"lazy" mmap for a device driver ?

hi,
i have a kernel module implementing a memory mapped special device
which exports a large block of memory to the process.
I see that when the process calls mmap(), my routine foo_mmap()
is called immediately once per page, even though the process is
not actually touching the pages. I believe this happens
through dev_pager_alloc().

Right now i can live with that because all the memory is allocated
at module load time, but i might want to have a sparse memory
region which is populated dynamically, so i was wondering if
there is a way to achieve this. I see there are two other
device routines, d_mmap2 and d_mmap_single, any pointer to
documentation or comments on how they differ ?

thanks
luigi