Re: Zone's in Linux

Gil Hamilton wrote:
Alexander Krizhanovsky <a.krizhanovsky@xxxxxxxxx> wrote in news:1187785854.294148.246500@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:


As far as I understood this comment x86 can't directly address memory
higher than 896Mb (I don't understand why here is wrote 900Mb):
So could you explain why we have only 896Mb of Normal Zone (that is
can't directly address whole 2Gb of physical memory)? Why these 2Gb
aren't fully arranged to NormalZone?


The issue is how much of the memory is visible in kernel *virtual memory* at a given time. There is a total visibility of 4GB of virtual memory. In the "normal" x86 linux kernel configuration, the virtual address space is subdivided between user-space which gets 3GB (address 0 through 0xbfffffff) and 1GB for the kernel address space (0xc0000000 through 0xffffffff). That is, the kernel only has 1GB of virtual memory space available for its use. Starting at 0xc0000000, the first X pages of physical memory are mapped 1-to-1 into kernel virtual memory space. This makes it convenient to access any low physical address: you just add PAGE_OFFSET (0xc0000000) to it.

ACK. My answer was context-free. In the context of a Linux kernel (and we're in comp.os.*linux*.development.system, after all), you're absolutely right.

Josef
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Josef Möllers (Pinguinpfleger bei FSC)
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