Re: [RFC] Tight check of pfn_valid on sparsemem

[Johannes Weiner]

On Tue, Jul 13, 2010 at 12:53:48AM +0900, Minchan Kim wrote:
> Kukjin, Could you test below patch?
> I don't have any sparsemem system. Sorry. 
> 
> -- CUT DOWN HERE --
> 
> Kukjin reported oops happen while he change min_free_kbytes
> http://www.spinics.net/lists/arm-kernel/msg92894.html
> It happen by memory map on sparsemem. 
> 
> The system has a memory map following as. 
>      section 0             section 1              section 2
> 0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000
> SECTION_SIZE_BITS 28(256M)
> 
> It means section 0 is an incompletely filled section.
> Nontheless, current pfn_valid of sparsemem checks pfn loosely. 
> 
> It checks only mem_section's validation.
> So in above case, pfn on 0x25000000 can pass pfn_valid's validation check.
> It's not what we want.
> 
> The Following patch adds check valid pfn range check on pfn_valid of sparsemem.

Look at the declaration of struct mem_section for a second.  It is
meant to partition address space uniformly into backed and unbacked
areas.

It comes with implicit size and offset information by means of
SECTION_SIZE_BITS and the section's index in the section array.

Now you are not okay with the _granularity_ but propose to change _the
model_ by introducing a subsection within each section and at the same
time make the concept of a section completely meaningless: its size
becomes arbitrary and its associated mem_map and flags will apply to
the subsection only.

My question is: if the sections are not fine-grained enough, why not
just make them?

The biggest possible section size to describe the memory population on
this machine accurately is 16M.  Why not set SECTION_SIZE_BITS to 24?
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