Hello all,
This is my first post to the kernelnewbie list, so here we go....
Ok, so I am very much a bottom up learner, so I am trying to understand the
kernel from the lowest level up. I am up to the point where I am trying to
understand the relationship between virtual memory to the physical memory
layout on the machine. What I am totally floundering on is the 3G/1G split
and the necessity of highmem. Ok, So I understand that the kernel is placed
in the second meg of physical memory and then this is mapped into the top 1G
of virtual address space. Where I get confused is everything after that :-)
I am failing to understand why all the physical memory is mapped into the
kernel space up to the highmem mark and the highmem area is then used to
temporarily map additional memory into kernel space. Is this done so that
the kernel can then use memory that it controls to hand out to user
processes? If so, why does the kernel need these areas of memory mapped into
its virtual address space? It seems like the kernel could keep a list of
all page frames that are available and then use this to map in page frames
for userspace memory without ever having it mapped into kernel memory space.
As an example. Let us start simple, where no highmem is needed, say a
machine with 512MB of memory. Accordding to my mental model the kernel would
have all of this space linearly mapped in kernel address space. The kernel
would then hand out pieces of memory by mapping them into user processe
space as requests are made. Is this correct? If so, please explain why the
kernel had to have this memory originally mapped into its own address space
in order to hand it out to a user process? Then, lets say we have a machine
with 1.5GB of memory. The kernel linearly maps 896MB of space into kernel
space and the leaves teh remaning 128MB for temporary mappings. When are
these temporary mappings done, and for what purpose. Would these temporary
mappings take place when user processes require space? Just as above, why
does the kernel need to temporarily map this space just to hand it out to
user processes?
I've read several tutorials but have yet to pin down exactly why memory is
setup as it is. Any help anyone might provide would be greatly appreciated.
Thanks,
Patrick
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