workload_dir() keeps the data file off tmpfs because shmem cannot be
reclaimed without swap. But statfs() only sees the top of a stacking
filesystem: with /tmp on overlayfs over tmpfs, as in virtme-ng guests,
the check passes and all 32 MiB are still charged as shmem. Whether the
test then passes depends on unrelated page cache in the cgroup; in a
full test_progs run it fails with:

  test_memcg_proactive_reclaim:FAIL:reclaimed unexpected reclaimed: actual 0 <= expected 0

Check memory.stat after writing the file and skip when most of the
data was charged as shmem.

An AI coding assistant (Claude) was used to find and analyze the
failure and to draft the fix and this changelog.

Fixes: 0f9b1e9ef584 ("selftests/bpf: Add bpf_proactive_reclaim test")
Signed-off-by: Kurt Lin <lylin1023@gmail.com>
---

Notes:
    Tested in a virtme-ng guest without swap: with /tmp on overlayfs over
    tmpfs the test now skips; on a disk-backed directory it passes.
    
    After writing 32 MiB, memory.stat shows shmem 32.0 MiB on overlayfs
    /tmp and on tmpfs, and 0.0 MiB on a disk-backed filesystem.

 .../bpf/prog_tests/memcg_proactive_reclaim.c  | 28 +++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/memcg_proactive_reclaim.c b/tools/testing/selftests/bpf/prog_tests/memcg_proactive_reclaim.c
index c22dcfabe856..c03550676041 100644
--- a/tools/testing/selftests/bpf/prog_tests/memcg_proactive_reclaim.c
+++ b/tools/testing/selftests/bpf/prog_tests/memcg_proactive_reclaim.c
@@ -46,6 +46,28 @@ static const char *workload_dir(void)
 	return NULL;
 }
 
+/*
+ * statfs() only sees the top of a stacking filesystem. When /tmp is an
+ * overlayfs whose upper layer lives on tmpfs, as in virtme-ng guests, the
+ * check above passes but the written data still ends up as shmem. Ask the
+ * cgroup how the pages were actually charged.
+ */
+static bool charged_as_shmem(void)
+{
+	static char buf[8192];
+	unsigned long long shmem;
+	char *p;
+
+	if (read_cgroup_file(CG_PATH, "memory.stat", buf, sizeof(buf)))
+		return false;
+
+	p = strstr(buf, "\nshmem ");
+	if (!p || sscanf(p, "\nshmem %llu", &shmem) != 1)
+		return false;
+
+	return shmem >= FILE_SIZE / 2;
+}
+
 void test_memcg_proactive_reclaim(void)
 {
 	struct memcg_proactive_reclaim *skel = NULL;
@@ -99,6 +121,12 @@ void test_memcg_proactive_reclaim(void)
 	if (!ASSERT_OK(fsync(data_fd), "fsync"))
 		goto out;
 
+	if (charged_as_shmem()) {
+		printf("%s:SKIP: %s is backed by shmem\n", __func__, dir);
+		test__skip();
+		goto out;
+	}
+
 	args.cgroup_id = cgroup_id;
 	args.size = FILE_SIZE;
 	skel->bss->reclaimed = 0;

base-commit: 0021f17128de5dc0b19f1a523f448df2440d8088
-- 
2.56.0