About SECTION_SIZE_BITS for Sparsemem

[Minchan Kim]

On Thu, Jul 15, 2010 at 5:49 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Wed, Jul 14, 2010 at 08:04:07PM +0900, Minchan Kim wrote:
>> On Wed, Jul 14, 2010 at 5:49 PM, Russell King - ARM Linux
>> <linux at arm.linux.org.uk> wrote:
>> > On Wed, Jul 14, 2010 at 08:59:06AM +0900, Minchan Kim wrote:
>> >> But I think Kukjin case's best solution is to make section size 16M, not 256M.
>> >> Regardless of this, Your idea is the direction we should proceed, I think.
>> >
>> > So what if someone decides to fit 8MB DRAMs to the board?
>> >
>>
>> Hmm. It might be a problem.
>> Should we consider all configuration?
>>
>> Then, in below cases ram size never can change?
>> How does it acked and merged?
>> I don't know below board can't configurable ram size. Just out of curiosity.
>
> All of these, SECTION_SIZE_BITS refers to the _spacing_ of memory, not
> the minimum size of DRAM which can ever be fitted.
>
> Reducing SECTION_SIZE_BITS is really not an option for some of these
> platforms - just look at where memory starts in physical space:
>
> arch/arm/mach-clps711x/include/mach/memory.h:#define PHYS_OFFSET UL(0xc0000000)
> arch/arm/mach-clps711x/include/mach/memory.h:#define SECTION_SIZE_BITS  24
> arch/arm/mach-lh7a40x/include/mach/memory.h:#define PHYS_OFFSET UL(0xc0000000)
> arch/arm/mach-lh7a40x/include/mach/memory.h:#define SECTION_SIZE_BITS   24
> arch/arm/mach-realview/include/mach/memory.h:#define PHYS_OFFSET UL(0x70000000)
> arch/arm/mach-realview/include/mach/memory.h:#define SECTION_SIZE_BITS  28
> arch/arm/mach-realview/include/mach/memory.h:#define PHYS_OFFSET UL(0x00000000)
> arch/arm/mach-rpc/include/mach/memory.h:#define PHYS_OFFSET UL(0x10000000)
> arch/arm/mach-rpc/include/mach/memory.h:#define SECTION_SIZE_BITS       26
> arch/arm/mach-s5pv210/include/mach/memory.h:#define PHYS_OFFSET UL(0x20000000)
> arch/arm/mach-s5pv210/include/mach/memory.h:#define SECTION_SIZE_BITS   28
> arch/arm/mach-sa1100/include/mach/memory.h:#define PHYS_OFFSET UL(0xc0000000)
> arch/arm/mach-sa1100/include/mach/memory.h:#define SECTION_SIZE_BITS    27
>
> If physical memory starts at 3GB, with SECTION_SIZE_BITS set at 24,
> there's 256 sections already.  Reducing SECTION_SIZE_BITS to the multiple
> of RAM size will increase the number of sections to many thousands.
> Clearly, sparsemem doesn't handle sparse memory very well, and is
> defficient.
>
> I don't know what the answer is to this; I'm totally fed up with being
> pushed into using things, and then later down the line being told that
> its all wrong and shouldn't be used like we are.  Maybe we should just
> delete these sub-architectures which use sparsemem and say to people
> that the kernel just doesn't support their memory layouts.
>
> <depressed>.
>

Sorry for late response.

Okay. I think Kame's recent solution is a good.
http://www.spinics.net/lists/linux-mm/msg06594.html
It doesn't need overriding pfn_valid, doesn't need more space.
And we can optimize it with overriding SECTION_MAP_LAST_BIT with
SECTION_MAP_HAS_HOLE  to avoid non-hole section overhead.
If we all agree, I will make the patch.

All agree?

-- 
Kind regards,
Minchan Kim