vmalloc size

[Subramaniam Appadodharana]

Hi Dave,

Thank you for taking your time to provide this example. This is indeed very
insightful.

On Mon, Jun 25, 2012 at 7:41 PM, Dave Hylands <dhylands at gmail.com> wrote:

> Hi Subbu,
>
> ...snip,,,
> >> __pa only works on kernel direct addresses.
> >>
> >> __pa doesn't work on the addresses from vmalloc
> >>
> >> Using __pa on VMALLOC_START or VMALLOC_END doesn't really make sense.
> >> If there were any physical memory there, it would be highmem.
> >>
> > Okay, but my intention was to just print out the pa for the VMALLOC_START
> > and END, just to know where they are.
> > And I thought they will work. no?
>
> You'll get some numbers, but they're meaningless.
>
> Let's take a concrete example (using ARM mappings)
>
> Suppose your machine has 1 GB of RAM with a physical addresses of
> 0x40000000 thru 0x7FFFFFFF
>
> Let's suppose that PHYS_OFFSET = 0xC0000000
> Let's suppose that VMALLOC_START is set to 0xE0000000 and VMALLOC_END
> is at 0xEFFFFFFF (256 Mb)
>
> Agreed!

> So what we have is that memory from 0xC0000000 thru 0xDFFFFFFF (512
> Mb) is kernel direct mapped memory and maps to physical addresses
> 0x40000000 thru 0x5FFFFFFF
>
> Agreed

> Physical memory from 0x60000000 thru 0x7FFFFFFF (512Mb) is considered
> high-mem. For the most part this is only usable by user-space. From
> kernel space, you need to use the kmap/kunmap functions to even access
> the memory.
>
> Agreed

> If you use __pa(VMALLOC_START) you'll ALWAYS (with the above example)
> get 0xE0000000.
> Calling __pa(VMALLOC_END) would return 0xEFFFFFFF which is some
> address from the middle of high-memory, essentially a meaningless
> number.
>
> Agreed this one too!!!


> However, if you call vmalloc and lets suppose that vmalloc just
> happens to return 0xE0000000. The physical address of the first page
> might be 0xD2345000.
>
> What's important is that the physical pages which back up the vmalloc
> area all come from the kernel direct mapped area. They won't ever be
> backed by pages from high-memory. So the physical addresses will all
> be in the range 0x40000000 thru 0x5FFFFFFF.
>
> This is the one that I had a completely wrong understanding of!!!!!!!
I understand from the above statement that  the vmalloc'ed virtual address
will _ALWAYS_  correspond to a  physical address from the lowmem region! I
was under the impression that the carved out region for the vmalloc,
is the one that would back any vmalloc'ed virtual address, which is
absolutely wrong by what you are saying.

Now this also means that increasing vmalloc inadvertently reduces lowmem.
Why is this designed such a way?
Essentially, the idea that we increase vmalloc is because we expect more
memory to be consumed via vmalloc
calls, and hence we might need more physical address backing. But
increasing vmalloc decreases low mem, which would also mean that we have
less backing.
Am I missing something here too :)?


You can use vmalloc_to_pfn to return the PFN of the page which is
> backing a vmalloc'd virtual address. (if the physical address of
> 0xE0000000 was 0xD2345000, then the pfn would be 0xD2345, assuming 4K
> pages).
>
Agreed on this too!  Thanks again for your time and patience.

>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
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