Preserve write permissions in migration entries

[Christoph Lameter]

I cleaned up the patch a bit and ran some tests.


This patch implements the preservation of the write permissions.
The preservation of write permission avoids unnecessary COW operations
following page migration.

Signed-off-by: Christoph Lameter <clameter@xxxxxxx>
Signed-Off-By: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Index: linux-2.6.17-rc1-mm2/include/linux/swap.h
===================================================================
--- linux-2.6.17-rc1-mm2.orig/include/linux/swap.h	2006-04-14 09:08:42.000000000 -0700
+++ linux-2.6.17-rc1-mm2/include/linux/swap.h	2006-04-14 10:01:27.000000000 -0700
@@ -33,8 +33,9 @@ static inline int current_is_kswapd(void
 #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
+#define MAX_SWAPFILES		((1 << MAX_SWAPFILES_SHIFT)-2)
+#define SWP_MIGRATION_READ	MAX_SWAPFILES
+#define SWP_MIGRATION_WRITE	(MAX_SWAPFILES + 1)
 #endif
 
 /*
Index: linux-2.6.17-rc1-mm2/include/linux/swapops.h
===================================================================
--- linux-2.6.17-rc1-mm2.orig/include/linux/swapops.h	2006-04-14 09:55:25.000000000 -0700
+++ linux-2.6.17-rc1-mm2/include/linux/swapops.h	2006-04-14 10:06:43.000000000 -0700
@@ -69,15 +69,22 @@ static inline pte_t swp_entry_to_pte(swp
 }
 
 #ifdef CONFIG_MIGRATION
-static inline swp_entry_t make_migration_entry(struct page *page)
+static inline swp_entry_t make_migration_entry(struct page *page, int write)
 {
 	BUG_ON(!PageLocked(page));
-	return swp_entry(SWP_TYPE_MIGRATION, page_to_pfn(page));
+	return swp_entry(write ? SWP_MIGRATION_WRITE : SWP_MIGRATION_READ,
+			 page_to_pfn(page));
 }
 
 static inline int is_migration_entry(swp_entry_t entry)
 {
-	return unlikely(swp_type(entry) == SWP_TYPE_MIGRATION);
+	return unlikely(swp_type(entry) == SWP_MIGRATION_READ ||
+			swp_type(entry) == SWP_MIGRATION_WRITE);
+}
+
+static inline int is_write_migration_entry(swp_entry_t entry)
+{
+	return swp_type(entry) == SWP_MIGRATION_WRITE;
 }
 
 static inline struct page *migration_entry_to_page(swp_entry_t entry)
Index: linux-2.6.17-rc1-mm2/mm/rmap.c
===================================================================
--- linux-2.6.17-rc1-mm2.orig/mm/rmap.c	2006-04-13 16:43:24.000000000 -0700
+++ linux-2.6.17-rc1-mm2/mm/rmap.c	2006-04-14 10:04:27.000000000 -0700
@@ -602,7 +602,7 @@ static int try_to_unmap_one(struct page 
 			 * pte is removed and then restart fault handling.
 			 */
 			BUG_ON(!migration);
-			entry = make_migration_entry(page);
+			entry = make_migration_entry(page, pte_write(pteval));
 		}
 		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
 		BUG_ON(pte_file(*pte));
Index: linux-2.6.17-rc1-mm2/mm/migrate.c
===================================================================
--- linux-2.6.17-rc1-mm2.orig/mm/migrate.c	2006-04-13 16:44:07.000000000 -0700
+++ linux-2.6.17-rc1-mm2/mm/migrate.c	2006-04-14 10:06:01.000000000 -0700
@@ -167,7 +167,10 @@ static void remove_migration_pte(struct 
 
 	inc_mm_counter(mm, anon_rss);
 	get_page(new);
-	set_pte_at(mm, addr, ptep, pte_mkold(mk_pte(new, vma->vm_page_prot)));
+	pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
+	if (is_write_migration_entry(entry))
+		pte = pte_mkwrite(pte);
+	set_pte_at(mm, addr, ptep, pte);
 	page_add_anon_rmap(new, vma, addr);
 out:
 	pte_unmap_unlock(pte, ptl);
-
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