Memory (re)mapping
Kernel memory sometimes needs to be remapped, either from kernel to user space, or from high memory to a low memory region (from kernel to kernel space). The common case is remapping the kernel memory to user space, but there are other cases, such as when we need to access high memory.
Understanding the use of kmap
The Linux kernel permanently maps 896 MB of its address space to the lower 896 MB of the physical memory (low memory). On a 4 GB system, there is only 128 MB left to the kernel to map the remaining 3.2 GB of physical memory (high memory). However, low memory is directly addressable by the kernel because of the permanent and one-to-one mapping. When it comes to high memory (memory preceding 896 MB), the kernel has to map the requested region of high memory into its address space, and the 128 MB mentioned previously is especially reserved for this. The function used to perform this trick is kmap(). The kmap() function is used to map a given page into...