Re: [PATCH v3 3/3] selftests: add zswapin and no zswap tests

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Feb 05, 2024 at 02:56:08PM -0800, Nhat Pham wrote:
> Add a selftest to cover the zswapin code path, allocating more memory
> than the cgroup limit to trigger swapout/zswapout, then reading the
> pages back in memory several times. This is inspired by a recently
> encountered 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 verify that when memory.zswap.max = 0, no pages can go
> to the zswap pool for the cgroup.
> 
> Suggested-by: Rik van Riel <riel@surriel.com>
> Suggested-by: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>

LGTM with a few nits below:
Acked-by: Yosry Ahmed <yosryahmed@google.com>

Thanks!

> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 120 +++++++++++++++++++-
>  1 file changed, 119 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index 32ce975b21d1..c263610a4a60 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -60,6 +60,27 @@ static long get_zswpout(const char *cgroup)
>  	return cg_read_key_long(cgroup, "memory.stat", "zswpout ");
>  }
>  
> +static int allocate_and_read_bytes(const char *cgroup, void *arg)
> +{
> +	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';
> +
> +	/* go through the allocated memory to (z)swap in and out pages */

nit: s/go/Go

> +	for (int i = 0; i < size; i += 4095) {
> +		if (mem[i] != 'a')
> +			ret = -1;
> +	}
> +
> +	free(mem);
> +	return ret;
> +}
> +
>  static int allocate_bytes(const char *cgroup, void *arg)
>  {
>  	size_t size = (size_t)arg;
> @@ -100,7 +121,6 @@ static int test_zswap_usage(const char *root)
>  	int ret = KSFT_FAIL;
>  	char *test_group;
>  
> -	/* Set up */

We removed this comment here.

>  	test_group = cg_name(root, "no_shrink_test");
>  	if (!test_group)
>  		goto out;
> @@ -133,6 +153,102 @@ static int test_zswap_usage(const char *root)
>  	return ret;
>  }
>  
> +/*
> + * Check that when memory.zswap.max = 0, no pages can go to the zswap pool for
> + * the cgroup.
> + */
> +static int test_swapin_nozswap(const char *root)
> +{
> +	int ret = KSFT_FAIL;
> +	char *test_group;
> +	long swap_peak, zswpout;
> +
> +	test_group = cg_name(root, "no_zswap_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", "0"))
> +		goto out;
> +
> +	/* Allocate and read more than memory.max to trigger swapin */
> +	if (cg_run(test_group, allocate_and_read_bytes, (void *)MB(32)))
> +		goto out;
> +
> +	/* Verify that pages are swapped out, but no zswap happened */
> +	swap_peak = cg_read_long(test_group, "memory.swap.peak");
> +	if (swap_peak < 0) {
> +		ksft_print_msg("failed to get cgroup's swap_peak\n");
> +		goto out;
> +	}
> +
> +	if (swap_peak == 0) {
> +		ksft_print_msg("pages should be swapped out\n");
> +		goto out;
> +	}

We can actually check that this number is >= 24M instead. Not a big
deal, but might as well.

> +
> +	zswpout = get_zswpout(test_group);
> +	if (zswpout < 0) {
> +		ksft_print_msg("failed to get zswpout\n");
> +		goto out;
> +	}
> +
> +	if (zswpout > 0) {
> +		ksft_print_msg("zswapout > 0 when memory.zswap.max = 0\n");
> +		goto out;
> +	}
> +
> +	ret = KSFT_PASS;
> +
> +out:
> +	cg_destroy(test_group);
> +	free(test_group);
> +	return ret;
> +}
> +
> +/* Simple test to verify the (z)swapin code paths */
> +static int test_zswapin(const char *root)
> +{
> +	int ret = KSFT_FAIL;
> +	char *test_group;
> +	long zswpin;
> +
> +	/* Set up */

Yet we added a similar one here :)

> +	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", "max"))
> +		goto out;
> +
> +	/* Allocate and read more than memory.max to trigger (z)swap in */
> +	if (cg_run(test_group, allocate_and_read_bytes, (void *)MB(32)))
> +		goto out;
> +
> +	zswpin = cg_read_key_long(test_group, "memory.stat", "zswpin ");
> +	if (zswpin < 0) {
> +		ksft_print_msg("failed to get zswpin\n");
> +		goto out;
> +	}
> +
> +	if (zswpin == 0) {
> +		ksft_print_msg("zswpin should not be 0\n");
> +		goto out;
> +	}

Same here, we can check that zswpin is at least 24M worth of events.
Again, not a big deal, but might as well.