On Mon 23-06-08 20:14:54, Hidehiro Kawai wrote: > Hi, > > I noticed a problem of this patch. Please see below. > > Jan Kara wrote: > > > On Tue 03-06-08 13:40:25, Hidehiro Kawai wrote: > > > >>Subject: [PATCH 4/5] jbd: fix error handling for checkpoint io > >> > >>When a checkpointing IO fails, current JBD code doesn't check the > >>error and continue journaling. This means latest metadata can be > >>lost from both the journal and filesystem. > >> > >>This patch leaves the failed metadata blocks in the journal space > >>and aborts journaling in the case of log_do_checkpoint(). > >>To achieve this, we need to do: > >> > >>1. don't remove the failed buffer from the checkpoint list where in > >> the case of __try_to_free_cp_buf() because it may be released or > >> overwritten by a later transaction > >>2. log_do_checkpoint() is the last chance, remove the failed buffer > >> from the checkpoint list and abort the journal > >>3. when checkpointing fails, don't update the journal super block to > >> prevent the journaled contents from being cleaned. For safety, > >> don't update j_tail and j_tail_sequence either > > 3. is implemented as described below. > (1) if log_do_checkpoint() detects an I/O error during > checkpointing, it calls journal_abort() to abort the journal > (2) if the journal has aborted, don't update s_start and s_sequence > in the on-disk journal superblock > > So, if the journal aborts, journaled data will be replayed on the > next mount. > > Now, please remember that some dirty metadata buffers are written > back to the filesystem without journaling if the journal aborted. > We are happy if all dirty metadata buffers are written to the disk, > the integrity of the filesystem will be kept. However, replaying > the journaled data can overwrite the latest on-disk metadata blocks > partly with old data. It would break the filesystem. Yes, it would. But how do you think it can happen that a metadata buffer will be written back to the filesystem when it is a part of running transaction? Note that checkpointing code specifically checks whether the buffer being written back is part of a running transaction and if so, it waits for commit before writing back the buffer. So I don't think this can happen but maybe I miss something... > My idea to resolve this problem is that we don't write out metadata > buffers which belong to uncommitted transactions if journal has > aborted. Although the latest filesystem updates will be lost, > we can ensure the integrity. It will also be effective for the > kernel panic in the middle of writing metadata buffers without > journaling (this would occur in the `mount -o errors=panic' case.) > > Which integrity or latest state should we choose? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
