Re: [PATCH 3/3] selftests: add test for zswapin

Yosry Ahmed
2024-01-30T01:24:35Z

On Mon, Jan 29, 2024 at 02:45:42PM -0800, Nhat Pham wrote:
> We recently encountered a kernel crash on the zswapin path in our
> internal kernel, which went undetected because of a lack of test
> coverage for this path. Add a selftest to cover this code path,
> allocating more memories than the cgroup limit to trigger

s/memories/memory

> swapout/zswapout, then reading the pages back in memories several times.
>
> Also add a variant of this test that runs with zswap disabled, to verify
> swapin correctness as well.
>
> Suggested-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
> tools/testing/selftests/cgroup/test_zswap.c | 67 ++++++++++++++++++++-
> 1 file changed, 65 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index 32ce975b21d1..86231c86dc89 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -60,17 +60,39 @@ 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)
> +static int allocate_bytes_and_read(const char *cgroup, void *arg, bool read)
> {
> size_t size = (size_t)arg;
> char *mem = (char *)malloc(size);
> + int ret = 0;
>
> if (!mem)
> return -1;
> for (int i = 0; i < size; i += 4095)
> mem[i] = 'a';
> +
> + if (read) {
> + /* cycle through the allocated memory to (z)swap in and out pages */
> + for (int t = 0; t < 5; t++) {

What benefit does the iteration serve here? I would guess one iteration
is enough to swap everything in at least once, no?

> + for (int i = 0; i < size; i += 4095) {
> + if (mem[i] != 'a')
> + ret = -1;
> + }
> + }
> + }
> +
> free(mem);
> - return 0;
> + return ret;
> +}
> +
> +static int allocate_bytes(const char *cgroup, void *arg)
> +{
> + return allocate_bytes_and_read(cgroup, arg, false);
> +}
> +
> +static int read_bytes(const char *cgroup, void *arg)
> +{
> + return allocate_bytes_and_read(cgroup, arg, true);
> }

I don't like how we reuse allocate_bytes_and_read(), we are not saving
much. Let's keep allocate_bytes() as-is and add a separate helper. Also,
I think allocate_and_read_bytes() is easier to read.

>
> static char *setup_test_group_1M(const char *root, const char *name)
> @@ -133,6 +155,45 @@ static int test_zswap_usage(const char *root)
> return ret;
> }
>
> +/* Simple test to verify the (z)swapin code paths */
> +static int test_zswapin_size(const char *root, char *zswap_size)
> +{
> + int ret = KSFT_FAIL;
> + char *test_group;
> +
> + /* Set up */
> + test_group = cg_name(root, "zswapin_test");
> + if (!test_group)
> + goto out;
> + if (cg_create(test_group))
> + goto out;
> + if (cg_write(test_group, "memory.max", "8M"))
> + goto out;
> + if (cg_write(test_group, "memory.zswap.max", zswap_size))
> + goto out;
> +
> + /* Allocate and read more than memory.max to trigger (z)swap in */
> + if (cg_run(test_group, read_bytes, (void *)MB(32)))
> + goto out;
> +
> + ret = KSFT_PASS;
> +
> +out:
> + cg_destroy(test_group);
> + free(test_group);
> + return ret;
> +}
> +
> +static int test_swapin(const char *root)
> +{
> + return test_zswapin_size(root, "0");
> +}

Why are we testing the no zswap case? I am all for testing but it seems
out of scope here. It would have been understandable if we are testing
memory.zswap.max itself, but we are not doing that.

FWIW, I think the tests here should really be separated from cgroup
tests, but I understand why they were added here. There is a lot of
testing for memcg interface and control for zswap, and a lot of nice
helpers present.