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>

diff --git a/mm/zswap.c b/mm/zswap.c
index 0ffcad0..5a63f78 100644 (file)
--- 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);