Tracing only write page-faults

Hello.
I want to trace all write page-faults from an application. I use the
pager that wait for page-fault and maps the fpage to thread calling
l4_ipc_send_and_wait setting L4_ITEM_MAP. For tracing only write
page-faults I map fpage as L4_FPAGE_RO if it was read-pf and then I
map the same fpage with L4_FPAGE_RW if the next write-pf occurs. (I
mean I map the same fpage with the different flags twice). But an
application doesn't continue run. Also I have tried to
l4_task_unmap(L4_FP_OTHER_SPACES) this fpage before mapping it as
L4_FPAGE_RW. It doesn't work. What should I use to change fpage's
rights to L4_FPAGE_RW for fpage that is already mapped as L4_FPAGE_RO
for an application?

Ivan Bludov