mm: add swap cache interface for swap reference

In a following patch, the usage of swap cache is recorded into swap_map.
This patch is for necessary interface changes to do that.

2 interfaces:

  - swapcache_prepare()
  - swapcache_free()

are added for allocating/freeing refcnt from swap-cache to existing swap
entries.  But implementation itself is not changed under this patch.  At
adding swapcache_free(), memcg's hook code is moved under
swapcache_free().  This is better than using scattered hooks.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Balbir Singh <balbir@in.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 26 insertions, 9 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 0132fbd..47ab191 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1097,7 +1097,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	shmem_swp_unmap(entry);
 unlock:
 	spin_unlock(&info->lock);
-	swap_free(swap);
+	swapcache_free(swap, NULL);
 redirty:
 	set_page_dirty(page);
 	if (wbc->for_reclaim)
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 1416e7e..19bdf30 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -162,11 +162,11 @@ int add_to_swap(struct page *page)
 			return 1;
 		case -EEXIST:
 			/* Raced with "speculative" read_swap_cache_async */
-			swap_free(entry);
+			swapcache_free(entry, NULL);
 			continue;
 		default:
 			/* -ENOMEM radix-tree allocation failure */
-			swap_free(entry);
+			swapcache_free(entry, NULL);
 			return 0;
 		}
 	}
@@ -188,8 +188,7 @@ void delete_from_swap_cache(struct page *page)
 	__delete_from_swap_cache(page);
 	spin_unlock_irq(&swapper_space.tree_lock);
 
-	mem_cgroup_uncharge_swapcache(page, entry);
-	swap_free(entry);
+	swapcache_free(entry, page);
 	page_cache_release(page);
 }
 
@@ -293,7 +292,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		/*
 		 * Swap entry may have been freed since our caller observed it.
 		 */
-		if (!swap_duplicate(entry))
+		if (!swapcache_prepare(entry))
 			break;
 
 		/*
@@ -317,7 +316,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		}
 		ClearPageSwapBacked(new_page);
 		__clear_page_locked(new_page);
-		swap_free(entry);
+		swapcache_free(entry, NULL);
 	} while (err != -ENOMEM);
 
 	if (new_page)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 312fafe..3187079 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -510,6 +510,16 @@ void swap_free(swp_entry_t entry)
 }
 
 /*
+ * Called after dropping swapcache to decrease refcnt to swap entries.
+ */
+void swapcache_free(swp_entry_t entry, struct page *page)
+{
+	if (page)
+		mem_cgroup_uncharge_swapcache(page, entry);
+	return swap_free(entry);
+}
+
+/*
  * How many references to page are currently swapped out?
  */
 static inline int page_swapcount(struct page *page)
@@ -1979,6 +1989,15 @@ bad_file:
 	goto out;
 }
 
+/*
+ * Called when allocating swap cache for exising swap entry,
+ */
+int swapcache_prepare(swp_entry_t entry)
+{
+	return swap_duplicate(entry);
+}
+
+
 struct swap_info_struct *
 get_swap_info_struct(unsigned type)
 {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2c4b945..52339dd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -470,8 +470,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
 		swp_entry_t swap = { .val = page_private(page) };
 		__delete_from_swap_cache(page);
 		spin_unlock_irq(&mapping->tree_lock);
-		mem_cgroup_uncharge_swapcache(page, swap);
-		swap_free(swap);
+		swapcache_free(swap, page);
 	} else {
 		__remove_from_page_cache(page);
 		spin_unlock_irq(&mapping->tree_lock);