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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] selftest/mm: fix pointer comparison in mremap_test Message-Id: <20251107160855.58891ac6df6854a3b608185f@linux-foundation.org> In-Reply-To: <6e07949b-d86f-46d8-a68c-9717cfb26084@kernel.org> References: <20251106104917.39890-1-ankitkhushwaha.linux@gmail.com> <6e07949b-d86f-46d8-a68c-9717cfb26084@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 58E0F1A0003 X-Stat-Signature: u56bywjbw4r5m8obniidirthwpranhdd X-Rspam-User: X-HE-Tag: 1762560537-511530 X-HE-Meta: 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 jJRQEVhh 56IlGz8r4SOA4HHpDAUZPSR86fCUixj6gf7s1/NP9e61Tb6RcCOMyhXIY13Ic5+Gd03Zluy5CXUNIIwIZdmdP75ZCVVwXrOx/Osw+ySdnLeOBndCZHfLqIF5KSwq420STmgau1lqklhgvr/Vm/4QdNHgphB6NlQ70f9yZ+S8uKE8WrOzvNPDRvi72X2xQLl8IJP4zsHcx4IGJCUV30/E3wxZd3pkGdvMc2ZQ30NgbT4jVZSZy9BBsUnI+cPRvkTUrkz90S6tSamK3LMs1sJBzFk2xummycVxU+3HrZcV2/Ilnhqa10iR2+W/y46pUT34W2pvV0P9z93RACxTC3tDuJ0/AqIPo/IJonS5L3pgQdQVzyuoFDxB8SXKgAiZ+TonKWoY3WBlXbQySqOw= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 7 Nov 2025 10:27:27 +0100 "David Hildenbrand (Red Hat)" wrote: > On 06.11.25 13:02, Ankit Khushwaha wrote: > > On Thu, Nov 06, 2025 at 12:18:57PM +0100, David Hildenbrand (Red Hat) wrote: > >> On 06.11.25 11:49, Ankit Khushwaha wrote: > >>> Pointer arthemitic with 'void * addr' and 'unsigned long long dest_alignment' > >>> triggers following warning: > >>> > >>> mremap_test.c:1035:31: warning: pointer comparison always evaluates to > >>> false [-Wtautological-compare] > >>> 1035 | if (addr + c.dest_alignment < addr) { > >>> | ^ > >>> > >>> typecasting 'addr' to 'unsigned long long' to fix pointer comparison. > >> > >> With which compiler are you seeing this? > > > > Hi David, > > > > clang version 20.1.8 (Fedora 20.1.8-4.fc42) raised this warning. > > > > To reproduce: > > make -C tools/testing/selftests/mm CC=clang > > Thanks, and thanks to Lorenzo for the details. > > Acked-by: David Hildenbrand (Red Hat) I must say, applying this would be an unhappy life event. if (void* + ulong < void*) makes perfect sense in a world which permits void* arithmetic (ie, ours). So what the heck is clang doing?? If we do void *addr2 = addr + c.dest_alignment; if (addr2 < addr) ... then which statement warns, and why?