1 files changed, 63 insertions, 15 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index d5d5060..cdb7146 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1849,6 +1849,20 @@ int btrfs_release_file(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+static int start_ordered_ops(struct inode *inode, loff_t start, loff_t end)
+{
+	int ret;
+
+	atomic_inc(&BTRFS_I(inode)->sync_writers);
+	ret = filemap_fdatawrite_range(inode->i_mapping, start, end);
+	if (!ret && test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT,
+			     &BTRFS_I(inode)->runtime_flags))
+		ret = filemap_fdatawrite_range(inode->i_mapping, start, end);
+	atomic_dec(&BTRFS_I(inode)->sync_writers);
+
+	return ret;
+}
+
 /*
  * fsync call for both files and directories.  This logs the inode into
  * the tree log instead of forcing full commits whenever possible.
@@ -1878,30 +1892,64 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	 * multi-task, and make the performance up.  See
 	 * btrfs_wait_ordered_range for an explanation of the ASYNC check.
 	 */
-	atomic_inc(&BTRFS_I(inode)->sync_writers);
-	ret = filemap_fdatawrite_range(inode->i_mapping, start, end);
-	if (!ret && test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT,
-			     &BTRFS_I(inode)->runtime_flags))
-		ret = filemap_fdatawrite_range(inode->i_mapping, start, end);
-	atomic_dec(&BTRFS_I(inode)->sync_writers);
+	ret = start_ordered_ops(inode, start, end);
 	if (ret)
 		return ret;
 
 	mutex_lock(&inode->i_mutex);
-
-	/*
-	 * We flush the dirty pages again to avoid some dirty pages in the
-	 * range being left.
-	 */
 	atomic_inc(&root->log_batch);
 	full_sync = test_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
 			     &BTRFS_I(inode)->runtime_flags);
+	/*
+	 * We might have have had more pages made dirty after calling
+	 * start_ordered_ops and before acquiring the inode's i_mutex.
+	 */
 	if (full_sync) {
+		/*
+		 * For a full sync, we need to make sure any ordered operations
+		 * start and finish before we start logging the inode, so that
+		 * all extents are persisted and the respective file extent
+		 * items are in the fs/subvol btree.
+		 */
 		ret = btrfs_wait_ordered_range(inode, start, end - start + 1);
-		if (ret) {
-			mutex_unlock(&inode->i_mutex);
-			goto out;
-		}
+	} else {
+		/*
+		 * Start any new ordered operations before starting to log the
+		 * inode. We will wait for them to finish in btrfs_sync_log().
+		 *
+		 * Right before acquiring the inode's mutex, we might have new
+		 * writes dirtying pages, which won't immediately start the
+		 * respective ordered operations - that is done through the
+		 * fill_delalloc callbacks invoked from the writepage and
+		 * writepages address space operations. So make sure we start
+		 * all ordered operations before starting to log our inode. Not
+		 * doing this means that while logging the inode, writeback
+		 * could start and invoke writepage/writepages, which would call
+		 * the fill_delalloc callbacks (cow_file_range,
+		 * submit_compressed_extents). These callbacks add first an
+		 * extent map to the modified list of extents and then create
+		 * the respective ordered operation, which means in
+		 * tree-log.c:btrfs_log_inode() we might capture all existing
+		 * ordered operations (with btrfs_get_logged_extents()) before
+		 * the fill_delalloc callback adds its ordered operation, and by
+		 * the time we visit the modified list of extent maps (with
+		 * btrfs_log_changed_extents()), we see and process the extent
+		 * map they created. We then use the extent map to construct a
+		 * file extent item for logging without waiting for the
+		 * respective ordered operation to finish - this file extent
+		 * item points to a disk location that might not have yet been
+		 * written to, containing random data - so after a crash a log
+		 * replay will make our inode have file extent items that point
+		 * to disk locations containing invalid data, as we returned
+		 * success to userspace without waiting for the respective
+		 * ordered operation to finish, because it wasn't captured by
+		 * btrfs_get_logged_extents().
+		 */
+		ret = start_ordered_ops(inode, start, end);
+	}
+	if (ret) {
+		mutex_unlock(&inode->i_mutex);
+		goto out;
 	}
 	atomic_inc(&root->log_batch);