[PATCH] ARM: Fix virtual kernel memory printing for sparsemem

[will.deacon@arm.com (Will Deacon)]

Hi Russell,

* Russell King wrote:
> On Thu, Mar 25, 2010 at 03:24:29PM +0000, Catalin Marinas wrote:
> > On Thu, 2010-03-25 at 15:10 +0000, Russell King - ARM Linux wrote:
> > > While this looks fine, I'd like to see a lot of Tested-by's against
> > > this before it's merged - we've had similar code in show_mem()
> > > which has proven to be quite problematical to get right for all the
> > > various different combinations we have.
> > >
> > > However, we also have the same method in show_mem() which we know
> > > works fine, so I'd also like to see the problem with using it in
> > > mem_init() fully analysed - rather than a "possibly because".
> >
> > I can remove the "possibly" part :). The page_count() is given a page
> > with some random flags and it thinks it's a compound page. It than tries
> > to access page->first which isn't set, hence the error.
> >
> > The original code assumes that for a given node the map is contiguous
> > and it calculates the page as (map + pfn). This is only true for
> > flatmem. With sparsemem the page calculation is a bit more complicated
> > but handled by pfn_to_page().
> 
> And show_mem() ?  It seems to suffer from the same problem.

Well if the page flags are being set randomly, there's a good chance that show_mem()
won't get as far as calling page_count(). Even then, if the _count field that ends up
being read happens to be aligned, it won't fail. If this is what is happening, I'd
expect show_mem to give incorrect results on platforms with SPARSEMEM enabled. Has this
been observed by anybody?

Will