@@ -1912,12 +1912,33 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
atomic_inc(&root->log_batch);
full_sync = test_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
&BTRFS_I(inode)->runtime_flags);
- if (full_sync) {
- ret = btrfs_wait_ordered_range(inode, start, end - start + 1);
- if (ret) {
- mutex_unlock(&inode->i_mutex);
- goto out;
- }
+ /*
+ * 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 their extent
+ * map to construct a file extent item for logging without waiting for
+ * the respective ordered operation to finish - these file extent items
+ * point 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.
+ */
+ ret = btrfs_wait_ordered_range(inode, start, end - start + 1);
+ if (ret) {
+ mutex_unlock(&inode->i_mutex);
+ goto out;
}
atomic_inc(&root->log_batch);
When the fsync callback (btrfs_sync_file) starts, it first waits for the writeback of any dirty pages to start and finish without holding the inode's mutex (to reduce contention). After this it acquires the inode's mutex and repeats that process via btrfs_wait_ordered_range only if we're doing a full sync (BTRFS_INODE_NEEDS_FULL_SYNC flag is set on the inode). This is not safe for a non full sync - we need to start and wait for writeback to finish of any pages that might have been made dirty before acquiring the inode's mutex and after that first step mentioned before. Why this is needed is explained by the following comment added to btrfs_sync_file: "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 their extent map to construct a file extent item for logging without waiting for the respective ordered operation to finish - these file extent items point 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." Signed-off-by: Filipe Manana <fdmanana@suse.com> --- fs/btrfs/file.c | 33 +++++++++++++++++++++++++++------ 1 file changed, 27 insertions(+), 6 deletions(-)