Btrfs: deal with convert_extent_bit errors to avoid fs corruption

When committing a transaction or a log, we look for btree extents that
need to be durably persisted by searching for ranges in a io tree that
have some bits set (EXTENT_DIRTY or EXTENT_NEW). We then attempt to clear
those bits and set the EXTENT_NEED_WAIT bit, with calls to the function
convert_extent_bit, and then start writeback for the extents.

That function however can return an error (at the moment only -ENOMEM
is possible, specially when it does GFP_ATOMIC allocation requests
through alloc_extent_state_atomic) - that means the ranges didn't got
the EXTENT_NEED_WAIT bit set (or at least not for the whole range),
which in turn means a call to btrfs_wait_marked_extents() won't find
those ranges for which we started writeback, causing a transaction
commit or a log commit to persist a new superblock without waiting
for the writeback of extents in that range to finish first.

Therefore if a crash happens after persisting the new superblock and
before writeback finishes, we have a superblock pointing to roots that
weren't fully persisted or roots that point to nodes or leafs that weren't
fully persisted, causing all sorts of unexpected/bad behaviour as we endup
reading garbage from disk or the content of some node/leaf from a past
generation that got cowed or deleted and is no longer valid (for this later
case we end up getting error messages like "parent transid verify failed on
X wanted Y found Z" when reading btree nodes/leafs from disk).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/transaction.c

@@ -76,6 +76,32 @@ void btrfs_put_transaction(struct btrfs_transaction *transaction)
 	}
 }
 
+static void clear_btree_io_tree(struct extent_io_tree *tree)
+{
+	spin_lock(&tree->lock);
+	while (!RB_EMPTY_ROOT(&tree->state)) {
+		struct rb_node *node;
+		struct extent_state *state;
+
+		node = rb_first(&tree->state);
+		state = rb_entry(node, struct extent_state, rb_node);
+		rb_erase(&state->rb_node, &tree->state);
+		RB_CLEAR_NODE(&state->rb_node);
+		/*
+		 * btree io trees aren't supposed to have tasks waiting for
+		 * changes in the flags of extent states ever.
+		 */
+		ASSERT(!waitqueue_active(&state->wq));
+		free_extent_state(state);
+		if (need_resched()) {
+			spin_unlock(&tree->lock);
+			cond_resched();
+			spin_lock(&tree->lock);
+		}
+	}
+	spin_unlock(&tree->lock);
+}
+
 static noinline void switch_commit_roots(struct btrfs_transaction *trans,
 					 struct btrfs_fs_info *fs_info)
 {
@@ -89,6 +115,7 @@ static noinline void switch_commit_roots(struct btrfs_transaction *trans,
 		root->commit_root = btrfs_root_node(root);
 		if (is_fstree(root->objectid))
 			btrfs_unpin_free_ino(root);
+		clear_btree_io_tree(&root->dirty_log_pages);
 	}
 	up_write(&fs_info->commit_root_sem);
 }
@@ -828,17 +855,38 @@ int btrfs_write_marked_extents(struct btrfs_root *root,
 
 	while (!find_first_extent_bit(dirty_pages, start, &start, &end,
 				      mark, &cached_state)) {
-		convert_extent_bit(dirty_pages, start, end, EXTENT_NEED_WAIT,
+		bool wait_writeback = false;
+
+		err = convert_extent_bit(dirty_pages, start, end,
+					 EXTENT_NEED_WAIT,
 					 mark, &cached_state, GFP_NOFS);
-		cached_state = NULL;
+		/*
+		 * convert_extent_bit can return -ENOMEM, which is most of the
+		 * time a temporary error. So when it happens, ignore the error
+		 * and wait for writeback of this range to finish - because we
+		 * failed to set the bit EXTENT_NEED_WAIT for the range, a call
+		 * to btrfs_wait_marked_extents() would not know that writeback
+		 * for this range started and therefore wouldn't wait for it to
+		 * finish - we don't want to commit a superblock that points to
+		 * btree nodes/leafs for which writeback hasn't finished yet
+		 * (and without errors).
+		 * We cleanup any entries left in the io tree when committing
+		 * the transaction (through clear_btree_io_tree()).
+		 */
+		if (err == -ENOMEM) {
+			err = 0;
+			wait_writeback = true;
+		}
+		if (!err)
 			err = filemap_fdatawrite_range(mapping, start, end);
 		if (err)
 			werr = err;
+		else if (wait_writeback)
+			werr = filemap_fdatawait_range(mapping, start, end);
+		cached_state = NULL;
 		cond_resched();
 		start = end + 1;
 	}
-	if (err)
-		werr = err;
 	return werr;
 }
 
@@ -862,8 +910,20 @@ int btrfs_wait_marked_extents(struct btrfs_root *root,
 
 	while (!find_first_extent_bit(dirty_pages, start, &start, &end,
 				      EXTENT_NEED_WAIT, &cached_state)) {
-		clear_extent_bit(dirty_pages, start, end, EXTENT_NEED_WAIT,
+		/*
+		 * Ignore -ENOMEM errors returned by clear_extent_bit().
+		 * When committing the transaction, we'll remove any entries
+		 * left in the io tree. For a log commit, we don't remove them
+		 * after committing the log because the tree can be accessed
+		 * concurrently - we do it only at transaction commit time when
+		 * it's safe to do it (through clear_btree_io_tree()).
+		 */
+		err = clear_extent_bit(dirty_pages, start, end,
+				       EXTENT_NEED_WAIT,
 				       0, 0, &cached_state, GFP_NOFS);
+		if (err == -ENOMEM)
+			err = 0;
+		if (!err)
 			err = filemap_fdatawait_range(mapping, start, end);
 		if (err)
 			werr = err;
@@ -919,17 +979,17 @@ static int btrfs_write_and_wait_marked_extents(struct btrfs_root *root,
 	return 0;
 }
 
-int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans,
+static int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans,
 				     struct btrfs_root *root)
 {
-	if (!trans || !trans->transaction) {
+	int ret;
-		struct inode *btree_inode;
+
-		btree_inode = root->fs_info->btree_inode;
+	ret = btrfs_write_and_wait_marked_extents(root,
-		return filemap_write_and_wait(btree_inode->i_mapping);
-	}
-	return btrfs_write_and_wait_marked_extents(root,
 					   &trans->transaction->dirty_pages,
 					   EXTENT_DIRTY);
+	clear_btree_io_tree(&trans->transaction->dirty_pages);
+
+	return ret;
 }
 
 /*

fs/btrfs/transaction.h

@@ -145,8 +145,6 @@ struct btrfs_trans_handle *btrfs_attach_transaction_barrier(
 					struct btrfs_root *root);
 struct btrfs_trans_handle *btrfs_start_ioctl_transaction(struct btrfs_root *root);
 int btrfs_wait_for_commit(struct btrfs_root *root, u64 transid);
-int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans,
-				     struct btrfs_root *root);
 
 void btrfs_add_dead_root(struct btrfs_root *root);
 int btrfs_defrag_root(struct btrfs_root *root);