[LTP] [PATCH v2 3/3] controllers/memcg: accept non-zero max_usage_in_bytes after reset

[Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>]

Several Linux kernel versions report a non-zero max_usage_in_bytes after
resetting the counter.  For example v5.4, v5.8, v5.10, v5.11, v5.12 and
5.13.0-rc5:

    memcg_max_usage_in_bytes_test 4 TINFO: Test reset memory.memsw.max_usage_in_bytes
    memcg_max_usage_in_bytes_test 4 TINFO: Running memcg_process --mmap-anon -s 4194304
    memcg_max_usage_in_bytes_test 4 TINFO: Warming up pid: 1416
    memcg_max_usage_in_bytes_test 4 TINFO: Process is still here after warm up: 1416
    memcg_max_usage_in_bytes_test 4 TFAIL: memory.memsw.max_usage_in_bytes is 4325376, 4194304 expected
    memcg_max_usage_in_bytes_test 4 TFAIL: memory.memsw.max_usage_in_bytes is 122880, 0 expected

It seems that recent Linux kernel still notices some memory allocation
by the memcg tool.  Accept therefore a range from 0 to 32 pages.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
---
 .../memcg/functional/memcg_max_usage_in_bytes_test.sh       | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/testcases/kernel/controllers/memcg/functional/memcg_max_usage_in_bytes_test.sh b/testcases/kernel/controllers/memcg/functional/memcg_max_usage_in_bytes_test.sh
index a940606cbd34..8f0fc33996f3 100755
--- a/testcases/kernel/controllers/memcg/functional/memcg_max_usage_in_bytes_test.sh
+++ b/testcases/kernel/controllers/memcg/functional/memcg_max_usage_in_bytes_test.sh
@@ -40,7 +40,11 @@ test_max_usage_in_bytes()
 
 	if [ $check_after_reset -eq 1 ]; then
 		echo 0 > $item
-		check_mem_stat $item 0
+		# Recent Linux kernels (at least v5.4) started reporting
+		# a non-zero max_usage_in_bytes after resetting the counter.
+		# The typical values are 0, 4096, 8096 and up to 122880.
+		# Cause is not known, so let's just be flexible.
+		check_mem_stat $item 0 $((PAGESIZE * 32))
 	fi
 
 	stop_memcg_process
-- 
2.27.0