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

From: wangjianjian (C)
Date: Thu Jul 11 2024 - 20:51:37 EST

On 2024/7/12 3:28, Andreas Dilger wrote:
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++;
Yes, I had ever thought about this, just curious why nobody encounter problems here.

Cheers, Andreas





--
Regards