Re: 1GB/2GB/3GB User Space Splitting Patch 2.6.8.1 (PSEUDO SPAM)

[Timothy Miller]

Roland Dreier wrote:
>     Timothy> My question is: Why can't we just shrink the kernel
>     Timothy> address space by that same amount, allowing the kernel
>     Timothy> address space plus the extra to fit into 1GB?
>
> If you have 1 GB of memory and want to map it all into the kernel's
> address space and the kernel has only 1 GB of address space total,
> then there is no room for anything else (such as address space to
> ioremap memory-mapped peripherals or space for vmalloc allocations).
> Therefore, if you want to have 1 GB of RAM mapped linearly into the
> kernel's address space, you need strictly more than 1 GB of kernel
> address space.  In practice, 1.25 GB of kernel address space (a
> PAGE_OFFSET value of 0xb0000000) works well with 1 GB of RAM.  That's
> what I run on my main desktop machine with 1 GB of RAM to avoid HIGHMEM.
>
>  - Roland


Ok, I understand now.  I think.  With 0xc0000000, you have 128M of 
highmem, right?  Why do you add 256M to the kernel address space?  Is 
there a further advantage to that?

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