Comment # 1 on bug 911585 from Michal Hocko

I am not a filesystem expert nor familiar with perl but it might be possible
that the mtime was not synced yet because perl didn't flush (msync) the file
yet and system does that automatically only after certain (configurable) time.

>From the memory management POV mtime is updated from page_mkwrite path. I can
see xfs calling block_page_mkwrite from its callback (xfs_vm_page_mkwrite) and
that one calls file_update_time which updates the timestamps for the inode
(unless it is marked as S_NOCMTIME which doesn't seem to be the case for xfs).
So the mtime is definitely updated.

Could you be more specific about what exactly you are doing with the timing,
please?