High memory mappings in kernel virtual address space

2
votes

The linear address beyond 896MB correspond to High memory region ZONE_HIGHMEM. So the page allocator functions will not work on this region, since they give the linear address of directly mapped page frames in ZONE_NORMAL and ZONE_DMA.

I am confused about these lines specified in Undertanding linux Kernel:

  1. What do they mean when they say "In 64 bit hardware platforms ZONE_HIGHMEM is always empty."

  2. What does this highlighted statement mean: "The allocation of high-memory page frames is done only through alloc_pages() function. These functions do not return linear address since they do not exist. Instead the functions return linear address of the page descriptor of the first allocated page frame. These linear addresses always exist, because all page descriptors are allocated in low memory once and forever during kernel initialization."

What are these Page descriptors and does the 896MB already have all page descriptors of entire RAM.

1
linuxmemory-managementlinux-kernelvirtual-memory

1 Answers

1
votes

  1. The x86-32 kernel needs high memory to access more than 1G of physical memory, as it is impossible to permanently map more than 2^{32} addresses within a 32-bit address space and the kernel/user split is 1G/3G. The x86-64 kernel has no such limitation, as the amount of physically-addressable memory (currently 256T) fits within its 64-bit address space and thus may always be permanently mapped. High memory is a hack. Ideally you don't need it. Indeed, the point of x86-64 is to be able to directly address all the memory you could possibly want. Taken from https://www.quora.com/Linux-Kernel/What-is-the-difference-between-high-memory-and-normal-memory

  2. I think page descriptor means struct page. And considering the sizeof struct page. Yes all of them can be stored in ZONE_NORMAL