Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Jul 29, 2010 at 10:46:13AM -0500, Christoph Lameter wrote:
> On Thu, 29 Jul 2010, Minchan Kim wrote:
> 
> > On Wed, Jul 28, 2010 at 12:02:16PM -0500, Christoph Lameter wrote:
> > > On Thu, 29 Jul 2010, Minchan Kim wrote:
> > > > invalid memmap pages will be freed by free_memmap and will be used
> > > > on any place. How do we make sure it has PG_reserved?
> > >
> > > Not present memmap pages make pfn_valid fail already since there is no
> > > entry for the page table (vmemmap) or blocks are missing in the sparsemem
> > > tables.
> > >
> > > > Maybe I don't understand your point.
> > >
> > > I thought we are worrying about holes in the memmap blocks containing page
> > > structs. Some page structs point to valid pages and some are not. The
> > > invalid page structs need to be marked consistently to allow the check.
> >
> > The thing is that memmap pages which contains struct page array on hole will be
> > freed by free_memmap in ARM. Please loot at arch/arm/mm/init.c.
> > And it will be used by page allocator as free pages.
> 
> Arg thats the solution to the mystery. freememmap() is arm specific hack!
> 
> Sparsemem allows you to properly handle holes already and then pfn_valid
> will work correctly.
> 
> Why are the ways to manage holes in the core not used by arm?

I did use ARCH_HAS_HOLES_MEMORYMODEL.
It is used by only ARM now. 
If you disable the config, it doesn't affect the core. 

> 
> sparsemem does a table lookup to determine valid and invalid sections of
> the memmp.
> 
The thing is valid section also have a invalid memmap. 
Maybe my description isn't enough. 
Please look at description and following URL. 

We already confirmed this problem. 
http://www.spinics.net/lists/arm-kernel/msg92918.html

== CUT 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 but ARM can free mem_map on hole
     to save memory space. So in above case, pfn on 0x25000000 can pass pfn_valid's
     validation check. It's not what we want.




-- 
Kind regards,
Minchan Kim