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

[Wang Jianjian]

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 ?


> Cheers,