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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1sFEDD-00000000658-3ikm; Thu, 06 Jun 2024 14:35:31 +0000 Date: Thu, 6 Jun 2024 15:35:31 +0100 From: Matthew Wilcox To: "Pankaj Raghav (Samsung)" Cc: Shuah Khan , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, gost.dev@samsung.com, mcgrof@kernel.org, linux-kselftest@vger.kernel.org, Zi Yan , Pankaj Raghav Subject: Re: [PATCH] selftests/mm: use volatile keyword to not optimize mmap read variable Message-ID: References: <20240606135835.600022-1-kernel@pankajraghav.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240606135835.600022-1-kernel@pankajraghav.com> X-Stat-Signature: 5hciiojqwegzu4hruirq7ynb8ns7r7od X-Rspamd-Queue-Id: 54501C0016 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1717684538-558106 X-HE-Meta: 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 Rog1MwKi Am1qO8CMDkuyH1Y7S2iMRs/eVZDaLwW0LKde7JCPC3OrWAW8BIDzEFWPaqFA1EUrMNeRcCcgvq6UId2mnj72m+QPGS58vcJZrkbNGKvJAalWXWJr1GpLDIR34xkLywymh/nQ3aRHH4NzwO30= 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 Thu, Jun 06, 2024 at 01:58:35PM +0000, Pankaj Raghav (Samsung) wrote: > +++ b/tools/testing/selftests/mm/split_huge_page_test.c > @@ -300,7 +300,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd, > char **addr) > { > size_t i; > - int __attribute__((unused)) dummy = 0; > + volatile int __attribute__((unused)) dummy = 0; The mistake made by whoever wrote this test was making 'dummy' a stack variable. That lets the compiler figure out that it's unused. If you make it a top-level variable (not static) so the compiler can't tell whether it's referenced by a different compilation unit, it can't make that deduction. And you don't need the stupid attibute or volatile on it.