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Sun, 9 Nov 2025 19:11:09 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id RoarJU3nEGlMWAAAD6G6ig (envelope-from ); Sun, 09 Nov 2025 19:11:09 +0000 Message-ID: Date: Sun, 9 Nov 2025 20:11:09 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] selftest/mm: fix pointer comparison in mremap_test To: Mike Rapoport , Andrew Morton , Nathan Chancellor Cc: "David Hildenbrand (Red Hat)" , Ankit Khushwaha , Lorenzo Stoakes , "Liam R. Howlett" , Suren Baghdasaryan , Michal Hocko , Shuah Khan , linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org References: <20251106104917.39890-1-ankitkhushwaha.linux@gmail.com> <6e07949b-d86f-46d8-a68c-9717cfb26084@kernel.org> <20251107160855.58891ac6df6854a3b608185f@linux-foundation.org> Content-Language: en-US From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspam-User: X-Rspamd-Queue-Id: 885B218001A X-Stat-Signature: okttx3nf6ex8mmo1tqh5o7hr9nh4kgmy X-HE-Tag: 1762715471-217201 X-HE-Meta: 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 4JZc8/Ln 8ToVrLsMudfRmfVdFW+5XnH1he+0otSgoN6aWQ5lsJFi1MdKvbIOgktCWc49MC7EeF8cJ8o9gO1DNmViOsPuKHHn9HHnDnI74t7MzarXrJ5EAdqNQ1NLn3Z/8OM/g3UqScCfDC+sefdhX3YjG0/UxUSGjfQRASnUExlN4sJjNKJfVDSXA3ds23gQlUUGrKHnXKNazniRukMOC23P+SIUzvCLnAeiJKCpx13okFE7RLJ53hbj3gX/s9ct6n1FvUvqASrfEYq94LM3xn7Iaw04yb0kiMMo4ETujJLIIXpTg2Ut7hZd8+cRewXDYBzl2FU4F/X75VOy3E3A54dHq1VRZgBZURnYEkTWD43x+iNsxWMB6A9ziGETqNKYWfWxI1ddv3GilY5vBtZD30QSerUTxX3tEBvKWd++srC6yCVFc8n7CMvI= 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 11/9/25 07:54, Mike Rapoport wrote: > (added Nathan for clang advice) > > On Fri, Nov 07, 2025 at 04:08:55PM -0800, Andrew Morton wrote: >> 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?? My (not very informed) guess would be something about undefined behavior because pointer arithmetic is strictly speaking only valid within an array, so void* + ulong is also still in the same array, and thus can't become smaller by an overflow, because overflow can't happen if we're still within the same valid array... But I don't know if this strictness is only applied to the warning itself or to the actual compilation too (does it eliminate everything as dead code then?) >> If we do >> >> void *addr2 = addr + c.dest_alignment; >> if (addr2 < addr) >> ... >> >> then which statement warns, and why? As the answer was that nothing warns, I'd think it just isn't able to warn if it's no longer part of the same statement. Whether it also means it's not eliminated as dead code anymore, dunno.