[PATCH v2] /dev/zero: also implement ->read

[Christoph Hellwig]

Christophe reported a major speedup due to avoiding the iov_iter
overhead, so just add this trivial function.

Suggested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Tested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---

Changes since v1:
 - fix the Suggested-by: tag
 - report the actually read bytes in case of a partial clear_user
 - remove an impossible to hit conditional

 drivers/char/mem.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index abd4ffdc8cdebc..94c2b556cf9728 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -726,6 +726,33 @@ 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);
+		size_t left;
+
+		left = clear_user(buf + cleared, chunk);
+		if (unlikely(left)) {
+			cleared += (chunk - left);
+			if (!cleared)
+				return -EFAULT;
+			break;
+		}
+		cleared += chunk;
+		count -= chunk;
+
+		if (signal_pending(current))
+			break;
+		cond_resched();
+	}
+
+	return cleared;
+}
+
 static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 {
 #ifndef CONFIG_MMU
@@ -921,6 +948,7 @@ 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,
-- 
2.28.0