fsmount:

Unlike open_tree(2) with OPEN_TREE_CLONE, fsmount() can only be called once in the lifetime of a filesystem instance to produce a mount object.

I don't understand what you meant here. This phrase in its current form is wrong.
Consider this scenario: we did this:
fsopen(...)
fsconfig(..., FSCONFIG_SET_STRING, "source", ...)
fsconfig(..., FSCONFIG_CMD_CREATE, ...)
fsmount(...)
fsopen(...)
fsconfig(..., FSCONFIG_SET_STRING, "source", ...)
fsconfig(..., FSCONFIG_CMD_CREATE, ...)
fsmount(...)

We used FSCONFIG_CMD_CREATE here as opposed to FSCONFIG_CMD_CREATE_EXCL, thus
it is possible that second fsmount will return mount for the same superblock.
Thus that statement "fsmount() can only be called once in the lifetime of a filesystem instance to produce a mount object"
is not true.

--
Askar Safin
https://types.pl/@safinaskar