[PATCH v2 2/2] selftests: cgroup: Report memory.current shortfall in alloc_pagecache_50M_check

[Sebastian Chlad <sebastianchlad@gmail.com>]

alloc_pagecache_50M_check() fails silently when memory.current is below
the 50M that was just written to the page cache. This is the check that
can trip when the page cache of the test cgroup is reclaimed under
global memory pressure, so print the observed value.

test_memcg_high() runs the same helper with memory.high set to 30M and
expects it to fail, so reporting there would print a misleading failure
message on every passing run. Split the body into
__alloc_pagecache_50M_check() with a report flag, and give
test_memcg_high() a quiet variant.

Assisted-by: claude-opus-5-5
Signed-off-by: Sebastian Chlad <sebastian.chlad@suse.com>
---
 .../selftests/cgroup/test_memcontrol.c        | 27 ++++++++++++++++---
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 8882c7faeb32..759e1640746d 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -222,7 +222,7 @@ static int alloc_anon_50M_check(const char *cgroup, void *arg)
 	return ret;
 }
 
-static int alloc_pagecache_50M_check(const char *cgroup, void *arg)
+static int __alloc_pagecache_50M_check(const char *cgroup, bool report)
 {
 	size_t size = MB(50);
 	int ret = -1;
@@ -237,14 +237,19 @@ static int alloc_pagecache_50M_check(const char *cgroup, void *arg)
 		goto cleanup;
 
 	current = cg_read_long(cgroup, "memory.current");
-	if (current < size)
+	if (current < size) {
+		if (report)
+			fprintf(stderr, "memory.current %ld < expected %zu\n",
+				current, size);
 		goto cleanup;
+	}
 
 	file = cg_read_key_long(cgroup, "memory.stat", "file ");
 	if (file < 0)
 		goto cleanup;
 
-	if (!values_close_report(file, current, 10))
+	if (report ? !values_close_report(file, current, 10) :
+		     !values_close(file, current, 10))
 		goto cleanup;
 
 	ret = 0;
@@ -254,6 +259,20 @@ static int alloc_pagecache_50M_check(const char *cgroup, void *arg)
 	return ret;
 }
 
+static int alloc_pagecache_50M_check(const char *cgroup, void *arg)
+{
+	return __alloc_pagecache_50M_check(cgroup, true);
+}
+
+/*
+ * Same as alloc_pagecache_50M_check(), but silent on failure, for callers
+ * that expect the allocation to be held below 50M.
+ */
+static int alloc_pagecache_50M_check_quiet(const char *cgroup, void *arg)
+{
+	return __alloc_pagecache_50M_check(cgroup, false);
+}
+
 /*
  * This test create a memory cgroup, allocates
  * some anonymous memory and some pagecache
@@ -735,7 +754,7 @@ static int test_memcg_high(const char *root)
 	if (cg_run(memcg, alloc_anon, (void *)MB(31)))
 		goto cleanup;
 
-	if (!cg_run(memcg, alloc_pagecache_50M_check, NULL))
+	if (!cg_run(memcg, alloc_pagecache_50M_check_quiet, NULL))
 		goto cleanup;
 
 	if (cg_run(memcg, alloc_pagecache_max_30M, NULL))
-- 
2.51.0