selftests: cgroup: fix alloc_anon_noexit() instantly freeing memory

Currently, alloc_anon_noexit() calls alloc_anon() which instantly frees
the allocated memory. alloc_anon_noexit() is usually used with
cg_run_nowait() to run a process in the background that allocates
memory. It makes sense for the background process to keep the memory
allocated and not instantly free it (otherwise there is no point of
running it in the background).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yosry Ahmed <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Chen Wandun <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: "Michal Koutn" <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Vaibhav Jain <[email protected]>
Cc: Wei Xu <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: Zefan Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
[kms: backport to 5.15]
Signed-off-by: Kelsey Steele <[email protected]>

Author: yosrym93

tools/testing/selftests/cgroup/test_memcontrol.c

@@ -210,13 +210,17 @@ static int alloc_pagecache_50M_noexit(const char *cgroup, void *arg)
 static int alloc_anon_noexit(const char *cgroup, void *arg)
 {
 	int ppid = getppid();
+	size_t size = (unsigned long)arg;
+	char *buf, *ptr;
 
-	if (alloc_anon(cgroup, arg))
-		return -1;
+	buf = malloc(size);
+	for (ptr = buf; ptr < buf + size; ptr += PAGE_SIZE)
+		*ptr = 0;
 
 	while (getppid() == ppid)
 		sleep(1);
 
+	free(buf);
 	return 0;
 }