Read/Write migration entries: Implement correct behavior in copy_one_pte

[Christoph Lameter <clameter@sgi.com>]

Note that this is again only a partial solution. mprotect() also has the
potential of changing the write status to read. Are there any additional
occurrences? Would you check and fix this one as well?

If we cannot get to all the locations or if these fixes get too extensive
then I think we better drop the preservation of write permissions and
tolerate the occurrence of some useless COW after migration.



Migration entries with write permission must become SWP_MIGRATION_READ
entries if a COW mapping is processed. The migration entries from which
the copy is being made must also become SWP_MIGRATION_READ. This mimicks
the copying of pte for an anonymous page.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.17-rc1-mm3/mm/memory.c
===================================================================
--- linux-2.6.17-rc1-mm3.orig/mm/memory.c	2006-04-18 10:58:33.000000000 -0700
+++ linux-2.6.17-rc1-mm3/mm/memory.c	2006-04-18 11:09:23.000000000 -0700
@@ -434,7 +434,9 @@ copy_one_pte(struct mm_struct *dst_mm, s
 	/* pte contains position in swap or file, so copy. */
 	if (unlikely(!pte_present(pte))) {
 		if (!pte_file(pte)) {
-			swap_duplicate(pte_to_swp_entry(pte));
+			swp_entry_t entry = pte_to_swp_entry(pte);
+
+			swap_duplicate(entry);
 			/* make sure dst_mm is on swapoff's mmlist. */
 			if (unlikely(list_empty(&dst_mm->mmlist))) {
 				spin_lock(&mmlist_lock);
@@ -443,6 +445,19 @@ copy_one_pte(struct mm_struct *dst_mm, s
 						 &src_mm->mmlist);
 				spin_unlock(&mmlist_lock);
 			}
+			if (is_migration_entry(entry) &&
+					is_cow_mapping(vm_flags)) {
+				page = migration_entry_to_page(entry);
+
+				/*
+				 * COW mappings require pages in both parent
+				*  and child to be set to read.
+				 */
+				entry = make_migration_entry(page,
+	`					SWP_MIGRATION_READ);
+				pte = swp_entry_to_pte(entry);
+				set_pte_at(src_mm, addr, src_pte, pte);
+			}
 		}
 		goto out_set_pte;
 	}

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