[LTP] [PATCH] memcontrol04: Fix test failure by keeping pagecache children alive

[Pavithra]

The test was failing with memory.current values much lower than expected:
  TFAIL: (A/B/C memory.current=6684672) ~= 34603008
  TFAIL: (A/B/D memory.current=5373952) ~= 17825792

Root cause:
Child processes allocating pagecache were exiting immediately after
allocation (via tst_reap_children()), causing the pagecache to be
freed before the test could measure memory.current values.

Fix:
Modified alloc_pagecache_in_child() to keep children alive during test:
- Added TEST_DONE checkpoint for child lifecycle coordination
- Parent waits for CHILD_IDLE checkpoint before proceeding
- Child signals CHILD_IDLE after allocation and fsync
- Child waits for TEST_DONE to keep memory allocated during test
- Modified cleanup_sub_groups() to wake waiting children before cleanup
- Changed alloc_anon_in_child() to use SAFE_WAITPID() for specific child

This matches the pattern used in memcontrol03.c and ensures pagecache
remains allocated during memory pressure testing, allowing correct
memory.current measurements.

Signed-off-by: Sachin Sant <sachinp@linux.ibm.com>, Pavithra <pavrampu@linux.ibm.com>
---
 .../kernel/controllers/memcg/memcontrol04.c   | 22 ++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/testcases/kernel/controllers/memcg/memcontrol04.c b/testcases/kernel/controllers/memcg/memcontrol04.c
index 715cc5bcd..d0188a1da 100644
--- a/testcases/kernel/controllers/memcg/memcontrol04.c
+++ b/testcases/kernel/controllers/memcg/memcontrol04.c
@@ -47,7 +47,8 @@ static struct tst_cg_group *leaf_cg[4];
 static int fd = -1;
 
 enum checkpoints {
-	CHILD_IDLE
+	CHILD_IDLE,
+	TEST_DONE,
 };
 
 enum trunk_cg {
@@ -67,6 +68,16 @@ static void cleanup_sub_groups(void)
 {
 	size_t i;
 
+	for (i = ARRAY_SIZE(leaf_cg); i > 0; i--) {
+		if (!leaf_cg[i - 1])
+			continue;
+
+		TST_CHECKPOINT_WAKE2(TEST_DONE,
+				     ARRAY_SIZE(leaf_cg) - 1);
+		tst_reap_children();
+		break;
+	}
+
 	for (i = ARRAY_SIZE(leaf_cg); i > 0; i--) {
 		if (!leaf_cg[i - 1])
 			continue;
@@ -88,7 +99,7 @@ static void alloc_anon_in_child(const struct tst_cg_group *const cg,
 	const pid_t pid = SAFE_FORK();
 
 	if (pid) {
-		tst_reap_children();
+		SAFE_WAITPID(pid, NULL, 0);
 		return;
 	}
 
@@ -107,7 +118,7 @@ static void alloc_pagecache_in_child(const struct tst_cg_group *const cg,
 	const pid_t pid = SAFE_FORK();
 
 	if (pid) {
-		tst_reap_children();
+		TST_CHECKPOINT_WAIT(CHILD_IDLE);
 		return;
 	}
 
@@ -117,6 +128,11 @@ static void alloc_pagecache_in_child(const struct tst_cg_group *const cg,
 		getpid(), tst_cg_group_name(cg), size);
 	alloc_pagecache(fd, size);
 
+	SAFE_FSYNC(fd);
+
+	TST_CHECKPOINT_WAKE(CHILD_IDLE);
+	TST_CHECKPOINT_WAIT(TEST_DONE);
+
 	exit(0);
 }
 
-- 
2.53.0