Re: [PATCH v4] ext4: fix fast commit inode enqueueing during a full journal commit

From: Andreas Dilger
Date: Thu Jul 11 2024 - 15:28:39 EST

On Jul 11, 2024, at 10:16 AM, Wang Jianjian <wangjianjian0@xxxxxxxxxxx> wrote:
> On 2024/7/11 23:16, Luis Henriques wrote:
>> On Thu, Jul 11 2024, wangjianjian (C) wrote:
>>
>>> On 2024/7/11 16:35, Luis Henriques (SUSE) wrote:
>>>> When a full journal commit is on-going, any fast commit has to be enqueued
>>>> into a different queue: FC_Q_STAGING instead of FC_Q_MAIN. This enqueueing
>>>> is done only once, i.e. if an inode is already queued in a previous fast
>>>> commit entry it won't be enqueued again. However, if a full commit starts
>>>> _after_ the inode is enqueued into FC_Q_MAIN, the next fast commit needs to
>>>> be done into FC_Q_STAGING. And this is not being done in function
>>>> ext4_fc_track_template().
>>>> This patch fixes the issue by re-enqueuing an inode into the STAGING queue
>>>> during the fast commit clean-up callback if it has a tid (i_sync_tid)
>>>> greater than the one being handled. The STAGING queue will then be spliced
>>>> back into MAIN.
>>>> This bug was found using fstest generic/047. This test creates several 32k
>>>> bytes files, sync'ing each of them after it's creation, and then shutting
>>>> down the filesystem. Some data may be loss in this operation; for example a
>>>> file may have it's size truncated to zero.
>>>> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@xxxxxxxxx>
>>>> ---
>>>> Hi!
>>>> v4 of this patch enqueues the inode into STAGING *only* if the current tid
>>>> is non-zero. It will be zero when doing an fc commit, and this would mean
>>>> to always re-enqueue the inode. This fixes the regressions caught by Ted
>>>> in v3 with fstests generic/472 generic/496 generic/643.
>>>> Also, since 2nd patch of v3 has already been merged, I've rebased this patch
>>>> to be applied on top of it.
>>>> fs/ext4/fast_commit.c | 10 ++++++++++
>>>> 1 file changed, 10 insertions(+)
>>>> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
>>>> index 3926a05eceee..facbc8dbbaa2 100644
>>>> --- a/fs/ext4/fast_commit.c
>>>> +++ b/fs/ext4/fast_commit.c
>>>> @@ -1290,6 +1290,16 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
>>>> EXT4_STATE_FC_COMMITTING);
>>>> if (tid_geq(tid, iter->i_sync_tid))
>>>> ext4_fc_reset_inode(&iter->vfs_inode);
>>>> + } else if (tid) {
>>>> + /*
>>>> + * If the tid is valid (i.e. non-zero) re-enqueue the
>>> one quick question about tid, if one disk is using long time and its tid get
>>> wrapped to 0, is it a valid seq? I don't find code handling this situation.
>> Hmm... OK. So, to answer to your question, the 'tid' is expected to wrap.
>> That's why we use:
>>
>> if (tid_geq(tid, iter->i_sync_tid))
> Yes, I know this.
>>
>> instead of:
>>
>> if (tid >= iter->i_sync_tid)
>>
>> (The second patch in v3 actually fixed a few places where the tid_*()
>> helpers weren't being used.)
>>
>> But your question shows me that my patch is wrong as '0' may actually be a
>> valid 'tid' value.
>
> Actually my question is, there are some place use '0' to check if a transaction is valid, e.g.
>
> In ext4_wait_for_tail_page_commit()
>
> 5218 while (1) {
> 5219 struct folio *folio = filemap_lock_folio(inode->i_mapping,
> 5220 inode->i_size >> PAGE_SHIFT);
> 5221 if (IS_ERR(folio))
> 5222 return;
> 5223 ret = __ext4_journalled_invalidate_folio(folio, offset,
> 5224 folio_size(folio) - offset);
> 5225 folio_unlock(folio);
> 5226 folio_put(folio);
> 5227 if (ret != -EBUSY)
> 5228 return;
> 5229 commit_tid = 0;
> 5230 read_lock(&journal->j_state_lock);
> 5231 if (journal->j_committing_transaction)
> 5232 commit_tid = journal->j_committing_transaction->t_tid;
> 5233 read_unlock(&journal->j_state_lock);
> 5234 if (commit_tid)
> 5235 jbd2_log_wait_commit(journal, commit_tid);
> 5236 }
> 5237 We only wait commit if tid is not zero.
>
> And in __jbd2_log_wait_for_space()
>
> 79 if (space_left < nblocks) {
> 80 int chkpt = journal->j_checkpoint_transactions != NULL;
> 81 tid_t tid = 0;
> 82
> 83 if (journal->j_committing_transaction)
> 84 tid = journal->j_committing_transaction->t_tid;
> 85 spin_unlock(&journal->j_list_lock);
> 86 write_unlock(&journal->j_state_lock);
> 87 if (chkpt) {
> 88 jbd2_log_do_checkpoint(journal);
> 89 } else if (jbd2_cleanup_journal_tail(journal) == 0) {
> 90 /* We were able to recover space; yay! */
> 91 ;
> 92 } else if (tid) {
> 93 /*
> 94 * jbd2_journal_commit_transaction() may want
> 95 * to take the checkpoint_mutex if JBD2_FLUSHED
> 96 * is set. So we need to temporarily drop it.
> 97 */
> 98 mutex_unlock(&journal->j_checkpoint_mutex);
> 99 jbd2_log_wait_commit(journal, tid);
> 100 write_lock(&journal->j_state_lock);
> 101 continue;
> We also only wait commit if tid is not zero.
>
> Does it mean all these have bugs if '0' is a valid 'tid' ?
>
> But on the other hand, if we don't consider sync and fsync, and default commit interval is 5s,
>
> time of tid wrap to 0 is nearly 680 years. However, we can run sync/fsync to make tid to increase
>
> more quickly in real world ?

The simple solution is that "0" is not a valid sequence. It looks like
this is a bug in jbd2_get_transaction() where it is incrementing the TID:

transaction->t_tid = journal->j_transaction_sequence++;

it should add a check to handle the wrap-around:

if (unlikely(transaction->t_tid == 0))
transaction->t_tid = journal->j_transaction_sequence++;

Cheers, Andreas





Attachment: signature.asc
Description: Message signed with OpenPGP