About SECTION_SIZE_BITS for Sparsemem

[Russell King - ARM Linux]

On Tue, Jul 13, 2010 at 10:50:35AM +0100, Mel Gorman wrote:
> On Tue, Jul 13, 2010 at 10:38:15AM +0100, Russell King - ARM Linux wrote:
> > On Tue, Jul 13, 2010 at 10:26:58AM +0100, Mel Gorman wrote:
> > > There is also an assumption that a section is fully populated or empty.
> > 
> > That is absolutely absurd. 
> 
> Arguably, violating the memory model by punching unexpected holes in it
> is a also absurd.

Well, it's something we've done since 1.2.xx, and actually it's a new
requirement that's evolved into the code that these page structs need
to be populated.  So it's arguably a regression in the MM code which
hasn't been detected.

> > So, I have a platform which has 256MB at
> > 64MB intervals in 4 chunks. I can fit 512kB to any slot. 
> 
> I'm afraid I'm not quite getting your example.
> 
> If the granularity of the banks is 64MB and the alignment is 256MB, I
> don't see what hole you'd be punching anyway.

I didn't say the alignment is 256MB.  I was referring to the *size* of
memory.

> Not necessarily, just don't punch holes within section boundaries when using
> sparsemem.

Err, you've just changed your story.

Let's take an example setup which is perfectly valid.  512kB at 0x10000000.
512kB at 0x14000000. 32MB at 0x1c000000.

You're saying that MAX_PHYSMEM_BITS=29 SECTION_SIZE_BITS=26 is wrong.

So, what do you suggest would be the correct sparsemem settings for this?
MAX_PHYSMEM_BITS=29 SECTION_SIZE_BITS=19 maybe?