Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

Christophe Leroy
2020-09-03T08:56:21Z

Le 03/09/2020 à 09:11, Christoph Hellwig a écrit :
>
> Except that we do not actually have such a patch. For normal user
> writes we only use ->write_iter if ->write is not present. But what
> shows up in the profile is that /dev/zero only has a read_iter op and
> not a normal read. I've added a patch below that implements a normal
> read which might help a tad with this workload, but should not be part
> of a regression.
>

With that patch below, throughput is 113.5MB/s (instead of 99.9MB/s).
So a 14% improvement. That's not bad.

Christophe

>
> ---
> 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,
>