x86_64 expertise needed re: BUG() at pageattr:107

["John W. Linville" <linville@tuxdriver.com>]

The x86_64 version of pageattr.c contains the following definition
of revert_page():

static void revert_page(unsigned long address, pgprot_t ref_prot)
{
       pgd_t *pgd;
       pmd_t *pmd;
       pte_t large_pte;

       pgd = pgd_offset_k(address);
       pmd = pmd_offset(pgd, address);
       BUG_ON(pmd_val(*pmd) & _PAGE_PSE);
       pgprot_val(ref_prot) |= _PAGE_PSE;
       large_pte = mk_pte_phys(__pa(address) & LARGE_PAGE_MASK, ref_prot);
       set_pte((pte_t *)pmd, large_pte);
}

Please notice the BUG_ON() (originally at line pageattr.c:107).
I notice that the x86 version of revert_page() has no similar check.

Would someone please explain:

	a) why is this a bug?;
	b) what is likely to have triggered it?; and,
	c) why is x86_64 different from x86?

I've Googled a few references to this problem (latest 10/10/2004),
but I've seen no solutions.  I do not currently have a reproducible
test case, although I have a report (which dates from around 2.6.7)
of seeing this problem when installing the e1000 driver module.

I'd be happy to see some educated guesses... :-)

Thanks for any input!

John
-- 
John W. Linville
linville@tuxdriver.com