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Linux is a bit unusual in that kernel memory is generally physically mapped and unless you use vmalloc any memory you allocate has to correspond to pages backed by RAM. This also ties into how file IO happens, swapping, and how Linux approach to IO is actually closer to Multics and OS/400 than OG Unix.

Many other systems instead default to using full power of virtual memory including swapping kernel space to disk, with only things explicitly need to be kept in ram being allocated from "non-paged" or "wired" memory.

EDIT: fixed spelling thanks to writing on phone.




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