From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Rapoport <rppt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH v3 1/4] lib/vsprintf: Avoid redundant work with 0 size
Date: Mon, 31 Jan 2022 22:42:43 +0200
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References: <20220131192308.608837-1-longman@redhat.com>
 <20220131192308.608837-2-longman@redhat.com>
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To: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Petr Mladek <pmladek-IBi9RG/b67k@public.gmane.org>, Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, Sergey Senozhatsky <senozhatsky-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>, Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>, Rasmus Villemoes <linux-qQsb+v5E8BnlAoU/VqSP6n9LOBIZ5rWg@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Ira Weiny <ira.weiny-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>, Rafael Aquini <aquini-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

On Mon, Jan 31, 2022 at 02:23:05PM -0500, Waiman Long wrote:
> For *scnprintf(), vsnprintf() is always called even if the input size is
> 0. That is a waste of time, so just return 0 in this case.
> 
> Note that vsnprintf() will never return -1 to indicate an error. So
> skipping the call to vsnprintf() when size is 0 will have no functional
> impact at all.
> 
> Signed-off-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Acked-by: David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Reviewed-by: Sergey Senozhatsky <senozhatsky-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> Acked-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>

Acked-by: Mike Rapoport <rppt-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>

> ---
>  lib/vsprintf.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3b8129dd374c..d419154b47bb 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2895,13 +2895,15 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args)
>  {
>  	int i;
>  
> +	if (unlikely(!size))
> +		return 0;
> +
>  	i = vsnprintf(buf, size, fmt, args);
>  
>  	if (likely(i < size))
>  		return i;
> -	if (size != 0)
> -		return size - 1;
> -	return 0;
> +
> +	return size - 1;
>  }
>  EXPORT_SYMBOL(vscnprintf);
>  
> -- 
> 2.27.0
> 

-- 
Sincerely yours,
Mike.