 @@ static const struct file_operations zero_fops = {
 	.llseek		= zero_lseek,
 	.write		= write_zero,
 	.read_iter	= read_iter_zero,
+	.read		= read_zero,
 	.write_iter	= write_iter_zero,
 	.mmap		= mmap_zero,
 	.get_unmapped_area = get_unmapped_area_zero,