Re: [RFC PATCH 1/2] ext4: fix fast commit inode enqueueing during a full journal commit

Luis Henriques
2024-05-22T13:21:52Z

On Wed 22 May 2024 12:35:45 PM +02, Jan Kara wrote;

> On Tue 21-05-24 16:45:34, 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().
>
> Ah, good catch.
>
>> This patch fixes the issue by simply re-enqueuing the inode from the MAIN
>> into the STAGING queue.
>>
>> 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@linux.dev>
>> ---
>> fs/ext4/fast_commit.c | 19 +++++++++++++------
>> 1 file changed, 13 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
>> index 87c009e0c59a..337b5289cf11 100644
>> --- a/fs/ext4/fast_commit.c
>> +++ b/fs/ext4/fast_commit.c
>> @@ -396,12 +396,19 @@ static int ext4_fc_track_template(
>> return ret;
>>
>> spin_lock(&sbi->s_fc_lock);
>> - if (list_empty(&EXT4_I(inode)->i_fc_list))
>> - list_add_tail(&EXT4_I(inode)->i_fc_list,
>> - (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
>> - sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING) ?
>> - &sbi->s_fc_q[FC_Q_STAGING] :
>> - &sbi->s_fc_q[FC_Q_MAIN]);
>> + if (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
>> + sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING) {
>> + if (list_empty(&EXT4_I(inode)->i_fc_list))
>> + list_add_tail(&EXT4_I(inode)->i_fc_list,
>> + &sbi->s_fc_q[FC_Q_STAGING]);
>> + else
>> + list_move_tail(&EXT4_I(inode)->i_fc_list,
>> + &sbi->s_fc_q[FC_Q_STAGING]);
>
> So I'm not sure this is actually safe. I'm concerned about the following
> race:
>
> Task1 Task2
>
> handle = ext4_journal_start(..)
> modify inode_X
> ext4_fc_track_inode(inode_X)
> ext4_fsync(inode_X)
> ext4_fc_commit()
> jbd2_fc_begin_commit()
> journal->j_flags |= JBD2_FAST_COMMIT_ONGOING;
> ...
> jbd2_journal_lock_updates()
> blocks waiting for handle of Task2
> modify inode_X
> ext4_fc_track_inode(inode_X)
> - moves inode out of FC_Q_MAIN
> ext4_journal_stop()
> fast commit proceeds but skips inode_X...

Hmm... I see, the problem is deeper that I thought.

> How we deal with a similar issue in jbd2 for ordinary buffers is that we
> just mark the buffer as *also* belonging to the next transaction (by
> setting jh->b_next_transaction) and during commit cleanup we move the bh to
> the appropriate list of the next transaction. Here, we could mark the inode
> as also being part of the next fast commit and during fastcommit cleanup we
> could move it to FC_Q_STAGING which is then spliced back to FC_Q_MAIN.

Yeah, I guess that would work. I'll need to add a new field to flag the
'next commit' in struct ext4_inode_info. I'll need to play a bit with it
and see what I can came up with. Thanks for the suggestion.

> Also Harshad has recently posted changes to fast commit code that modify
> how fast commits are serialized (in particular jbd2_journal_lock_updates()
> is gone). I didn't read them yet but your change definitely needs a careful
> verification against those changes to make sure we don't introduce new data
> integrity issues.
>

Right, I saw his patchset only after sending my RFC (and I should have
probably included him on the CC as well; probably get_maintainer.pl isn't
picking his email).

I'll need to look at those changes too, which will probably take me some
time as most of that code isn't familiar to me.

Thanks a lot for your review, Jan. Much appreciated.

Cheers,
--
Luis

>> + } else {
>> + if (list_empty(&EXT4_I(inode)->i_fc_list))
>> + list_add_tail(&EXT4_I(inode)->i_fc_list,
>> + &sbi->s_fc_q[FC_Q_MAIN]);
>> + }
>> spin_unlock(&sbi->s_fc_lock);
>>
>> return ret;
>
> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR