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b=F7aihoziRiv6mHe2u8ZO/m74t67Vxpw3JJ0ZBNtIDAf3ae4hDtDSkxecj1KkyhUAe 8DYzLinGmCU2Ui+NNUi6bOEhV/UaTSSIrS0NFQlY0CyVlsXitk4xZspPOMvtMDRs10 LquAgTIpZNCHokNh4+x2Hmf31O3o5Pk2k56vRFmc= Date: Sun, 19 May 2024 17:03:57 -0700 From: Andrew Morton To: Dev Jain Cc: shuah@kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Anshuman.Khandual@arm.com, sjayaram@akamai.com, stable@vger.kernel.org Subject: Re: [PATCH 2/2] selftests/mm: compaction_test: Fix trivial test success and reduce probability of OOM-killer invocation Message-Id: <20240519170357.757d30aac192c686f10d709c@linux-foundation.org> In-Reply-To: <20240515093633.54814-3-dev.jain@arm.com> References: <20240515093633.54814-1-dev.jain@arm.com> <20240515093633.54814-3-dev.jain@arm.com> 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-Queue-Id: 422771C000A X-Stat-Signature: ku7p5uq1cib6181kspcxiyqwpdpi7y5m X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1716163439-307199 X-HE-Meta: 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 r9S6XDCb Uv6usio2IRUZ1zZvJsgyqqcFIauAYwvkzE6PCi0BovVttfFLAtG1L+OAbqNiSW5zuSs3DOZR+aUV78TT/0/0FtgKr4e6mY6iqz0sOUQ/kpOa3HEE8AQAc4HMoWPSb11CI+ZlnBB4WDAsJjxr3uncXCTfCvc2Umgjg3Xoe3UGbK5GHrvWUBpv7yS/c53Op8EsBrnPvFLa14e/7Y8AWlhCTngPXG5kCGdQyrahRGpm12rxKxzA+/JuJcKtBXRAbFMSVaCpZBwmkPT/oZRdLJW0Vx7al+WVOXSlGAcp3T6AgjpyG+TTkOCu4MCFyUrVyH2cXLrpNN8xTnkgj80I= 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 Wed, 15 May 2024 15:06:33 +0530 Dev Jain wrote: > Reset nr_hugepages to zero before the start of the test. > > If a non-zero number of hugepages is already set before the start of the > test, the following problems arise: > > - The probability of the test getting OOM-killed increases. > Proof: The test wants to run on 80% of available memory to prevent > OOM-killing (see original code comments). Let the value of mem_free at the > start of the test, when nr_hugepages = 0, be x. In the other case, when > nr_hugepages > 0, let the memory consumed by hugepages be y. In the former > case, the test operates on 0.8 * x of memory. In the latter, the test > operates on 0.8 * (x - y) of memory, with y already filled, hence, memory > consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 * x. Q.E.D > > - The probability of a bogus test success increases. > Proof: Let the memory consumed by hugepages be greater than 25% of x, > with x and y defined as above. The definition of compaction_index is > c_index = (x - y)/z where z is the memory consumed by hugepages after > trying to increase them again. In check_compaction(), we set the number > of hugepages to zero, and then increase them back; the probability that > they will be set back to consume at least y amount of memory again is > very high (since there is not much delay between the two attempts of > changing nr_hugepages). Hence, z >= y > (x/4) (by the 25% assumption). > Therefore, > c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3 > hence, c_index can always be forced to be less than 3, thereby the test > succeeding always. Q.E.D > > NOTE: This patch depends on the previous one. > > -int check_compaction(unsigned long mem_free, unsigned int hugepage_size) > +int check_compaction(unsigned long mem_free, unsigned int hugepage_size, > + int initial_nr_hugepages) > { > int fd, ret = -1; > int compaction_index = 0; > - char initial_nr_hugepages[10] = {0}; > char nr_hugepages[10] = {0}; > + char init_nr_hugepages[10] = {0}; > + > + sprintf(init_nr_hugepages, "%d", initial_nr_hugepages); Well, [10] isn't really large enough. "-1111111111" requires 12 chars, with the trailing \0. And I'd suggest an unsigned type and a %u - negative initial_nr_hugepages doesn't make a lot of sense. > > +int set_zero_hugepages(int *initial_nr_hugepages) > +{ > + int fd, ret = -1; > + char nr_hugepages[10] = {0}; Ditto? > + fd = open("/proc/sys/vm/nr_hugepages", O_RDWR | O_NONBLOCK); > + if (fd < 0) { > + ksft_print_msg("Failed to open /proc/sys/vm/nr_hugepages: %s\n", > + strerror(errno)); > + goto out; > + } > + > + if (read(fd, nr_hugepages, sizeof(nr_hugepages)) <= 0) { > + ksft_print_msg("Failed to read from /proc/sys/vm/nr_hugepages: %s\n", > + strerror(errno)); > + goto close_fd; > + } > + > + lseek(fd, 0, SEEK_SET); > + > + /* Start with the initial condition of 0 huge pages */ > + if (write(fd, "0", sizeof(char)) != sizeof(char)) { > + ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n", > + strerror(errno)); > + goto close_fd; > + } > + > + *initial_nr_hugepages = atoi(nr_hugepages); > + ret = 0; > + > + close_fd: > + close(fd); > + > + out: > + return ret; > +}