linux-kernel - Re: [PATCH 02/16] lib/bitmap: don't call bitmap

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Message-ID: <YtXLUHQKIpMjoJna@smile.fi.intel.com>
Date:   Tue, 19 Jul 2022 00:06:24 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dennis Zhou <dennis@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Ingo Molnar <mingo@...hat.com>,
        Isabella Basso <isabbasso@...eup.net>,
        John Fastabend <john.fastabend@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        KP Singh <kpsingh@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Mel Gorman <mgorman@...e.de>, Miroslav Benes <mbenes@...e.cz>,
        Nathan Chancellor <nathan@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
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        Song Liu <songliubraving@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yonghong Song <yhs@...com>,
        linux-mm@...ck.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 02/16] lib/bitmap: don't call bitmap_set() with len == 0

On Mon, Jul 18, 2022 at 12:28:30PM -0700, Yury Norov wrote:
> bitmap_parselist() format allows passing 0-length regions, but because
> len == 0 is not covered by small_const_nbits() macro, we have to call
> __bitmap_set() and do some prologue calculations just for nothing.

...

>  static void bitmap_set_region(const struct region *r, unsigned long *bitmap)
>  {
> -	unsigned int start;
> +	unsigned int start, len;
>  
> -	for (start = r->start; start <= r->end; start += r->group_len)
> -		bitmap_set(bitmap, start, min(r->end - start + 1, r->off));
> +	for (start = r->start; start <= r->end; start += r->group_len) {
> +		len = min(r->end - start + 1, r->off);
> +		if (len > 0)
> +			bitmap_set(bitmap, start, len);

Maybe I'm missing something, but how small_const_nbits() can possible be useful
here? I mean in which cases the compiler would be able to prove that len is a
const with < sizeof(unsigned long) bits?

> +	}
>  }

-- 
With Best Regards,
Andy Shevchenko