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What does the virtual kernel memory layout in Linux?
When a HIGHMEM page is mapped using kmap (), virtual addresses are assigned from here. These are fix-mapped linear addresses which can refer to any physical address in the RAM, not just the last 1 GB like the lowmem addresses.
What is the memory layout for ARM Linux?
Kernel Memory Layout on ARM Linux. This document describes the virtual memory layout which the Linux kernel uses for ARM processors. It indicates which regions are free for platforms to use, and which are used by generic code. The ARM CPU is capable of addressing a maximum of 4GB virtual memory space, and this must be shared between user space
How is high memory accessed by the kernel?
User space (high memory) can be accessed by the user and, if necessary, also by the kernel. Kernel space (low memory) can only be accessed by the kernel.
How big is the lowmem range in Linux?
In decimal notation, that’s 3’255’914 (3179 kB). The lowmem range can be used by the kernel to directly access physical addresses. This is not the full 1 GB, because the kernel always requires at least 128 MB of linear addresses to implement noncontiguous memory allocation and fix-mapped linear addresses.
What are negative addresses in a virtual memory map?
Complete virtual memory map with 4-level page tables ¶ Negative addresses such as “-23 TB” are absolute addresses in bytes, counted down from the top of the 64-bit address space. It’s easier to understand the layout when seen both in absolute addresses and in distance-from-top notation.
What causes hole between user space and kernel address?
This causes hole between user space and kernel addresses if you interpret them as unsigned. The direct mapping covers all memory in the system up to the highest memory address (this means in some cases it can also include PCI memory holes).
How is memory randomized in the Linux kernel?
The mappings are not part of any other kernel PGD and are only available during EFI runtime calls. Note that if CONFIG_RANDOMIZE_ MEMORY is enabled, the direct mapping of all physical memory, vmalloc/ioremap space and virtual memory map are randomized.