Question on virtual memory layout: lowmem with memory hole

Russell King - ARM Linux wrote on Friday, August 26, 2011 12:22 AM:

On Fri, Aug 26, 2011 at 12:06:20AM +0530, Pedanekar, Hemant wrote:

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Russell King - ARM Linux wrote on Thursday, August 25, 2011 3:53 PM:

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On Thu, Aug 25, 2011 at 09:35:07AM +0530, Pedanekar, Hemant wrote:

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E.g., on OMAP3 with mem=32M at 0x80000000 mem=8M at 0x87800000

[...]

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So, larger the hole, more address space will be unusable - is that correct?

Correct.

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This also
means vmalloc space is lower compared to when a single mem=40M is passed.

Huh.  Either your maths is wrong or...

Here's case 1:

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     vmalloc : 0xc8800000 - 0xf8000000   ( 760 MB) And case 2:
     vmalloc : 0xc3000000 - 0xf8000000   ( 848 MB)

Looks to me like case 1, vmalloc space is _higher_ not _lower_.  That's
expected because you told the kernel it had more memory in case 1.

Sorry, my mistake - I actually meant "vmalloc space is _smaller_ compared
to when a single mem=40M is passed" though the actual physical RAM
available is same in both the cases.

Again, that's expected.

We require a 1:1 relationship between virtual and physical addresses for
efficiency - having non-linear translation means we'd need a lookup table,
and as these translations are heavily used, that will impact
performance.

Thanks for all the clarifications.

   Hemant