在Linux内核中取消映射“两次”映射的页面

Question

I use kmap to get the first virtual address of a low-memory page, inside a Linux Kernel module.

What happens if I call kunmap after that mapping? Is the persistent page mapping totally deleted or just some mapping counter is decreased? (should be 2 before the unmapping)

Answer 1

There is no mapping done by kmap if page belongs to low memory and hence there is no action done by kunmap too, but calling them is harmless as these checks are handled in there implementation.

First about kmap

kmap checks if page is below highmem_start_page(ie lowmemory page) as pages from lowmem are already visible and do not need to be mapped. If the page is already in low memory kmap simply returns the address of it.

Now about kunmap

kunmap checks if page is below highmem_start_page. If it is, the page already exists in low memory and needs no further handling, hence nop.