From: Christoph Lameter <clameter@sgi.com>

Add migration swap type and functions to handle migration entries

SWP_TYPE_MIGRATION is a special swap type that encodes the pfn of the page in
the swp_offset.  SWP_TYPE_MIGRATION swap entries are only set for a pte while
the corresponding page is locked.  Migration entries are removed while the
page is still locked.  Therefore the processing for this special type of swap
page can be simple.

Only freeing and duplication operations are supported for copy_page_range and
zap_range.  The freeing of this type of entry is ignored and we also simply do
nothing on duplication relying on the reverse maps to track replications of
the pte.

If do_swap_page encounters a migration entry then it simply redoes the fault
until the migration entry has gone away.  We used to take a page count on the
old page which frequently caused the page migration code to retry again. 
Redoing the fault immediately avoids migration retries.

Migration entry related operations work even if CONFIG_SWAP has not been
switched on.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 include/linux/swap.h    |    6 ++++++
 include/linux/swapops.h |   32 ++++++++++++++++++++++++++++++++
 mm/memory.c             |    6 ++++++
 mm/swapfile.c           |    6 ++++++
 4 files changed, 50 insertions(+)

diff -puN include/linux/swap.h~swapless-v2-add-migration-swap-entries include/linux/swap.h
--- devel/include/linux/swap.h~swapless-v2-add-migration-swap-entries	2006-04-13 17:09:53.000000000 -0700
+++ devel-akpm/include/linux/swap.h	2006-04-13 17:09:53.000000000 -0700
@@ -29,7 +29,13 @@ static inline int current_is_kswapd(void
  * the type/offset into the pte as 5/27 as well.
  */
 #define MAX_SWAPFILES_SHIFT	5
+#ifndef CONFIG_MIGRATION
 #define MAX_SWAPFILES		(1 << MAX_SWAPFILES_SHIFT)
+#else
+/* Use last entry for page migration swap entries */
+#define MAX_SWAPFILES		((1 << MAX_SWAPFILES_SHIFT)-1)
+#define SWP_TYPE_MIGRATION	MAX_SWAPFILES
+#endif
 
 /*
  * Magic header for a swap area. The first part of the union is
diff -puN include/linux/swapops.h~swapless-v2-add-migration-swap-entries include/linux/swapops.h
--- devel/include/linux/swapops.h~swapless-v2-add-migration-swap-entries	2006-04-13 17:09:53.000000000 -0700
+++ devel-akpm/include/linux/swapops.h	2006-04-13 17:09:53.000000000 -0700
@@ -67,3 +67,35 @@ static inline pte_t swp_entry_to_pte(swp
 	BUG_ON(pte_file(__swp_entry_to_pte(arch_entry)));
 	return __swp_entry_to_pte(arch_entry);
 }
+
+#ifdef CONFIG_MIGRATION
+static inline swp_entry_t make_migration_entry(struct page *page)
+{
+	BUG_ON(!PageLocked(page));
+	return swp_entry(SWP_TYPE_MIGRATION, page_to_pfn(page));
+}
+
+static inline int is_migration_entry(swp_entry_t entry)
+{
+	return swp_type(entry) == SWP_TYPE_MIGRATION;
+}
+
+static inline struct page *migration_entry_to_page(swp_entry_t entry)
+{
+	struct page *p = pfn_to_page(swp_offset(entry));
+	/*
+	 * Any use of migration entries may only occur while the
+	 * corresponding page is locked
+	 */
+	BUG_ON(!PageLocked(p));
+	BUG_ON(!is_migration_entry(entry));
+	return p;
+}
+#else
+
+#define make_migration_entry(page) swp_entry(0, 0)
+#define is_migration_entry(swp) 0
+#define migration_entry_to_page(swp) NULL
+
+#endif
+
diff -puN mm/memory.c~swapless-v2-add-migration-swap-entries mm/memory.c
--- devel/mm/memory.c~swapless-v2-add-migration-swap-entries	2006-04-13 17:09:53.000000000 -0700
+++ devel-akpm/mm/memory.c	2006-04-13 17:09:53.000000000 -0700
@@ -1879,6 +1879,12 @@ static int do_swap_page(struct mm_struct
 		goto out;
 
 	entry = pte_to_swp_entry(orig_pte);
+
+	if (unlikely(is_migration_entry(entry))) {
+		yield();
+		goto out;
+	}
+
 	page = lookup_swap_cache(entry);
 	if (!page) {
  		swapin_readahead(entry, address, vma);
diff -puN mm/swapfile.c~swapless-v2-add-migration-swap-entries mm/swapfile.c
--- devel/mm/swapfile.c~swapless-v2-add-migration-swap-entries	2006-04-13 17:09:53.000000000 -0700
+++ devel-akpm/mm/swapfile.c	2006-04-13 17:09:53.000000000 -0700
@@ -395,6 +395,9 @@ void free_swap_and_cache(swp_entry_t ent
 	struct swap_info_struct * p;
 	struct page *page = NULL;
 
+	if (is_migration_entry(entry))
+		return;
+
 	p = swap_info_get(entry);
 	if (p) {
 		if (swap_entry_free(p, swp_offset(entry)) == 1) {
@@ -1702,6 +1705,9 @@ int swap_duplicate(swp_entry_t entry)
 	unsigned long offset, type;
 	int result = 0;
 
+	if (is_migration_entry(entry))
+		return 1;
+
 	type = swp_type(entry);
 	if (type >= nr_swapfiles)
 		goto bad_file;
_