cgroup: fix racy check in alloc_pagecache_max_30M() helper function

alloc_pagecache_max_30M() in the cgroup memcg tests performs a 50MB
pagecache allocation, which it expects to be capped at 30MB due to the
calling process having a memory.high setting of 30MB.  After the
allocation, the function contains a check that verifies that MB(29) <
memory.current <= MB(30).  This check can actually fail
non-deterministically.

The testcases that use this function are test_memcg_high() and
test_memcg_max(), which set memory.min and memory.max to 30MB respectively
for the cgroup under test.  The allocation can slightly exceed this number
in both cases, and for memory.max, the process performing the allocation
will not have the OOM killer invoked as it's performing a pagecache
allocation.  This patchset therefore updates the above check to instead
use the verify_close() helper function.

Link: https://lkml.kernel.org/r/20220423155619.3669555-6-void@manifault.comSigned-off-by: David Vernet <void@manifault.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/cgroup/test_memcontrol.c

@@ -568,9 +568,14 @@ static int alloc_pagecache_max_30M(const char *cgroup, void *arg)
 {
 	size_t size = MB(50);
 	int ret = -1;
-	long current;
+	long current, high, max;
 	int fd;
 
+	high = cg_read_long(cgroup, "memory.high");
+	max = cg_read_long(cgroup, "memory.max");
+	if (high != MB(30) && max != MB(30))
+		goto cleanup;
+
 	fd = get_temp_fd();
 	if (fd < 0)
 		return -1;
@@ -579,7 +584,7 @@ static int alloc_pagecache_max_30M(const char *cgroup, void *arg)
 		goto cleanup;
 
 	current = cg_read_long(cgroup, "memory.current");
-	if (current <= MB(29) || current > MB(30))
+	if (!values_close(current, MB(30), 5))
 		goto cleanup;
 
 	ret = 0;