General MM Notes

Linux kernel space has 4 kinds of memory mapping
1. direct mapping
2. vmalloc mapping
3. kmap mapping
4. fixmap mapping

direct mapping is used for low memory pages which are used to store permanent data structures, page tables, interrupt tables and so on.

vmalloc mapping is used for non-contiguous memory mapping. i can use it any way possible. if you have a space reserved in vmalloc space and a bunch of pages in your hand, you can map those pages to that vmalloc space by modifying the kernel page tables. (example usage: by insmod to store the modules)

kmap mapping is kind of the vmalloc space but only one pgd entry in master kernel page table is dedicated for kmap virtual address.

fixmap mapping is similar to kmap mapping. (example usage: map a page table residing on a high mem page)

all these 4 kinds of memory virtual and physical pages are given as soon as the mapping request is placed and served. to page fault here means something terribly wrong going on inside.

user process pages are similar to kernel vmalloc space except they are in virtual address in user space and the physical pages are not allocated and mapped as long as there is page fault generated on those virtual addresss spaces.