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Wed, 6 Aug 2025 10:34:58 +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 ve+1LdIvk2gyNQAAD6G6ig (envelope-from ); Wed, 06 Aug 2025 10:34:58 +0000 Date: Wed, 6 Aug 2025 11:34:57 +0100 From: Pedro Falcato To: Lorenzo Stoakes Cc: Zi Yan , wang lian , Andrew Morton , linux-mm@kvack.org, David Hildenbrand , Wei Yang , Christian Brauner , Jann Horn , Kairui Song , Liam Howlett , Mark Brown , SeongJae Park , Shuah Khan , Vlastimil Babka , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH] selftests/mm: fix FORCE_READ to read input value correctly. Message-ID: References: <20250805175140.241656-1-ziy@nvidia.com> <7eaa1660-06d3-49b9-ba5e-28df1c35dcd1@lucifer.local> <6E5CCF4D-CB63-4E4C-BEDA-6FE533E0DFCD@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 1n6aix7ns17idm78i5a4es6i7wqj43ay X-Rspam-User: X-Rspamd-Queue-Id: 7B9A2C000A X-Rspamd-Server: rspam02 X-HE-Tag: 1754476502-824271 X-HE-Meta: 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 hAJzFKHG d+2sNiRTkCCvmBkzx7a+r1mzftXC1ywOl+hmG15nFadMWKnmETJQ3bsuMvvas8J869akEdTutRuwRpO8QRr03+RYwVy6gVeZwmikvSWh4nff8m86QJGRQZgg610VniNW+Z6lvw8Qio6/4VJJIHJblM81t6e9NoC0d2FvfetV2BeM5HgxvhvzoXjYQuKWUSdgd2vTp8V9EG8HzsszMqOO8VvD13iwntCA8e1Idlbh4t4p5Wb/ITokvZHcKMXj9MHpaDpv12QWfJxeyuA7Sdtc26a18n7r/pKJxFK+9bkM4dhxSM9YQ+b6VkKcmssEphtBsqNvKJruyTbmJ5BRrqnHGuBaea9VkeHPJwZygH2zwt0R1ZHEIDlilra/PsBDUd5AIgtA/b1UZE1SSHFzdwTsXJycJ3xwGlbsg5xjA6lC03ppeX4q/8ucTdSFV3ZjzfciPZC2x7/CBtoZxtTGG/4yXDSjiJMqWbhBF0d81 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 Tue, Aug 05, 2025 at 08:21:23PM +0100, Lorenzo Stoakes wrote: > +cc Pedro > > On Tue, Aug 05, 2025 at 03:09:54PM -0400, Zi Yan wrote: > > On 5 Aug 2025, at 15:00, Lorenzo Stoakes wrote: > > > > > On Tue, Aug 05, 2025 at 01:51:40PM -0400, Zi Yan wrote: > > >> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h > > >> index c20298ae98ea..b55d1809debc 100644 > > >> --- a/tools/testing/selftests/mm/vm_util.h > > >> +++ b/tools/testing/selftests/mm/vm_util.h > > >> @@ -23,7 +23,7 @@ > > >> * anything with it in order to trigger a read page fault. We therefore must use > > >> * volatile to stop the compiler from optimising this away. > > >> */ > > >> -#define FORCE_READ(x) (*(volatile typeof(x) *)x) > > >> +#define FORCE_READ(x) (*(const volatile typeof(x) *)&(x)) > > > > > > NIT: but wonder if const is necessary, and also (as discussed off-list > > > > I just used READ_ONCE() code, but it is not necessary. > > It's not end of the world though. > > > > > > again :) will this work with a (void) prefixed, just to a. make it clear > > > we're reading but discarding and b. to avoid any possible compiler warning > > > on this? > > > > Adding (void) makes no difference, at least from godbolt. > I disagree with adding (void), because volatile being properly propagated into the type should hide any Wunused-value warnings (because volatile reads can have side effects, so discarding a read is most definitely valid). And as I was seeing in https://godbolt.org/z/jnWsET1vx yesterday, GCC (and clang) can silently drop the volatile qualifier For Some Reason. -- Pedro