I don't think Song Liu ever intended to make filesystem-oblivious writable THPs work. The intent (at least from my side!) was always to make filesystems support arbitrary-sized "pages" ... which turned into folios when it became apparent that we needed a way to distinguish PAGE_SIZE pages from allocated chunks of memory. So I think it's fair to see "folios now exist" as being the fulfillment of "writable huge pages".

Song's patches were an important part of the process of getting folios going. It's not the order I would have done it in, but since Song had already done it, it got some of the infrastructure in place that we needed. We cooperated on this work, after the session at the 2019 LSFMM (https://lwn.net/Articles/789159/) we started a fortnightly phone call which continues to this day (although Song no longer participates).

By the way, the page cache was already aware of THPs before Song's patches. shmem supported THPs. It was really filesystems that were (and remain!) the limiting factor.