linux-ext4 - Re: [PATCH v5] ext4: fix fast commit inode enqueueing during a full journal commit

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Message-ID: <20240718110011.p2sq5hdy57nqkpxg@quack3>
Date: Thu, 18 Jul 2024 13:00:11 +0200
From: Jan Kara <jack@...e.cz>
To: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Cc: Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
	Jan Kara <jack@...e.cz>,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] ext4: fix fast commit inode enqueueing during a full
 journal commit

On Wed 17-07-24 18:22:20, 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 when doing a full commit.  However,
> to prevent a race with a fast-commit, the clean-up callback has to be called
> with the journal locked.
> 
> 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.
> 
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
> Hi!
> 
> And here's another attempt to fix this bug.  The most significant change is
> that now it doesn't assume a 'special' meaning for a tid of '0'.  Which is
> a wrong assumption as Jan has shown.
> 
> I've also added a Suggested-by: tag, although Jan pretty much owns this
> patch -- I have simply tested it and sent it out!
> 
>  fs/ext4/fast_commit.c | 15 ++++++++++++++-
>  fs/jbd2/journal.c     |  2 +-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> index 3926a05eceee..df71fd5b1fed 100644
> --- a/fs/ext4/fast_commit.c
> +++ b/fs/ext4/fast_commit.c
> @@ -1288,8 +1288,21 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
>  		list_del_init(&iter->i_fc_list);
>  		ext4_clear_inode_state(&iter->vfs_inode,
>  				       EXT4_STATE_FC_COMMITTING);
> -		if (tid_geq(tid, iter->i_sync_tid))
> +		if (tid_geq(tid, iter->i_sync_tid)) {
>  			ext4_fc_reset_inode(&iter->vfs_inode);
> +		} else if (full) {
> +			/*
> +			 * We are called after a full commit, inode has been
> +			 * modified while the commit was running. Re-enqueue
> +			 * the inode into STAGING, which will then be splice
> +			 * back into MAIN. This cannot happen during
> +			 * fastcommit because the journal is locked all the
> +			 * time in that case (and tid doesn't increase so
> +			 * tid check above isn't reliable).
> +			 */
> +			list_add_tail(&EXT4_I(&iter->vfs_inode)->i_fc_list,
> +				      &sbi->s_fc_q[FC_Q_STAGING]);
> +		}
>  		/* Make sure EXT4_STATE_FC_COMMITTING bit is clear */
>  		smp_mb();
>  #if (BITS_PER_LONG < 64)
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 1ebf2393bfb7..291a431f8aaf 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -740,9 +740,9 @@ EXPORT_SYMBOL(jbd2_fc_begin_commit);
>   */
>  static int __jbd2_fc_end_commit(journal_t *journal, tid_t tid, bool fallback)
>  {
> -	jbd2_journal_unlock_updates(journal);
>  	if (journal->j_fc_cleanup_callback)
>  		journal->j_fc_cleanup_callback(journal, 0, tid);
> +	jbd2_journal_unlock_updates(journal);
>  	write_lock(&journal->j_state_lock);
>  	journal->j_flags &= ~JBD2_FAST_COMMIT_ONGOING;
>  	if (fallback)
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR