Memory management questions

Memory management questions

[Tyler]

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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Re: Memory management questions

[Sean Neakums]

Tyler <tyler@agat.net> writes:

> 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/
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