MongoDB checks whether THP and THP Defrag are enabled by checking the contents of files present under the '/sys/kernel/mm/transparent_hugepage' directory.
There have been reports that these files can still exist and have the relevant configuration set to 'always' even if THP has been disabled via the boot-time parameter, as described in the preferred method in the online documentation.
Here are some relevant links to RHEL support cases:
- How to use, monitor, and disable transparent hugepages in Red Hat Enterprise Linux 6? (Note though the test at the end checks for Hugepage support, not Transparent Hugepage support)
- Disabling transparent hugepages (THP) on Red Hat Enterprise Linux 6 is not taking effect.
Finally, here is what I believe is the reference article for THP support in Linux: Documentation/vm/transhuge.txt