Btrfs: Fix free block discard calls down to the block layer

This is a patch to fix discard semantic to make Btrfs work with FTL and SSD.
We can improve FTL's performance by telling it which sectors are freed by file
system. But if we don't tell FTL the information of free sectors in proper
time, the transaction mechanism of Btrfs will be destroyed and Btrfs could not
roll back the previous transaction under the power loss condition.

There are some problems in the old implementation:
1, In __free_extent(), the pinned down extents should not be discarded.
2, In free_extents(), the free extents are all pinned, so they need to
be discarded in transaction committing time instead of free_extents().
3, The reserved extent used by log tree should be discard too.

This patch change discard behavior as follows:
1, For the extents which need to be free at once,
   we discard them in update_block_group().
2, Delay discarding the pinned extent in btrfs_finish_extent_commit()
   when committing transaction.
3, Remove discarding from free_extents() and __free_extent()
4, Add discard interface into btrfs_free_reserved_extent()
5, Discard sectors before updating the free space cache, otherwise,
   FTL will destroy file system data.

fs/btrfs/extent-tree.c

@@ -876,6 +876,38 @@ static void btrfs_issue_discard(struct block_device *bdev,
 }
 #endif
 
+static int btrfs_discard_extent(struct btrfs_root *root, u64 bytenr,
+				u64 num_bytes)
+{
+#ifdef BIO_RW_DISCARD
+	int ret;
+	u64 map_length = num_bytes;
+	struct btrfs_multi_bio *multi = NULL;
+
+	/* Tell the block device(s) that the sectors can be discarded */
+	ret = btrfs_map_block(&root->fs_info->mapping_tree, READ,
+			      bytenr, &map_length, &multi, 0);
+	if (!ret) {
+		struct btrfs_bio_stripe *stripe = multi->stripes;
+		int i;
+
+		if (map_length > num_bytes)
+			map_length = num_bytes;
+
+		for (i = 0; i < multi->num_stripes; i++, stripe++) {
+			btrfs_issue_discard(stripe->dev->bdev,
+					    stripe->physical,
+					    map_length);
+		}
+		kfree(multi);
+	}
+
+	return ret;
+#else
+	return 0;
+#endif
+}
+
 static int noinline free_extents(struct btrfs_trans_handle *trans,
 				 struct btrfs_root *extent_root,
 				 struct list_head *del_list)
@@ -1069,10 +1101,6 @@ search:
 		for (pos = cur, n = pos->next; pos != end;
 		     pos = n, n = pos->next) {
 			struct pending_extent_op *tmp;
-#ifdef BIO_RW_DISCARD
-			u64 map_length;
-			struct btrfs_multi_bio *multi = NULL;
-#endif
 			tmp = list_entry(pos, struct pending_extent_op, list);
 
 			/*
@@ -1084,28 +1112,6 @@ search:
 						 tmp->del);
 			BUG_ON(ret);
 
-#ifdef BIO_RW_DISCARD
-			map_length = tmp->num_bytes;
-			ret = btrfs_map_block(&info->mapping_tree, READ,
-					      tmp->bytenr, &map_length, &multi,
-					      0);
-			if (!ret) {
-				struct btrfs_bio_stripe *stripe;
-				int i;
-
-				stripe = multi->stripes;
-
-				if (map_length > tmp->num_bytes)
-					map_length = tmp->num_bytes;
-
-				for (i = 0; i < multi->num_stripes;
-				     i++, stripe++)
-					btrfs_issue_discard(stripe->dev->bdev,
-							    stripe->physical,
-							    map_length);
-				kfree(multi);
-			}
-#endif
 			list_del_init(&tmp->list);
 			unlock_extent(&info->extent_ins, tmp->bytenr,
 				      tmp->bytenr + tmp->num_bytes - 1,
@@ -1965,6 +1971,11 @@ static int update_block_group(struct btrfs_trans_handle *trans,
 			spin_unlock(&cache->space_info->lock);
 			if (mark_free) {
 				int ret;
+
+				ret = btrfs_discard_extent(root, bytenr,
+							   num_bytes);
+				WARN_ON(ret);
+
 				ret = btrfs_add_free_space(cache, bytenr,
 							   num_bytes);
 				WARN_ON(ret);
@@ -2104,8 +2115,12 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans,
 					    EXTENT_DIRTY);
 		if (ret)
 			break;
+
+		ret = btrfs_discard_extent(root, start, end + 1 - start);
+
 		btrfs_update_pinned_extents(root, start, end + 1 - start, 0);
 		clear_extent_dirty(unpin, start, end, GFP_NOFS);
+
 		if (need_resched()) {
 			mutex_unlock(&root->fs_info->pinned_mutex);
 			cond_resched();
@@ -2113,7 +2128,7 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans,
 		}
 	}
 	mutex_unlock(&root->fs_info->pinned_mutex);
-	return 0;
+	return ret;
 }
 
 static int finish_current_insert(struct btrfs_trans_handle *trans,
@@ -2458,10 +2473,6 @@ static int __free_extent(struct btrfs_trans_handle *trans,
 	if (refs == 0) {
 		u64 super_used;
 		u64 root_used;
-#ifdef BIO_RW_DISCARD
-		u64 map_length = num_bytes;
-		struct btrfs_multi_bio *multi = NULL;
-#endif
 
 		if (pin) {
 			mutex_lock(&root->fs_info->pinned_mutex);
@@ -2496,25 +2507,6 @@ static int __free_extent(struct btrfs_trans_handle *trans,
 		ret = update_block_group(trans, root, bytenr, num_bytes, 0,
 					 mark_free);
 		BUG_ON(ret);
-#ifdef BIO_RW_DISCARD
-		/* Tell the block device(s) that the sectors can be discarded */
-		ret = btrfs_map_block(&root->fs_info->mapping_tree, READ,
-				      bytenr, &map_length, &multi, 0);
-		if (!ret) {
-			struct btrfs_bio_stripe *stripe = multi->stripes;
-			int i;
-
-			if (map_length > num_bytes)
-				map_length = num_bytes;
-
-			for (i = 0; i < multi->num_stripes; i++, stripe++) {
-				btrfs_issue_discard(stripe->dev->bdev,
-						    stripe->physical,
-						     map_length);
-			}
-			kfree(multi);
-		}
-#endif
 	}
 	btrfs_free_path(path);
 	finish_current_insert(trans, extent_root, 0);
@@ -3112,16 +3104,21 @@ again:
 int btrfs_free_reserved_extent(struct btrfs_root *root, u64 start, u64 len)
 {
 	struct btrfs_block_group_cache *cache;
+	int ret = 0;
 
 	cache = btrfs_lookup_block_group(root->fs_info, start);
 	if (!cache) {
 		printk(KERN_ERR "Unable to find block group for %Lu\n", start);
 		return -ENOSPC;
 	}
+
+	ret = btrfs_discard_extent(root, start, len);
+
 	btrfs_add_free_space(cache, start, len);
 	put_block_group(cache);
 	update_reserved_extents(root, start, len, 0);
-	return 0;
+
+	return ret;
 }
 
 int btrfs_reserve_extent(struct btrfs_trans_handle *trans,