Pseudo file not showing swap information

I want to know if my container is using swap memory, and how much. But the pseudo file doesn’t have this information. Here is the memory.stat file:

cache 688128
rss 520192
rss_huge 0
mapped_file 0
dirty 0
writeback 0
pgpgin 2572
pgpgout 2277
pgfault 2513
pgmajfault 0
inactive_anon 548864
active_anon 12288
inactive_file 630784
active_file 16384
unevictable 0
hierarchical_memory_limit 9223372036854771712
total_cache 688128
total_rss 520192
total_rss_huge 0
total_mapped_file 0
total_dirty 0
total_writeback 0
total_pgpgin 2572
total_pgpgout 2277
total_pgfault 2513
total_pgmajfault 0
total_inactive_anon 548864
total_active_anon 12288
total_inactive_file 630784
total_active_file 16384

Is there some configuration option I have to add?