[PATCH bpf-next] selftests/bpf: Skip memcg_proactive_reclaim when data lands in shmem

[PATCH bpf-next] selftests/bpf: Skip memcg_proactive_reclaim when data lands in shmem

[Kurt Lin]

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

Re: [PATCH bpf-next] selftests/bpf: Skip memcg_proactive_reclaim when data lands in shmem

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential
issues to consider:
- [Low] selftests/bpf: workload_dir() never falls back to . when /tmp is
  overlayfs
- [Low] selftests/bpf: unflushed per-CPU memcg stats read in
  charged_as_shmem()
--

commit e2cf55e88beac1906e9ded044c8e29f6b8bf9980
Author: Kurt Lin <lylin1023@gmail.com>

selftests/bpf: Skip memcg_proactive_reclaim when data lands in shmem

This commit skips the memcg_proactive_reclaim selftest when memory.stat
shows that the test file's pages were charged as shmem, such as when /tmp
is an overlayfs backed by tmpfs.

> 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 c22dcfabe856e..c035506760416 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;
> +}

[Severity: Low]
Can charged_as_shmem() read 0 for shmem when per-CPU memcg stats have not
been flushed yet?

Reading memory.stat calls __mem_cgroup_flush_stats(memcg, false), which skips
the rstat flush unless pending updates exceed MEMCG_CHARGE_BATCH *
num_online_cpus():

mm/memcontrol.c:memcg_vmstats_needs_flush() {
	return atomic_long_read(&vmstats->stats_updates) >
		MEMCG_CHARGE_BATCH * num_online_cpus();
}

On a 64K-page kernel with 24 or more online CPUs (or a 4K-page kernel with
384 or more CPUs), writing 32 MiB of shmem does not cross that threshold, so
charged_as_shmem() sees shmem as 0 and fails to skip the test.  Retrying
until the "file " counter in memory.stat reaches at least FILE_SIZE / 2
(across the 2s periodic flusher window, as in cgroup_iter_memcg.c) would
ensure the stats are flushed before checking shmem.

>  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;
> +	}

[Severity: Low]
Does jumping straight to out here prevent falling back to "." when /tmp is
overlayfs over tmpfs?

workload_dir() checks dirs[] = { "/tmp", "." } so the test can use the
current working directory when /tmp is on tmpfs.  When /tmp is an overlayfs
over tmpfs, statfs("/tmp", &st) sees OVERLAYFS_SUPER_MAGIC and returns
"/tmp" on the first iteration, and this check then skips the whole test
without ever trying "." even if test_progs is running from a writable
disk-backed directory.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113180159.602033-1-lylin1023@gmail.com?part=1