kern_execve() locks text vnode exclusive to be able to set and clear VV_TEXT flag. VV_TEXT is mutually exclusive with the v_writecount > 0 condition.
The change removes VV_TEXT, replacing it with the condition v_writecount < -1, and put v_writecount under the vnode interlock. Each text reference decrements v_writecount. To clear the text reference, it is recorded in the vm_map_entry backed by the text file as MAP_ENTRY_VN_TEXT flag, and v_writecount is incremented on the map entry removal. Now, the operations like VOP_ADD_WRITECOUNT() and VOP_SET_TEXT() check that v_writecount does not contradict the desired change. vn_writecheck() is now racy and its use was eliminated everywhere except access, atomic check for writeability and increment of v_writecount is performed by the VOP.
nullfs bypasses v_writecount to the lower vnode always.