[PATCH] mm: unmapped page migration avoid unmap+remap overhead

[Hugh Dickins <hughd@google.com>]

Page migration's __unmap_and_move(), and rmap's try_to_unmap(),
were created for use on pages almost certainly mapped into userspace.
But nowadays compaction often applies them to unmapped page cache pages:
which may exacerbate contention on i_mmap_rwsem quite unnecessarily,
since try_to_unmap_file() makes no preliminary page_mapped() check.

Now check page_mapped() in __unmap_and_move(); and avoid repeating the
same overhead in rmap_walk_file() - don't remove_migration_ptes() when
we never inserted any.

(The PageAnon(page) comment blocks now look even sillier than before,
but clean that up on some other occasion.  And note in passing that
try_to_unmap_one() does not use a migration entry when PageSwapCache,
so remove_migration_ptes() will then not update that swap entry to
newpage pte: not a big deal, but something else to clean up later.)

Davidlohr remarked in "mm,fs: introduce helpers around the i_mmap_mutex"
conversion to i_mmap_rwsem, that "The biggest winner of these changes
is migration": a part of the reason might be all of that unnecessary
taking of i_mmap_mutex in page migration; and it's rather a shame that
I didn't get around to sending this patch in before his - this one is
much less useful after Davidlohr's conversion to rwsem, but still good.

Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/migrate.c |   28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

--- 3.18-rc7/mm/migrate.c	2014-10-19 22:12:56.809625067 -0700
+++ linux/mm/migrate.c	2014-11-30 20:17:51.205187663 -0800
@@ -746,7 +746,7 @@ static int fallback_migrate_page(struct
  *  MIGRATEPAGE_SUCCESS - success
  */
 static int move_to_new_page(struct page *newpage, struct page *page,
-				int remap_swapcache, enum migrate_mode mode)
+				int page_was_mapped, enum migrate_mode mode)
 {
 	struct address_space *mapping;
 	int rc;
@@ -784,7 +784,7 @@ static int move_to_new_page(struct page
 		newpage->mapping = NULL;
 	} else {
 		mem_cgroup_migrate(page, newpage, false);
-		if (remap_swapcache)
+		if (page_was_mapped)
 			remove_migration_ptes(page, newpage);
 		page->mapping = NULL;
 	}
@@ -798,7 +798,7 @@ static int __unmap_and_move(struct page
 				int force, enum migrate_mode mode)
 {
 	int rc = -EAGAIN;
-	int remap_swapcache = 1;
+	int page_was_mapped = 0;
 	struct anon_vma *anon_vma = NULL;
 
 	if (!trylock_page(page)) {
@@ -870,7 +870,6 @@ static int __unmap_and_move(struct page
 			 * migrated but are not remapped when migration
 			 * completes
 			 */
-			remap_swapcache = 0;
 		} else {
 			goto out_unlock;
 		}
@@ -910,13 +909,17 @@ static int __unmap_and_move(struct page
 	}
 
 	/* Establish migration ptes or remove ptes */
-	try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+	if (page_mapped(page)) {
+		try_to_unmap(page,
+			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+		page_was_mapped = 1;
+	}
 
 skip_unmap:
 	if (!page_mapped(page))
-		rc = move_to_new_page(newpage, page, remap_swapcache, mode);
+		rc = move_to_new_page(newpage, page, page_was_mapped, mode);
 
-	if (rc && remap_swapcache)
+	if (rc && page_was_mapped)
 		remove_migration_ptes(page, page);
 
 	/* Drop an anon_vma reference if we took one */
@@ -1017,6 +1020,7 @@ static int unmap_and_move_huge_page(new_
 {
 	int rc = 0;
 	int *result = NULL;
+	int page_was_mapped = 0;
 	struct page *new_hpage;
 	struct anon_vma *anon_vma = NULL;
 
@@ -1047,12 +1051,16 @@ static int unmap_and_move_huge_page(new_
 	if (PageAnon(hpage))
 		anon_vma = page_get_anon_vma(hpage);
 
-	try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+	if (page_mapped(hpage)) {
+		try_to_unmap(hpage,
+			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+		page_was_mapped = 1;
+	}
 
 	if (!page_mapped(hpage))
-		rc = move_to_new_page(new_hpage, hpage, 1, mode);
+		rc = move_to_new_page(new_hpage, hpage, page_was_mapped, mode);
 
-	if (rc != MIGRATEPAGE_SUCCESS)
+	if (rc != MIGRATEPAGE_SUCCESS && page_was_mapped)
 		remove_migration_ptes(hpage, hpage);
 
 	if (anon_vma)