[PATCH] selftests/cgroup: test_zswap: skip test_no_kmem_bypass if debugfs is unavailable

[PATCH] selftests/cgroup: test_zswap: skip test_no_kmem_bypass if debugfs is unavailable

[Wilson Felipe Pereira]

test_no_kmem_bypass() needs to read
/sys/kernel/debug/zswap/stored_pages via get_zswap_stored_pages() to
verify that compressed pages are charged to the memcg.

When running in an environment where debugfs is not mounted or
CONFIG_DEBUG_FS is disabled, get_zswap_stored_pages() fails, causing the
loop to terminate early and report a false negative (KSFT_FAIL).

Selftests should not fail when if debugfs is unavailable, and it
should print a message when it is skipped.

Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
---
 tools/testing/selftests/cgroup/test_zswap.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 49b36ee791606..c4f7b66f1b206 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -566,12 +566,19 @@ static int test_no_kmem_bypass(const char *root)
 	int child_status;
 	char *test_group = NULL;
 	pid_t child_pid;
+	size_t stored_pages;
 
 	/* Read sys info and compute test values accordingly */
 	if (sysinfo(&sys_info) != 0)
 		return KSFT_FAIL;
-	if (sys_info.totalram > 5000000000)
+	if (sys_info.totalram > 5000000000) {
+		ksft_print_msg("requires less than 5000000000 total ram\n");
 		return KSFT_SKIP;
+	}
+	if (get_zswap_stored_pages(&stored_pages)) {
+		ksft_print_msg("debugfs at /sys/kernel/debug is required\n");
+		return KSFT_SKIP;
+	}
 	values = mmap(0, sizeof(struct no_kmem_bypass_child_args), PROT_READ |
 			PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
 	if (values == MAP_FAILED)
@@ -604,7 +611,6 @@ static int test_no_kmem_bypass(const char *root)
 	/* Try to wakeup kswapd and let it push child memory to zswap */
 	set_min_free_kb(min_free_kb_high);
 	for (int i = 0; i < 20; i++) {
-		size_t stored_pages;
 		char *trigger_allocation = malloc(trigger_allocation_size);
 
 		if (!trigger_allocation)
-- 
2.55.0.508.g3f0d502094-goog

Re: [PATCH] selftests/cgroup: test_zswap: skip test_no_kmem_bypass if debugfs is unavailable

[Anshuman Khandual]

On Sat, Aug 01, 2026 at 04:12:46AM +0000, Wilson Felipe Pereira wrote:
> test_no_kmem_bypass() needs to read
> /sys/kernel/debug/zswap/stored_pages via get_zswap_stored_pages() to
> verify that compressed pages are charged to the memcg.
> 
> When running in an environment where debugfs is not mounted or
> CONFIG_DEBUG_FS is disabled, get_zswap_stored_pages() fails, causing the
> loop to terminate early and report a false negative (KSFT_FAIL).
> 
> Selftests should not fail when if debugfs is unavailable, and it
> should print a message when it is skipped.

Agreed.

> 
> Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index 49b36ee791606..c4f7b66f1b206 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -566,12 +566,19 @@ static int test_no_kmem_bypass(const char *root)
>  	int child_status;
>  	char *test_group = NULL;
>  	pid_t child_pid;
> +	size_t stored_pages;
>  
>  	/* Read sys info and compute test values accordingly */
>  	if (sysinfo(&sys_info) != 0)
>  		return KSFT_FAIL;
> -	if (sys_info.totalram > 5000000000)
> +	if (sys_info.totalram > 5000000000) {
> +		ksft_print_msg("requires less than 5000000000 total ram\n");

Agreed - should not silently just skip the test.

>  		return KSFT_SKIP;
> +	}
> +	if (get_zswap_stored_pages(&stored_pages)) {
> +		ksft_print_msg("debugfs at /sys/kernel/debug is required\n");

Could the error message here match that in gup_test.c for consistency
and probably later on these could be factored out in a macro constant

ksft_print_msg("mount debugfs at /sys/kernel/debug\n");

> +		return KSFT_SKIP;
> +	}
>  	values = mmap(0, sizeof(struct no_kmem_bypass_child_args), PROT_READ |
>  			PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
>  	if (values == MAP_FAILED)
> @@ -604,7 +611,6 @@ static int test_no_kmem_bypass(const char *root)
>  	/* Try to wakeup kswapd and let it push child memory to zswap */
>  	set_min_free_kb(min_free_kb_high);
>  	for (int i = 0; i < 20; i++) {
> -		size_t stored_pages;
>  		char *trigger_allocation = malloc(trigger_allocation_size);
>  
>  		if (!trigger_allocation)
> -- 
> 2.55.0.508.g3f0d502094-goog
> 

Otherwise LGTM.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>