I'm trying to understand the Linux 3/1 split (or 2/2, 1/3, any) and how mapping to physical memory work. Let's assume x86.
What I don't understand in particular is why the kernel's 1GiB in va[3GiB, 4GiB) is always mapped to pa[0, 1GiB]. The split is at (virtual)
What if I have more memory? What if I have less? Where does all memory for user-space go?
From TLDP I understand that the bottom physical 1GiB is always for the kernel (why?). High memory is used (by this post) when the virtual address space is smaller than the physical address space, because the memory is a lot and it would be wasted otherwise (right?); in x86-64 it is not being used because the virtual address space is anormous.
One thing to keep always the kernel there might be that on context switches
current remains the same and there's no need to change
This answer says:
The High Memory is the segment of memory that user-space programs can address. It cannot touch Low Memory.
Low Memory is the segment of memory that the Linux kernel can address directly. If the kernel must access High Memory, it has to map it into its own address space first.
Are people overloading the terms "low memory" and "high memory"?
Finally, LDD3 says:
The kernel cannot directly manipulate memory that is not mapped into the kernel's address space. The kernel, in other words, needs its own virtual address for any memory it must touch directly. Thus, for many years, the maximum amount of physical memory that could be handled by the kernel was the amount that could be mapped into the kernel's portion of the virtual address space, minus the space needed for the kernel code itself. As a result, x86-based Linux systems could work with a maximum of a little under 1 GB of physical memory.
Does this refer to the fact that a pointer
p in the kernel must be hold a virtual address, not a physical one, as mapping always applies? Why this "1GiB of physical memory" restriction?