Christophe reported a major speedup due to avoiding the iov_iter
overhead, so just add this trivial function.
Suggested-by: Christophe Leroy <christophe.le...@csgroup.eu>
Signed-off-by: Christoph Hellwig <h...@lst.de>
Tested-by: Christophe Leroy <christophe.le...@csgroup.eu>
---
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
