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

Christophe Leroy
2020-09-03T16:10:17Z

Le 03/09/2020 à 17:59, Christoph Hellwig a écrit :
> 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.
>
> Christophe Leroy <christophe.leroy@csgroup.eu>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
> drivers/char/mem.c | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index abd4ffdc8cdebc..1dc99ab158457a 100644
> --- 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,
>