Question on virtual memory layout: lowmem with memory hole

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

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

quoted

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

quoted

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

(CASE 1)
 Memory: 32MB 8MB = 40MB total
 Memory: 28408k/28408k available, 12552k reserved, 0K highmem
 Virtual kernel memory layout:
     vector  : 0xffff0000 - 0xffff1000   (   4 kB)
     fixmap  : 0xfff00000 - 0xfffe0000   ( 896 kB)
     DMA     : 0xffc00000 - 0xffe00000   (   2 MB)
     vmalloc : 0xc8800000 - 0xf8000000   ( 760 MB)
     lowmem  : 0xc0000000 - 0xc8000000   ( 128 MB)
     modules : 0xbf000000 - 0xc0000000   (  16 MB)
       .text : 0xc0008000 - 0xc05ac2c8   (5777 kB)
       .init : 0xc05ad000 - 0xc05f8780   ( 302 kB)
       .data : 0xc05fa000 - 0xc06838d0   ( 551 kB)
        .bss : 0xc06838f4 - 0xc0bd8a14   (5461 kB)

Is this expected? 88MB space between two 'mem's seems to have lost.

Yes.  The memory layout lines gives an overview of the virtual memory
address space _regions_. 

What it's saying is that the virtual addresses from 0xc0000000 - 0xc8000000
are used for lowmem.  That may not be fully populated, but that's what
the address range is reserved for.

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.