mm/zswap: refactor the get/put routines

The refcount routine was not fit the kernel get/put semantic exactly,
There were too many judgement statements on refcount and it could be
minus.

This patch does the following:

 - move refcount judgement to zswap_entry_put() to hide resource free function.

 - add a new function zswap_entry_find_get(), so that callers can use
   easily in the following pattern:

     zswap_entry_find_get
     .../* do something */
     zswap_entry_put

 - to eliminate compile error, move some functions declaration

This patch is based on Minchan Kim <minchan@kernel.org> 's idea and suggestion.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Cc: Seth Jennings <sjennings@variantweb.net>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 88 insertions, 94 deletions

diff --git a/mm/zswap.c b/mm/zswap.c
index 0ffcad03baea..5a63f78a5601 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -217,6 +217,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
         if (!entry)
                 return NULL;
         entry->refcount = 1;
+        RB_CLEAR_NODE(&entry->rbnode);
         return entry;
 }
 
@@ -225,19 +226,6 @@ static void zswap_entry_cache_free(struct zswap_entry *entry)
         kmem_cache_free(zswap_entry_cache, entry);
 }
 
-/* caller must hold the tree lock */
-static void zswap_entry_get(struct zswap_entry *entry)
-{
-        entry->refcount++;
-}
-
-/* caller must hold the tree lock */
-static int zswap_entry_put(struct zswap_entry *entry)
-{
-        entry->refcount--;
-        return entry->refcount;
-}
-
 /*********************************
 * rbtree functions
 **********************************/
@@ -285,6 +273,61 @@ static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
         return 0;
 }
 
+static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
+{
+        if (!RB_EMPTY_NODE(&entry->rbnode)) {
+                rb_erase(&entry->rbnode, root);
+                RB_CLEAR_NODE(&entry->rbnode);
+        }
+}
+
+/*
+ * Carries out the common pattern of freeing and entry's zsmalloc allocation,
+ * freeing the entry itself, and decrementing the number of stored pages.
+ */
+static void zswap_free_entry(struct zswap_tree *tree,
+                        struct zswap_entry *entry)
+{
+        zbud_free(tree->pool, entry->handle);
+        zswap_entry_cache_free(entry);
+        atomic_dec(&zswap_stored_pages);
+        zswap_pool_pages = zbud_get_pool_size(tree->pool);
+}
+
+/* caller must hold the tree lock */
+static void zswap_entry_get(struct zswap_entry *entry)
+{
+        entry->refcount++;
+}
+
+/* caller must hold the tree lock
+* remove from the tree and free it, if nobody reference the entry
+*/
+static void zswap_entry_put(struct zswap_tree *tree,
+                        struct zswap_entry *entry)
+{
+        int refcount = --entry->refcount;
+
+        BUG_ON(refcount < 0);
+        if (refcount == 0) {
+                zswap_rb_erase(&tree->rbroot, entry);
+                zswap_free_entry(tree, entry);
+        }
+}
+
+/* caller must hold the tree lock */
+static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
+                                pgoff_t offset)
+{
+        struct zswap_entry *entry = NULL;
+
+        entry = zswap_rb_search(root, offset);
+        if (entry)
+                zswap_entry_get(entry);
+
+        return entry;
+}
+
 /*********************************
 * per-cpu code
 **********************************/
@@ -368,18 +411,6 @@ static bool zswap_is_full(void)
                 zswap_pool_pages);
 }
 
-/*
- * Carries out the common pattern of freeing and entry's zsmalloc allocation,
- * freeing the entry itself, and decrementing the number of stored pages.
- */
-static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
-{
-        zbud_free(tree->pool, entry->handle);
-        zswap_entry_cache_free(entry);
-        atomic_dec(&zswap_stored_pages);
-        zswap_pool_pages = zbud_get_pool_size(tree->pool);
-}
-
 /*********************************
 * writeback code
 **********************************/
@@ -503,7 +534,7 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
         struct page *page;
         u8 *src, *dst;
         unsigned int dlen;
-        int ret, refcount;
+        int ret;
         struct writeback_control wbc = {
                 .sync_mode = WB_SYNC_NONE,
         };
@@ -518,13 +549,12 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
 
         /* find and ref zswap entry */
         spin_lock(&tree->lock);
-        entry = zswap_rb_search(&tree->rbroot, offset);
+        entry = zswap_entry_find_get(&tree->rbroot, offset);
         if (!entry) {
                 /* entry was invalidated */
                 spin_unlock(&tree->lock);
                 return 0;
         }
-        zswap_entry_get(entry);
         spin_unlock(&tree->lock);
         BUG_ON(offset != entry->offset);
 
@@ -566,42 +596,35 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
         zswap_written_back_pages++;
 
         spin_lock(&tree->lock);
-
         /* drop local reference */
-        zswap_entry_put(entry);
-        /* drop the initial reference from entry creation */
-        refcount = zswap_entry_put(entry);
+        zswap_entry_put(tree, entry);
 
