Add ephemeral items

Ephemeral items exist to reference external values.  They're going to be
used by the page cache to reference dirty pages for writeback.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/counters.h

@@ -25,6 +25,8 @@
 	EXPAND_COUNTER(compact_segment_read)	\
 	EXPAND_COUNTER(compact_segment_written)	\
 	EXPAND_COUNTER(item_create)		\
+	EXPAND_COUNTER(item_create_ephemeral)	\
+	EXPAND_COUNTER(item_update_ephemeral)	\
 	EXPAND_COUNTER(item_lookup_hit)		\
 	EXPAND_COUNTER(item_lookup_miss)	\
 	EXPAND_COUNTER(item_delete)		\

kmod/src/item.c

@@ -64,7 +64,8 @@ struct cached_item {
 	};
 
 	long dirty;
-	unsigned deletion:1;
+	unsigned deletion:1,
+	         ephemeral:1;
 
 	struct scoutfs_key_buf *key;
 
@@ -87,7 +88,8 @@ static void free_item(struct super_block *sb, struct cached_item *item)
 {
 	if (!IS_ERR_OR_NULL(item)) {
 		scoutfs_key_free(sb, item->key);
-		scoutfs_kvec_kfree(item->val);
+		if (!item->ephemeral)
+			scoutfs_kvec_kfree(item->val);
 		kfree(item);
 	}
 }
@@ -344,7 +346,7 @@ static void erase_item(struct super_block *sb, struct item_cache *cac,
  * The caller is responsible for marking the newly inserted item dirty.
  */
 static int insert_item(struct super_block *sb, struct item_cache *cac,
-		       struct cached_item *ins)
+		       struct cached_item *ins, bool overwrite)
 {
 	struct rb_root *root = &cac->items;
 	struct cached_item *item;
@@ -369,7 +371,7 @@ restart:
 				item->dirty |= RIGHT_DIRTY;
 			node = &(*node)->rb_right;
 		} else {
-			if (!item->deletion)
+			if (!item->deletion && !overwrite)
 				return -EEXIST;
 
 			/* sadly there's no augmented replace */
@@ -767,7 +769,7 @@ int scoutfs_item_create(struct super_block *sb, struct scoutfs_key_buf *key,
 		return -ENOMEM;
 
 	spin_lock_irqsave(&cac->lock, flags);
-	ret = insert_item(sb, cac, item);
+	ret = insert_item(sb, cac, item, false);
 	if (!ret) {
 		scoutfs_inc_counter(sb, item_create);
 		mark_item_dirty(cac, item);
@@ -780,6 +782,74 @@ int scoutfs_item_create(struct super_block *sb, struct scoutfs_key_buf *key,
 	return ret;
 }
 
+/*
+ * Ephemeral items are slightly magical and used to track file contents
+ * without copying the data into an allocated value.
+ *
+ * Their value kvec clones the callers which means they reference
+ * external data.  They're freed after items are copied into segments so
+ * that callers can know that no items reference their structures after
+ * a commit finishes.
+ *
+ * They forcefully clobber any existing item at their key without
+ * reading the existing item.
+ */
+int scoutfs_item_create_ephemeral(struct super_block *sb,
+			          struct scoutfs_key_buf *key,
+				  struct kvec *val)
+{
+	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
+	struct item_cache *cac = sbi->item_cache;
+	struct cached_item *item;
+	unsigned long flags;
+	int ret;
+
+	item = alloc_item(sb, key, NULL);
+	if (!item)
+		return -ENOMEM;
+
+	scoutfs_kvec_clone(item->val, val);
+	item->ephemeral = 1;
+
+	spin_lock_irqsave(&cac->lock, flags);
+
+	ret = insert_item(sb, cac, item, true);
+	BUG_ON(ret);
+
+	scoutfs_inc_counter(sb, item_create_ephemeral);
+	mark_item_dirty(cac, item);
+
+	spin_unlock_irqrestore(&cac->lock, flags);
+
+	return ret;
+}
+
+/*
+ * Update the value for an ephemeral item if it exists.
+ */
+void scoutfs_item_update_ephemeral(struct super_block *sb,
+				   struct scoutfs_key_buf *key,
+				   struct kvec *val)
+{
+	struct scoutfs_sb_info *sbi = SCOUTFS_SB(sb);
+	struct item_cache *cac = sbi->item_cache;
+	struct cached_item *item;
+	unsigned long flags;
+
+	spin_lock_irqsave(&cac->lock, flags);
+
+	item = find_item(sb, &cac->items, key);
+	if (item && item->ephemeral) {
+		trace_printk("updating ephemeral item %p\n", item);
+		scoutfs_inc_counter(sb, item_update_ephemeral);
+		clear_item_dirty(cac, item);
+		scoutfs_kvec_clone(item->val, val);
+		mark_item_dirty(cac, item);
+	}
+
+	spin_unlock_irqrestore(&cac->lock, flags);
+}
+
 /*
  * Allocate an item with the key and value and add it to the list of
  * items to be inserted as a batch later.  The caller adds in sort order
@@ -852,7 +922,7 @@ int scoutfs_item_insert_batch(struct super_block *sb, struct list_head *list,
 
 	list_for_each_entry_safe(item, tmp, list, entry) {
 		list_del(&item->entry);
-		if (insert_item(sb, cac, item))
+		if (insert_item(sb, cac, item, false))
 			list_add(&item->entry, list);
 	}
 
@@ -1314,7 +1384,7 @@ int scoutfs_item_dirty_seg(struct super_block *sb, struct scoutfs_segment *seg)
 		del = item;
 		item = next_dirty(item);
 
-		if (del->deletion)
+		if (del->deletion || del->ephemeral)
 			erase_item(sb, cac, del);
 
 		nr_items--;

kmod/src/item.h

@@ -23,9 +23,15 @@ int scoutfs_item_insert(struct super_block *sb, struct scoutfs_key_buf *key,
 		        struct kvec *val);
 int scoutfs_item_create(struct super_block *sb, struct scoutfs_key_buf *key,
 		        struct kvec *val);
+int scoutfs_item_create_ephemeral(struct super_block *sb,
+			          struct scoutfs_key_buf *key,
+				  struct kvec *val);
 int scoutfs_item_dirty(struct super_block *sb, struct scoutfs_key_buf *key);
 int scoutfs_item_update(struct super_block *sb, struct scoutfs_key_buf *key,
 			struct kvec *val);
+void scoutfs_item_update_ephemeral(struct super_block *sb,
+				   struct scoutfs_key_buf *key,
+				   struct kvec *val);
 void scoutfs_item_delete_dirty(struct super_block *sb,
 			       struct scoutfs_key_buf *key);
 int scoutfs_item_delete_many(struct super_block *sb,