Re: [patch][rfc] 4/5: remap ZERO_PAGE mappings

[Nick Piggin]

4/5
Remap ZERO_PAGE ptes when remapping memory. This is currently just an
optimisation for MIPS, which is the only architecture with multiple
zero pages - it now retains the mapping it needs for good cache performance,
and as well do_wp_page is now able to always correctly detect and
optimise zero page COW faults.

In future, this becomes required in order to always be able to detect
whether a pte points to a ZERO_PAGE using only the pte, vaddr pair.

Signed-off-by: Nick Piggin <npiggin@xxxxxxx>

Index: linux-2.6/mm/mremap.c
===================================================================
--- linux-2.6.orig/mm/mremap.c
+++ linux-2.6/mm/mremap.c
@@ -141,6 +141,10 @@ move_one_page(struct vm_area_struct *vma
 			if (dst) {
 				pte_t pte;
 				pte = ptep_clear_flush(vma, old_addr, src);
+				/* ZERO_PAGE can be dependant on virtual addr */
+				if (pfn_valid(pte_pfn(pte)) &&
+					pte_page(pte) == ZERO_PAGE(old_addr))
+					pte = pte_wrprotect(mk_pte(ZERO_PAGE(new_addr), new_vma->vm_page_prot));
 				set_pte_at(mm, new_addr, dst, pte);
 			} else
 				error = -ENOMEM;