pgd table entry pointing null after process switching

hi, linux folks, I'm seeing an 'unable to handle kernel paging request' after loading busybox elf loading. The context table is now pointing a new pgd table and the releavant pgd table entry is pointing null. What could be the cause? the address to access is frame buffer that I had linux allocate from consistent memory region. (I'm borrowing arm dma_alloc_writecombine conde in arch/arm/mm/dma-mappping.c for sparc.) Something is wrong duing the process address space allocation. (I'm reading a book section while doing this..) Any hint will be greatly appreciated. Thanks! Chan