         /*
-         * There are three possible values for refcount here:
-         * (1) refcount is 1, load is in progress, unlink from rbtree,
-         *     load will free
-         * (2) refcount is 0, (normal case) entry is valid,
-         *     remove from rbtree and free entry
-         * (3) refcount is -1, invalidate happened during writeback,
-         *     free entry
-         */
-        if (refcount >= 0) {
-                /* no invalidate yet, remove from rbtree */
-                rb_erase(&entry->rbnode, &tree->rbroot);
-        }
+        * There are two possible situations for entry here:
+        * (1) refcount is 1(normal case),  entry is valid and on the tree
+        * (2) refcount is 0, entry is freed and not on the tree
+        *     because invalidate happened during writeback
+        *  search the tree and free the entry if find entry
+        */
+        if (entry == zswap_rb_search(&tree->rbroot, offset))
+                zswap_entry_put(tree, entry);
         spin_unlock(&tree->lock);
-        if (refcount <= 0) {
-                /* free the entry */
-                zswap_free_entry(tree, entry);
-                return 0;
-        }
-        return -EAGAIN;
 
+        goto end;
+
+        /*
+        * if we get here due to ZSWAP_SWAPCACHE_EXIST
+        * a load may happening concurrently
+        * it is safe and okay to not free the entry
+        * if we free the entry in the following put
+        * it it either okay to return !0
+        */
 fail:
         spin_lock(&tree->lock);
-        refcount = zswap_entry_put(entry);
-        if (refcount <= 0) {
-                /* invalidate happened, consider writeback as success */
-                zswap_free_entry(tree, entry);
-                ret = 0;
-        }
+        zswap_entry_put(tree, entry);
         spin_unlock(&tree->lock);
+
+end:
         return ret;
 }
 
@@ -685,11 +708,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
                 if (ret == -EEXIST) {
                         zswap_duplicate_entry++;
                         /* remove from rbtree */
-                        rb_erase(&dupentry->rbnode, &tree->rbroot);
-                        if (!zswap_entry_put(dupentry)) {
-                                /* free */
-                                zswap_free_entry(tree, dupentry);
-                        }
+                        zswap_rb_erase(&tree->rbroot, dupentry);
+                        zswap_entry_put(tree, dupentry);
                 }
         } while (ret == -EEXIST);
         spin_unlock(&tree->lock);
@@ -718,17 +738,16 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
         struct zswap_entry *entry;
         u8 *src, *dst;
         unsigned int dlen;
-        int refcount, ret;
+        int ret;
 
         /* find */
         spin_lock(&tree->lock);
-        entry = zswap_rb_search(&tree->rbroot, offset);
+        entry = zswap_entry_find_get(&tree->rbroot, offset);
         if (!entry) {
                 /* entry was written back */
                 spin_unlock(&tree->lock);
                 return -1;
         }
-        zswap_entry_get(entry);
         spin_unlock(&tree->lock);
 
         /* decompress */
@@ -743,22 +762,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
         BUG_ON(ret);
 
         spin_lock(&tree->lock);
-        refcount = zswap_entry_put(entry);
-        if (likely(refcount)) {
-                spin_unlock(&tree->lock);
-                return 0;
-        }
+        zswap_entry_put(tree, entry);
         spin_unlock(&tree->lock);
 
-        /*
-         * We don't have to unlink from the rbtree because
-         * zswap_writeback_entry() or zswap_frontswap_invalidate page()
-         * has already done this for us if we are the last reference.
-         */
-        /* free */
-
-        zswap_free_entry(tree, entry);
-
         return 0;
 }
 
@@ -767,7 +773,6 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
 {
         struct zswap_tree *tree = zswap_trees[type];
         struct zswap_entry *entry;
-        int refcount;
 
         /* find */
         spin_lock(&tree->lock);
@@ -779,20 +784,12 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
         }
 
         /* remove from rbtree */
-        rb_erase(&entry->rbnode, &tree->rbroot);
+        zswap_rb_erase(&tree->rbroot, entry);
 
         /* drop the initial reference from entry creation */
-        refcount = zswap_entry_put(entry);
+        zswap_entry_put(tree, entry);
 
         spin_unlock(&tree->lock);
-
-        if (refcount) {
-                /* writeback in progress, writeback will free */
-                return;
-        }
-
-        /* free */
-        zswap_free_entry(tree, entry);
 }
 
 /* frees all zswap entries for the given swap type */
@@ -806,11 +803,8 @@ static void zswap_frontswap_invalidate_area(unsigned type)
 
         /* walk the tree and free everything */
         spin_lock(&tree->lock);
-        rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode) {
-                zbud_free(tree->pool, entry->handle);
-                zswap_entry_cache_free(entry);
-                atomic_dec(&zswap_stored_pages);
-        }
+        rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode)
+                zswap_free_entry(tree, entry);
         tree->rbroot = RB_ROOT;
         spin_unlock(&tree->lock);