Hi, On Thu 28-01-10 09:23:43, 丁定华 wrote: > As you wrote, if T2!=T1, then T2 is committing transaction while T1 > is running transaction, and if T1 complete commit, we don't care > about the content of buffers. But there is a prerequisite -->"T1 > complete commit", if T1 start commit and another transaction T3 > becomes the new running transaction, T3 may need to reuse T2 log > space and force checkpoint, and since we have clean the BH_dirty bit > of buffers after T2 commits, so T2 may be freed before T1 complete > commit, and unfortunately, T1 doesn't complete commit, so after > replay, updates of T2 get lost, fs becomes inconsistent. Hmm, you are right. That could probably happen. It only hits ext3/4 in data=journaled mode but the bug is there. But it's hard to fix it in a reasonable way - if we would just leave the dirty bit set, we will have aliasing problems - buffer B is attached to some page which used to be from file F, so unmap_underlying_metadata will not find it because it looks only into page cache the block device, not to the pagecaches of individual files. So if we reallocate the block of B for some other file G before the buffer B is checkpointed, we have two dirty buffers representing the same block and thus data corruption can happen. Maybe we could handle them by setting b_next_transaction to the transaction that deleted the buffer (in jbd2_journal_unmap_buffer) and setting buffer freed like we do now. Commit code would handle freed buffers like: If b_next_transaction is set, file buffer to forget list of the next transaction. If b_next_transaction isn't set, clear all dirty bits. What do you think? Honza > 2010/1/27 Jan Kara <jack@xxxxxxx>: > > Hi, > > > > On Wed 27-01-10 10:32:18, 丁定华 wrote: > >> I'm a little confused about BH_Freed bit. The only place it is set > >> is journal_unmap_buffer, which is called by jbd2_journal_invalidatepage when > >> we want to truncate a file. Since jbd2_journal_invalidatepage is called > >> outside of transaction, We can't make sure whether the "add to orphan" > >> operation belongs to committing transaction or not, so we can't touch the > >> buffer belongs to committing transaction, instead BH_Freed bit is set to > >> indicate that this buffer can be discarded in running transaction. But i > >> think we shouldn't clear BH_JBDdirty in jbd2_journal_commit_transaction, as > >> following codes does: > >> /* A buffer which has been freed while still being > >> * journaled by a previous transaction may end up still > >> * being dirty here, but we want to avoid writing back > >> * that buffer in the future now that the last use has > >> * been committed. That's not only a performance gain, > >> * it also stops aliasing problems if the buffer is left > >> * behind for writeback and gets reallocated for another > >> * use in a different page. */ > >> if (buffer_freed(bh)) { > >> clear_buffer_freed(bh); > >> clear_buffer_jbddirty(bh); > >> } > >> Note that, *We can't make sure "current running transaction" can complete > >> commit work.* If we clear BH_JBDdirty bit here, this buffer may be freed > >> here, the log space of older transaction may be freed before the "current > >> running transaction" complete commit work, and if this happends, filesystem > >> will be inconsistent. > > Let me sketch the situation here: > > The file F gets truncated. The inode is added to orphan list in some > > transaction T1, only then jbd2_journal_invalidatepage can be called. > > As you wrote above, it can happen that jbd2_journal_invalidatepage on > > buffer B runs when some transaction T2 containing B is being committed and > > in that case we set BH_Freed. If T2 != T1 - i.e., T2 is being committed > > and T1 is the running transaction, note that we clear the dirty bit only > > when T2 is fully committed and we are processing forget list. So buffer has > > been properly written to T2 and we just won't write it in the transaction > > T1. And that is fine because as soon as transaction T1 finishes commit, we > > don't care about what happens with buffers of F because the fact that F is > > truncated is recorded and in case of crash we finish truncate during > > journal replay. And if we crash before T1 finishes commit, we don't care > > about contents of T1 either. If T2 == T1, the above reasoning applies as > > well and the situation is even simpler. > > > > Honza > > -- > > Jan Kara <jack@xxxxxxx> > > SUSE Labs, CR > > > > > > -- > 丁定华 -- 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