linux-kernel - Re: [PATCH 03/16] lib/test_bitmap: don't test bitmap

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Message-ID: <36677eab-6407-afd1-4cbf-a90be9554c8b@rasmusvillemoes.dk>
Date:   Tue, 9 Aug 2022 14:37:22 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Yury Norov <yury.norov@...il.com>, 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>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 03/16] lib/test_bitmap: don't test bitmap_set if nbits ==
 0

On 18/07/2022 21.28, Yury Norov wrote:
> Don't test bitmap_set(bitmap, start, 0) as it's useless, most probably
> a sign of error in real code, 

No it's not. The nbits can easily be the result of some computation that
ended up resulting in 0 being the right number to copy (or set, or
whatnot), and it's not unreasonable to _not_ check in the caller for
that special case, but rather rely on bitmap_set() to behave sanely - it
has perfectly well-defined semantics to "set 0 bits starting at @start".

The same way that memset() and memcpy() and memcmp() and countless other
functions have perfectly well-defined semantics with a length of 0, and
we don't add caller-side checks for those either.

NAK on this series.

Rasmus