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d=pankajraghav.com; s=MBO0001; t=1717705115; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ohDaG0hOCzORTv2Q7Osb5vtnPE6ReKAWi9dBZ7ab1xc=; b=O1ZmJ6RlSaIOZnPqzA910/zSq4CkE+BrWOeNVx/EF5z6ns2s5l8/eGtXfKm/IgSB7aQgWh tX4O09EFjB0RqCkS8+vnZwUKX42BvYRExY1SAvJSJg6aFJdwdSnhEJ+Yi0SLMXNqmCGHgP AR10WbEyD+zM4kInasIOD27qTCyeC18NseUz9ZlG/IpX7BlbQMOCt/eCGu3oXMsIPoGEeD A92YRz07AVoOe/9WTPvYABVKoYDSWHdLSc6z+rP5/0drGs6GgGAqdNIOkDRgFma3bMXNln pBG4Y5gAHVzaL8EhZ+Sd0OQeJk8xBrfEqDLIBlDO4e6QYQMMQ4nFlXZpD8nw7g== Date: Thu, 6 Jun 2024 20:18:31 +0000 From: "Pankaj Raghav (Samsung)" To: David Hildenbrand Cc: Shuah Khan , Andrew Morton , willy@infradead.org, 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: <20240606201831.ywmawi7xl6jgj3p4@quentin> References: <20240606135835.600022-1-kernel@pankajraghav.com> <5b4e7ef2-3ced-4d4a-989c-e99b06598d32@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 835B8A0008 X-Stat-Signature: tsgmyfwub3hqgk7jd85pp7pmffb9386e X-HE-Tag: 1717705120-743296 X-HE-Meta: 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 vnX2TMqe JDEHOzDHyTkt+BlZ+Y5JJmmZHXT+n9+ODArG45Trdzh5Ot6cjItkho3eOCdZMKc8kW8LopUU63V9UtZnbaky6lfK7TKJr5hRU+wsIKBePb5UFidigXgMJ5KvVQFTPaCWoVMmN2tJj3iAOy2qIYOY5QLOXi2Ghc7n0bKakN4Fb8wzt/FwBRfXgzFhKIu0jxSjXzxBBPwmQvT8f/wWGhB/FwKvP5nUE0ABvEHLOzD1OF6CoYXUeJ2HahOWRSaZ28o2jblBLYL7c5uIi06JRKP8Rzo60Bdy9H7UfPwbZS6WpYIIKsO4FbNjP4/PhLONvPhYITNEegiUC7gQpcyiJd7emMAKsi8RYiQn8CmXWQp1p96iQRSs= 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 05:57:21PM +0200, David Hildenbrand wrote: > On 06.06.24 17:56, David Hildenbrand wrote: > > On 06.06.24 15:58, Pankaj Raghav (Samsung) wrote: > > > From: Pankaj Raghav > > > > > > create_pagecache_thp_and_fd() in split_huge_page_test.c used the > > > variable dummy to perform mmap read. > > > > > > However, this test was skipped even on XFS which has large folio > > > support. The issue was compiler (gcc 13.2.0) was optimizing out the > > > dummy variable, therefore, not creating huge page in the page cache. > > > > > > Add volatile keyword to force compiler not to optimize out the loop > > > where we read from the mmaped addr. > > > > > > Signed-off-by: Pankaj Raghav > > > --- > > > tools/testing/selftests/mm/split_huge_page_test.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c > > > index d3c7f5fb3e7b..c573a58f80ab 100644 > > > --- a/tools/testing/selftests/mm/split_huge_page_test.c > > > +++ 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; > > > srand(time(NULL)); > > > > > > base-commit: d97496ca23a2d4ee80b7302849404859d9058bcd > > > > The rick we do in some other tests is: > > > > char *tmp; > > > > tmp = *whatever; > > asm volatile("" : "+r" (tmp)); > > char tmp; of course. See cow.c as an example. Thanks David! I remember also seeing this when I grepped for volatile in the selftests directory. Willy gave the idea of making it as a global variable [1]. But your trick also works :) diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c index d3c7f5fb3e7b..9c957703c1f7 100644 --- a/tools/testing/selftests/mm/split_huge_page_test.c +++ b/tools/testing/selftests/mm/split_huge_page_test.c @@ -341,6 +341,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd, for (size_t i = 0; i < fd_size; i++) dummy += *(*addr + i); + asm volatile("" : "+r" (dummy)); if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) { ksft_print_msg("No large pagecache folio generated, please provide a filesystem supporting large folio\n"); I am fine with either solutions. But using the trick asm volatile is more cleaner than making it a global variable IMO and makes it more uniform across the other mm tests. Let me know what others think. [1] https://lore.kernel.org/linux-mm/20240606154428.672643-1-kernel@pankajraghav.com/