Hi, Since some time I am trying to fix Linux Test Project tests around memory cgroups: https://lists.linux.it/pipermail/ltp/2021-June/023259.html The trouble I have, for example with memcg_max_usage_in_bytes_test.sh is that on recent kernels (v4.15+) on x86_64, the memory group reports max usage as higher than process' anonymous mapping. The test works like this: 1. Fork a process, signal it to mmap 4 MB (PROT_WRITE | PROT_READ, AP_PRIVATE | MAP_ANONYMOUS) and touch the memory. 2. Add the process to control group. 3. Signal it to munmap the region and immediately mmap again the same 4 MB (with touching the memory). 4. Check the counters and reset them. 5. munmap 6. Check the counters Mentioned memcg_max_usage_in_bytes_test.sh checks the counters of memory.memsw.max_usage_in_bytes which are: a. early kernels: 4 MB (so only the mmap) b. v4.15, v5.4 kernel: 4 MB + 32 pages c. v5.11 kernel: 4 MB + 32 pages + 2 pages I tweaked the mmap() size to smaller values and then the accounting is even different. For example mmap of 1 up to 32 pages the memory.memsw.max_usage_in_bytes is always 131072. After final munmap (point 5 above), the test expects the memcg_max_usage_in_bytes to be =0, however it is usually 8 or 132 kB. Which kind of points that process is charged for something not related to that memory map directly. The questions: How accurate are now the cgroup counters? I understood they should charge only pages allocated by the process, so why mmap(4 kB) causes max_usage_in_bytes=132 kB? Why mmap(4 MB) causes max_usage_in_bytes=4 MB + 34 pages? What is being accounted there (stack guards?)? Or maybe the entire LTP test checking so carefully memcg limits is useless? The v5.4 kernel config is here: https://kernel.ubuntu.com/~kernel-ppa/config/focal/linux-azure/5.4.0-1039.41/amd64-config.flavour.azure Best regards, Krzysztof
