[PATCH v2] selftests/cgroup: test_zswap: skip test_no_kmem_bypass if debugfs is unavailable

[Wilson Felipe Pereira <wfelipe@google.com>]

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 if debugfs is unavailable, and it should print
a message when it is skipped.

While I'm here, also add a warning message if the test is being skipped
due to totalram size and make the check for totalram more readable.

Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
---
v1 -> v2:
  - Check for debugfs availability using access("/sys/kernel/debug/zswap/stored_pages", R_OK)
    instead of calling get_zswap_stored_pages() (Yosry Ahmed).
  - Update debugfs print message to exactly match gup_test.c (Anshuman Khandual).
  - Add GB(x) macro to cgroup_util.h (Yosry Ahmed). Cast to unsigned long long
    to prevent 32-bit truncation without altering existing MB(x) signedness.
  - Update RAM check to GB(4) to match comments and format totalram output
    with 1 decimal place (%.1fGB) (Yosry Ahmed).
  - Fix commit message typo (Yosry Ahmed).
  - Moved stored_pages back inside the for loop.

v1: https://lore.kernel.org/all/20260801041255.752039-1-wfelipe@google.com/

 .../selftests/cgroup/lib/include/cgroup_util.h      |  1 +
 tools/testing/selftests/cgroup/test_zswap.c         | 13 +++++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
index febc1723d0903..c0f07226b222f 100644
--- a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
+++ b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
@@ -7,6 +7,7 @@
 #endif

 #define MB(x) (x << 20)
+#define GB(x) ((unsigned long long)(x) << 30)

 #define USEC_PER_SEC	1000000L
 #define NSEC_PER_SEC	1000000000L
diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 49b36ee791606..a836337440040 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -20,6 +20,7 @@ static int page_size;

 #define PATH_ZSWAP "/sys/module/zswap"
 #define PATH_ZSWAP_ENABLED "/sys/module/zswap/parameters/enabled"
+#define PATH_ZSWAP_STORED_PAGES "/sys/kernel/debug/zswap/stored_pages"

 static int read_int(const char *path, size_t *value)
 {
@@ -55,7 +56,7 @@ static int read_min_free_kb(size_t *value)

 static int get_zswap_stored_pages(size_t *value)
 {
-	return read_int("/sys/kernel/debug/zswap/stored_pages", value);
+	return read_int(PATH_ZSWAP_STORED_PAGES, value);
 }

 static long get_cg_wb_count(const char *cg)
@@ -570,8 +571,16 @@ static int test_no_kmem_bypass(const char *root)
 	/* 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 > GB(4)) {
+		ksft_print_msg(
+			"requires less than 4GB total ram, sys_info.totalram: %.1fGB\n",
+			(double)sys_info.totalram / GB(1));
 		return KSFT_SKIP;
+	}
+	if (access(PATH_ZSWAP_STORED_PAGES, R_OK)) {
+		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)
--
2.55.0.679.g6767b8d81c-goog