The vm_page_free_toq function moves a page into the free queue, and disassociates it from its object. If the page is held, wired, already free, or its busy count is not zero, the system will panic. If the PG_ZERO flag is set on the page, it is placed at the end of the free queue; otherwise, it is placed at the front.
If the pages object is of type OBJT_VNODE and it is the last page associated with the object, the underlying vnode may be freed.
The vm_page_free and vm_page_free_zero functions both call vm_page_free_toq to actually free the page, but vm_page_free_zero sets the PG_ZERO flag and vm_page_free clears the PG_ZERO flag prior to the call to vm_page_free_toq.
The vm_page_try_to_free function verifies that the page is not held, wired, busy or dirty, and if so, marks the page as busy, drops any protection that may be set on the page, and frees it.
vm_page_try_to_free returns 1 if it is able to free the page; otherwise, 0 is returned.
vm_page_busy(9), vm_page_hold(9), vm_page_wire(9)