On Wed, Jan 11 2006, J.A. Magallon wrote:

It's in -mm now.

If they don't, they should not touch the option...

I think the help text is good enough, but it would definitely be nice with a fuller description of what exactly low and high memory is and the implications of the various settings.

It tells you that the remaining memory is available as high memory, so it's not lost of course. It also tells you that accessing this high memory is indeed possible, but it's a little more costly since it needs to be mapped temporarily into the kernel address space.

No worries, perhaps you will be the one writing the Documentation/ bit to accompany this then :-)

Basically the option boils down to how much virtual address space you want to assign to the kernel and user space. The kernel can always access all of memory, but in some cases part of that memory will be available as high memory that needs to be mapped in first (see references to kmap() and kmap_atomic() in the kernel). So whether changing the mapping or using highmem is the best option for you, depends entirely on what you run on that machine. If you require a huge user address space, then you don't want to change away from the 3/1 user/kernel default setting. However, if you don't need the full 3G of adress space to user apps, then you are better off increasing the kernel address space range to get rid of the high memory mapping.

For the "typical" case of 1GB machine, using the _OPT setting to just move the offset slightly is a really good choice as it only removes a little bit of the user address range.