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I have been trying to run through an exercise to open a filepointer for /dev/mem in user space and navigate over all of the mem_map entries (struct page entries). In order to get started on this, I wrote a kernel module to display start of memmap logical address and also its physical address. I used the functions pfn_to_page(0) to get the logical address and __pa(pfn_to_page(0)) to get its corresponding physical address.
The output of this kernel module is something like this
When I try to use mmap to map to the physical address above it works fine but when I try to read values from that map_base, my process gets killed and when I look into /var/log/messages there is a message that says bad PGD.
My question is why is physical address (0000620000000000) beyond 4gb, I have only 2GB memory but its a 64 bit machine. Am I missing something here?
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