Memory management questions

DISCUSSION Tyler 2 participants 2 messages Jun 12, 2004 – Jun 13, 2004 Memory management

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Tyler

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2004-06-12 19:11 UTC #0 lore raw

Hi all,

I've always thinked that paging or virtual memory was practical to avoid memory fragmentation. I thinked that you can map contiguous virtual pages to non contiguous physical page frames. But let's take a look at the macros __va(x) and __pa(x) :
#define __pa(x) ((unsigned long)x-PAGE_OFFSET)
#define __va(x) ((unsigned long)x+PAGE_OFFSET)
PAGE_OFFSET is a constant. For me, this means that virtual contiguous adresses have to be mapped to contiguous physical adresses. Am I wrong ?:)

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Sean Neakums

2004-06-13 13:55 UTC #1 lore raw

Quoted 9 lines · Tyler

I've always thinked that paging or virtual memory was practical to avoid memory fragmentation. I thinked that you can map contiguous virtual pages to non contiguous physical page frames.
But let's take a look at the macros __va(x) and __pa(x) :
#define __pa(x) ((unsigned long)x-PAGE_OFFSET)
#define __va(x) ((unsigned long)x+PAGE_OFFSET)
PAGE_OFFSET is a constant. For me, this means that virtual contiguous adresses have to be mapped to contiguous physical adresses. Am I wrong ?:)

IIRC PAGE_OFFSET is the value added to a physical address to obtain the address in the *kernel's* identity mapping of physical memory. The macros above look like they're used to convert between the two. I don't think they have much directly to do with userspace virtual addresses, whose mappings to physical memory are obtained via the page tables.

I'm VM-tarded, though, so the above may be a bunch of hooey.

A good place to start with the Linux VM is probably Mel Gorman's VM documentation, which although focused on 2.4 should give you a good start. Which reminds me, I should read this myself.

	http://www.skynet.ie/~mel/projects/vm/