[photon-torpedo]

The author mentions that they also wrote a backup program (bup), and for backup programs it would be very convenient if directory mtimes would get updated like this (recursively up to the root), as it would allow to skip scanning the entire filesystem for changed files (which in my experience is where backup programs spend most of their time).

[junon]

I don't remember if mtime is updated on each write call or just on fopen but I could see this being a huge performance overhead, namely for applications that are FS bound. I wonder if io_uring would help the situation though since it's mainly geared toward filesystem operations.

[Doxin]

No reason a hypothetical recursive mtime needs to be atomic. The kernel could just stick it in a buffer somewhere and deal with that sort of think out-of-band and in batches. You'd probably need some filesystem journaling trickery if you want to make sure the recursive mtime always updates eventually when a file is modified.