We recently encountered a bug that makes all zswap store attempt fail. Specifically, after:

"141fdeececb3 mm/zswap: delay the initialization of zswap"

if we build a kernel with zswap disabled by default, then enabled after the swapfile is set up, the zswap tree will not be initialized. As a result, all zswap store calls will be short-circuited. We have to perform another swapon to get zswap working properly again.

Fortunately, this issue has since been fixed by the patch that kills frontswap:

"42c06a0e8ebe mm: kill frontswap"

which performs zswap_swapon() unconditionally, i.e always initializing the zswap tree.

This test add a sanity check that ensure zswap storing works as intended.

Signed-off-by: Nhat Pham nphamcs@gmail.com --- tools/testing/selftests/cgroup/test_zswap.c | 48 +++++++++++++++++++++ 1 file changed, 48 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c index 49def87a909b..c99d2adaca3f 100644 --- a/tools/testing/selftests/cgroup/test_zswap.c +++ b/tools/testing/selftests/cgroup/test_zswap.c @@ -55,6 +55,11 @@ static int get_zswap_written_back_pages(size_t *value) return read_int("/sys/kernel/debug/zswap/written_back_pages", value); }

+static long get_zswpout(const char *cgroup) +{ + return cg_read_key_long(cgroup, "memory.stat", "zswpout "); +} + static int allocate_bytes(const char *cgroup, void *arg) { size_t size = (size_t)arg; @@ -68,6 +73,48 @@ static int allocate_bytes(const char *cgroup, void *arg) return 0; }

+/* + * Sanity test to check that pages are written into zswap. + */ +static int test_zswap_usage(const char *root) +{ + long zswpout_before, zswpout_after; + int ret = KSFT_FAIL; + char *test_group; + + /* Set up */ + test_group = cg_name(root, "no_shrink_test"); + if (!test_group) + goto out; + if (cg_create(test_group)) + goto out; + if (cg_write(test_group, "memory.max", "1M")) + goto out; + + zswpout_before = get_zswpout(test_group); + if (zswpout_before < 0) { + ksft_print_msg("Failed to get zswpout\n"); + goto out; + } + + /* Allocate more than memory.max to push memory into zswap */ + if (cg_run(test_group, allocate_bytes, (void *)MB(4))) + goto out; + + /* Verify that pages come into zswap */ + zswpout_after = get_zswpout(test_group); + if (zswpout_after <= zswpout_before) { + ksft_print_msg("zswpout does not increase after test program\n"); + goto out; + } + ret = KSFT_PASS; + +out: + cg_destroy(test_group); + free(test_group); + return ret; +} + /* * When trying to store a memcg page in zswap, if the memcg hits its memory * limit in zswap, writeback should not be triggered. @@ -235,6 +282,7 @@ struct zswap_test { int (*fn)(const char *root); const char *name; } tests[] = { + T(test_zswap_usage), T(test_no_kmem_bypass), T(test_no_invasive_cgroup_shrink), };