The kernel page cache is an XArray [1] (data structure here [2]) and dirty page pointers are annotated with the dirty tag when the page is dirtied [3]. There is a per-filesystem callback that usually always ends up here [4], where you can see the the above data structure [2] entry for that page is being tagged as dirty.

Edit: fixed typo

[1] https://docs.kernel.org/core-api/xarray.html

[2] https://elixir.bootlin.com/linux/latest/source/include/linux...

[3] https://elixir.bootlin.com/linux/latest/source/mm/page-write...

[4] https://elixir.bootlin.com/linux/latest/source/mm/page-write...