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.

Fixes: bd67d5c15cc1 ("Test compaction of mlocked memory") Cc: stable@vger.kernel.org Signed-off-by: Dev Jain dev.jain@arm.com --- tools/testing/selftests/mm/compaction_test.c | 72 ++++++++++++++------ 1 file changed, 50 insertions(+), 22 deletions(-)

diff --git a/tools/testing/selftests/mm/compaction_test.c b/tools/testing/selftests/mm/compaction_test.c index c5be395f8363..2ae059989771 100644 --- a/tools/testing/selftests/mm/compaction_test.c +++ b/tools/testing/selftests/mm/compaction_test.c @@ -82,12 +82,15 @@ int prereq(void) return -1; }

-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);

/* We want to test with 80% of available memory. Else, OOM killer comes in to play */ @@ -101,23 +104,6 @@ int check_compaction(unsigned long mem_free, unsigned int hugepage_size) goto out; }

- if (read(fd, initial_nr_hugepages, sizeof(initial_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; - } - - lseek(fd, 0, SEEK_SET); - /* Request a large number of huge pages. The Kernel will allocate as much as it can */ if (write(fd, "100000", (6*sizeof(char))) != (6*sizeof(char))) { @@ -140,8 +126,8 @@ int check_compaction(unsigned long mem_free, unsigned int hugepage_size)

lseek(fd, 0, SEEK_SET);

- if (write(fd, initial_nr_hugepages, strlen(initial_nr_hugepages)) - != strlen(initial_nr_hugepages)) { + if (write(fd, init_nr_hugepages, strlen(init_nr_hugepages)) + != strlen(init_nr_hugepages)) { ksft_print_msg("Failed to write value to /proc/sys/vm/nr_hugepages: %s\n", strerror(errno)); goto close_fd; @@ -165,6 +151,42 @@ int check_compaction(unsigned long mem_free, unsigned int hugepage_size) return ret; }

+int set_zero_hugepages(int *initial_nr_hugepages) +{ + int fd, ret = -1; + char nr_hugepages[10] = {0}; + + 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; +}

int main(int argc, char **argv) { @@ -175,6 +197,7 @@ int main(int argc, char **argv) unsigned long mem_free = 0; unsigned long hugepage_size = 0; long mem_fragmentable_MB = 0; + int initial_nr_hugepages;

ksft_print_header();

@@ -183,6 +206,10 @@ int main(int argc, char **argv)

ksft_set_plan(1);

+ /* start the test without hugepages reducing mem_free */ + if (set_zero_hugepages(&initial_nr_hugepages)) + return ksft_exit_fail(); + lim.rlim_cur = RLIM_INFINITY; lim.rlim_max = RLIM_INFINITY; if (setrlimit(RLIMIT_MEMLOCK, &lim)) @@ -226,7 +253,8 @@ int main(int argc, char **argv) entry = entry->next; }

- if (check_compaction(mem_free, hugepage_size) == 0) + if (check_compaction(mem_free, hugepage_size, + initial_nr_hugepages) == 0) return ksft_exit_pass();

return ksft_exit_fail();