From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Shevchenko Subject: Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size Date: Mon, 31 Jan 2022 13:22:42 +0200 Message-ID: References: <20220129205315.478628-1-longman@redhat.com> <20220129205315.478628-2-longman@redhat.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1643628639; x=1675164639; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=IrCfoF7eVv9DU6g4PgHgEV5cLq0Eof4NwIuHTMoZg8E=; b=dYUK0g8hidnaRro/VYR1kcNEvPpOtfYedQMtUIUXT/kSOxPuSixeQ+JH 3pawBbmGx9snj+aq7xvPMJIUgy45aQaPzGOa24Fdd8dp2+v5/O9YCQlSy Gk1cXXXJR+703azR6a/Rco1OirRU2/hnaW/1O6qnfu5/hfWQRAzWhTDnI NFeN4wc/ip6Vu4qFGcAlbpaycTj18UWgwIOiIbrLSFyls/u9m8jMR7s1K OwUwKcBr7yRYopHzLhVMkbh5dJ1hxYJYC1OD7EA80pvsR19bWGKPuVsgc aOplCg+lPJ3iEf7csgzxzeFPQk6+mLaeFKhvUoCj0XbNv+uuvcwRIWTxJ w==; Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Rasmus Villemoes Cc: David Rientjes , Waiman Long , Johannes Weiner , Michal Hocko , Vladimir Davydov , Andrew Morton , Petr Mladek , Steven Rostedt , Sergey Senozhatsky , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Ira Weiny , Rafael Aquini On Mon, Jan 31, 2022 at 12:02:29PM +0100, Rasmus Villemoes wrote: > On 31/01/2022 11.34, Andy Shevchenko wrote: > > On Mon, Jan 31, 2022 at 12:30:33PM +0200, Andy Shevchenko wrote: > >> On Mon, Jan 31, 2022 at 12:25:09PM +0200, Andy Shevchenko wrote: > >>> On Sun, Jan 30, 2022 at 12:49:37PM -0800, David Rientjes wrote: > >>>> On Sat, 29 Jan 2022, 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. > >>> > >>> Why do you think it's not legit? > >> > >> I have to elaborate. > >> > >> For *nprintf() the size=0 is quite useful to have. > >> For *cnprintf() the size=0 makes less sense, but, if we read `man snprintf()`: > >> > >> The functions snprintf() and vsnprintf() do not write more than size bytes > >> (including the terminating null byte ('\0')). If the output was truncated due > >> to this limit, then the return value is the number of characters (excluding > >> the terminating null byte) which would have been written to the final string > >> if enough space had been available. Thus, a return value of size or more > >> means that the output was truncated. (See also below under NOTES.) > >> > >> If an output error is encountered, a negative value is returned. > >> > >> Note the last sentence there. You need to answer to it in the commit message > >> why your change is okay and it will show that you thought through all possible > >> scenarios. > > > > Also it seems currently the kernel documentation is not aligned with the code > > > > "If @size is == 0 the function returns 0." > > > > It should mention the (theoretical?) possibility of getting negative value, > > if vsnprintf() returns negative value. > > > > The kernel's vsnprintf _will never_ return a negative value. There is > way too much code which relies on that. It also has to work from any > context, so we'll never do any memory allocation or anything else that > could possibly force us to error out, and even if we encounter some > impossible situation, we do not return a negative value, but just stop > the output where we are. Yep, I see the code. My comments more or less are related to the (better) commit message which may include what you just said. > So yes, micro-optimizing [v]scnprintf() is completely valid, but I've > never bothered to send the patch because the use case for scnprintf() is > primarily the > > ret += scnprintf(buf + ret, size - ret, ...); > > pattern, with ret starting out at 0 and size being some non-zero number. > When given a non-zero size, scnprintf() is guaranteed to return > something _strictly less_ than that value; that invariant guarantees > that the size-ret expression never becomes 0. So if scnprintf() is > properly used, I can't think of any situation where size will be 0, > hence I see that patch as correct-but-mostly-pointless. Good remark and again commit message probably should elaborate this as well. -- With Best Regards, Andy Shevchenko