/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.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index abd4ffdc8cdebc4ade04d4b77875bd894b063863..1dc99ab158457a25c4c53eb183c1034f3bf9d74e 100644 (file)
--- a/drivers/char/mem.c
+++ b/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,