[matt_kleviar]

#define virt_to_page(kaddr) (mem_map + (__pa(KADDR) >> PAGE_SHIFT))
1) why do we need it at te first point ?
2) why does it only talk bout kernel virtual addresses, what about processes virtual addresses ?
how processe's virtual addresses can have access to "struct pages" using mem_map array ?

[digvijay.gahlot]

1. It is used to convert from kernel logical addresses to Physical struct page pointer.

There is a one to one mapping for kernel logical address and physical address. So what this macro does is
__pa(kaddr) returns the physical address of the kaddr,
then __pa(kaddr) >> PAGE_SHIFT shifts the physical address by PAGE_SHIFT bits to get the physical PAGE FRAME NUMBER
mem_map stores struct pages for whole physical memory in the system, as it is an array so
mem_map + (__pa(kaddr) >> PAGE_SHIFT)
gives the correct struct page pointer corresponding to the PAGE FRAME NUMBER.

Your second question is too confusing clearify a little more.

[matt_kleviar]

Hi,
what you told about mem_map array storing of styruct pages is understood.
but I would like to ask questions little further.

1) how does user's virtual address can have access to struct pages ?
2) if mem_mao is storing entire map if the physical memory then only kernel virtual addresses use it.. how does user space address use it ?