Re: regression: 2f569af (CONFIG_HIGHPTE vs. sub-page page tables.)

["Uwe Kleine-König" <Uwe.Kleine-Koenig@digi.com>]

Hello Martin,

Martin Schwidefsky wrote:
> On Mon, 2008-02-25 at 14:26 +0100, Uwe Kleine-König wrote:
> > I see the following:
> > 
> > 	/ # for i in 1 2 3; do grep PageTables /proc/meminfo; done
> > 	PageTables:          4 kB
> > 	PageTables:   4294967292 kB
> > 	PageTables:   4294967284 kB
> > 
> > and I bisected it down to 2f569af (CONFIG_HIGHPTE vs. sub-page page
> > tables.)  This still happens in 2.6.25-rc3.
> > 
> > I have not investigated further, so I cannot tell if it's only the
> > output in meminfo that is broken.
> > 
> > This is on ARCH=arm, on a Digi cc9p9360 with ns9xxx_defconfig.
> 
> Hmm, not good. The number obviously went negative. There is an imbalance
> in the number of pgtable_page_ctor vs. pgtable_page_dtor. Could you try
> this patch and watch for warnings?
It doesn't trigger.

And your assumption that there is an imbalance between pgtable_page_ctor
and pgtable_page_dtor is wrong, too.  I added some debug output to both
of the two functions and for one grep both are executed seven times.

Moreover a "grep PageTables /proc/meminfo" resulted in 

	9x __dec_zone_state with item == NR_PAGETABLE
	7x __inc_zone_state with item == NR_PAGETABLE

All those 16 calles where done via __(inc|dec)_zone_page_state.

... some time later ...

I got it.  This code is currently in free_pgd_slow():

        pte = pmd_page(*pmd);
        pmd_clear(pmd);
        dec_zone_page_state(virt_to_page((unsigned long *)pgd), NR_PAGETABLE);
        pte_lock_deinit(pte);
        pte_free(mm, pte);
        pmd_free(mm, pmd);

So because (since 2f569af) pte_free does dec_zone_page_state and
pte_lock_deinit, these two happen twice here.

Removing the calls to dec_zone_page_state() and pte_lock_deinit() in
free_pgd_slow() fixed it for me.

For a complete fix we might want to change the type of pte?

Best regards
Uwe

-- 
Uwe Kleine-König, Software Engineer
Digi International GmbH Branch Breisach, Küferstrasse 8, 79206 Breisach, Germany
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