get_pteptr mystery

Hollis Blanchard hollis at
Thu Dec 19 11:06:23 EST 2002

I need to mark the first kernel code page writeable because I need to
write to 0xc00000fc. Can someone tell me why the following code fails?
get_pteptr returns 0.

	addr = 0xc00000fc;
	if (0 != get_pteptr(&init_mm, addr, &ptep)) {
		/* mark it writable */
		*ptep = pte_mkwrite(*ptep);
		/* flush this page from hw TLB */
		flush_tlb_range(&init_mm, addr, addr+1);
	} else {
		printk(KERN_ERR "couldn't get PTE!\n");

I've also tried using pgd_offset_k/pmd_offset/pte_offset by hand, but it
seems that the first time any error checking is done is in pte_offset,
and that fails. Specifically
	pmd_offset(pgd_offset_k(0xc00000fc)) = c0158000
	((pmd *)c0158000)->pmd = 0

My code runs at init time, after mm initialization but before init. Does
that mean I can't use init_mm...?

All I want is the pte pointer for 0xc00000fc. Advice welcome. :)

PowerPC Linux
IBM Linux Technology Center

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