The patch titled
Subject: selftests/mm: compaction_test: fix bogus test success and reduce probability of OOM-killer invocation
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
selftests-mm-compaction_test-fix-bogus-test-success-and-reduce-probability-of-oom-killer-invocation.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-compaction_test-fix-bogus-test-success-and-reduce-probability-of-oom-killer-invocation.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Dev Jain <dev.jain@xxxxxxx>
Subject: selftests/mm: compaction_test: fix bogus test success and reduce probability of OOM-killer invocation
Date: Tue, 21 May 2024 13:13:58 +0530
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
Link: https://lkml.kernel.org/r/20240521074358.675031-4-dev.jain@xxxxxxx
Fixes: bd67d5c15cc1 ("Test compaction of mlocked memory")
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Sri Jayaramappa <sjayaram@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
tools/testing/selftests/mm/compaction_test.c | 71 +++++++++++------
1 file changed, 49 insertions(+), 22 deletions(-)
--- a/tools/testing/selftests/mm/compaction_test.c~selftests-mm-compaction_test-fix-bogus-test-success-and-reduce-probability-of-oom-killer-invocation
+++ a/tools/testing/selftests/mm/compaction_test.c
@@ -82,13 +82,16 @@ int prereq(void)
return -1;
}
-int check_compaction(unsigned long mem_free, unsigned long hugepage_size)
+int check_compaction(unsigned long mem_free, unsigned long hugepage_size,
+ unsigned long initial_nr_hugepages)
{
unsigned long nr_hugepages_ul;
int fd, ret = -1;
int compaction_index = 0;
- char initial_nr_hugepages[20] = {0};
char nr_hugepages[20] = {0};
+ char init_nr_hugepages[20] = {0};
+
+ sprintf(init_nr_hugepages, "%lu", initial_nr_hugepages);
/* We want to test with 80% of available memory. Else, OOM killer comes
in to play */
@@ -102,23 +105,6 @@ int check_compaction(unsigned long mem_f
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))) {
@@ -146,8 +132,8 @@ int check_compaction(unsigned long mem_f
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;
@@ -171,6 +157,41 @@ int check_compaction(unsigned long mem_f
return ret;
}
+int set_zero_hugepages(unsigned long *initial_nr_hugepages)
+{
+ int fd, ret = -1;
+ char nr_hugepages[20] = {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 = strtoul(nr_hugepages, NULL, 10);
+ ret = 0;
+
+ close_fd:
+ close(fd);
+
+ out:
+ return ret;
+}
int main(int argc, char **argv)
{
@@ -181,6 +202,7 @@ int main(int argc, char **argv)
unsigned long mem_free = 0;
unsigned long hugepage_size = 0;
long mem_fragmentable_MB = 0;
+ unsigned long initial_nr_hugepages;
ksft_print_header();
@@ -189,6 +211,10 @@ 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))
+ ksft_exit_fail();
+
lim.rlim_cur = RLIM_INFINITY;
lim.rlim_max = RLIM_INFINITY;
if (setrlimit(RLIMIT_MEMLOCK, &lim))
@@ -232,7 +258,8 @@ 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)
ksft_exit_pass();
ksft_exit_fail();
_
Patches currently in -mm which might be from dev.jain@xxxxxxx are
selftests-mm-compaction_test-fix-bogus-test-success-on-aarch64.patch
selftests-mm-compaction_test-fix-incorrect-write-of-zero-to-nr_hugepages.patch
selftests-mm-compaction_test-fix-bogus-test-success-and-reduce-probability-of-oom-killer-invocation.patch
