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generation that got cowed or deleted and is no longer valid (for this later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>

2 files changed, 76 insertions, 18 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index dcaae3616728..04dbc800c209 100644
--- a/fs/btrfs/transaction.c
+++ b/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,
-                                   mark, &cached_state, GFP_NOFS);
-                cached_state = NULL;
-                err = filemap_fdatawrite_range(mapping, start, end);
+                bool wait_writeback = false;
+
+                err = convert_extent_bit(dirty_pages, start, end,
+                                         EXTENT_NEED_WAIT,
+                                         mark, &cached_state, GFP_NOFS);
+                /*
+                 * 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;
 }
 
@@ -862,9 +910,21 @@ 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,
-                                 0, 0, &cached_state, GFP_NOFS);
-                err = filemap_fdatawait_range(mapping, start, end);
+                /*
+                 * 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;
                 cond_resched();
@@ -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) {
-                struct inode *btree_inode;
-                btree_inode = root->fs_info->btree_inode;
-                return filemap_write_and_wait(btree_inode->i_mapping);
-        }
-        return btrfs_write_and_wait_marked_extents(root,
+        int ret;
+
+        ret = btrfs_write_and_wait_marked_extents(root,
                                            &trans->transaction->dirty_pages,
                                            EXTENT_DIRTY);
+        clear_btree_io_tree(&trans->transaction->dirty_pages);
+
+        return ret;
 }
 
 /*
diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index d8f40e1a5d2d..b3f5b40aab22 100644
--- a/fs/btrfs/transaction.h
+++ b/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);