from http://www.linux-mips.org/wiki/Highmem

> Overview
> --------
> Highmem is a method that increases the usable address space.
> Originally developed for Intel i386 where it enables the use of more than approx.
> 970MB of memory. Most of the code is portable to other architectures.
>

メモリのおよそ970MB以上の使用を可能にするIntel i386の為に開発された。


> Memory Managment on Linux/MIPS
> ------------------------------
> 32-bit Linux/MIPS kernels assume the entire low memory to be accessible through
> KSEG0 which limits lowmem to at most 512MB. Typical MIPS systems reserve part of
> that address space for other uses so a lowmem limit of 256MB is a common value.
> Memory beyond that can be used by 64-bit kernels or 32-bit kernels with highmem
> support enabled.
>

32-bit Linux/MIPSカーネルは、低位メモリ全体が、最大でも512MBにlowmemを制限するKSEG0を介してアクセス可能と仮定する。
典型的なMIPSシステムは、そのアドレス空間の一部を他の使用の為に予約するので、256MBのlowmemの限界が共通の値である。


> Problems
> --------
> Linux allocates a large array named mem_map[] in lowmem. On 32-bit kernels it
> contains one 64-byte entry per page of memory. Entries are allocated even for
> reserved memory ranges.
> This consumption of lowmem means memory pressure on lowmem will rise with the
> amount of highmem.
> In practice at most a ratio of 1:4 to 1:8 has been found to perform well.
>
> Highmem is accessed through temporary mappings.
> Creation and teardown of such mappings are are not for free and mappings are
> only permitted to exist for a short time. This means any kernel software using
> highmem needs to be written with special care or performance and stability of
> the entire system are at stake.
>
> Certain systems where memory may be outside the low 4GB physical address space
> will need to enable support for 64-bit physical addresses in pagetables also.
> This will nearly double the size of pagetables. Again pagetables resize in
> lowmem; the can be a large consumer of lowmem for system that make heavy use of
> mmap.
>
> While creation and teardown of mappings already are a significant overhead the
> cacheflushes necessary to deal with cache aliases would dwarf that.
> Therefore Linux/MIPS supports highmem only on systems that don't suffer from
> cache aliases. A highmem kernel will boot on such a system but only use the
> lowmem.
>
> Alternatives
> ------------
> 64-bit kernels use XKPHYS to access their lowmem.
> Due to the size of XKPHYS the entire physical memory is directly accessible so
> highmem isn't needed with 64-bit kernels.
> 64-bit kernels have somewhat larger code and data resulting in a slight slowdown.
> This however is by far preferable over the impact of highmem.
>
> See Also
> --------
> Jaguar ATX for some creative abuse of the kernel's highmem facilities.