[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.