On x86 kernel is normally split into 3GB (user) : 1 GB (Kernel) spaces. Kernel can only directly manipulate 1 GB (around 889 MB) from the PAGE_OFFSET (0xC0000000). The user space memofy below PAGE_OFFSET kernel can't directly access, it considers this memory as high memory. Kernel needs to kmap()/kmap_atomic() map user page and access that region.
If the memory is above 4 GB with page extension, it is clear that kernel can't access it as kernel uses "void *" and/or "unsigned long" to hold the address. But it is not clear for me why kernel's can't directly access memory below PAGE_OFFSET ( 0-3GB) directly. Regards Kshemendra
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