linux-ext4 - Re: [PATCH -next v3] jbd2: Fix data missing when reusing bh which is ready to be checkpointed

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Date:   Mon, 9 Jan 2023 16:29:23 +0100
From:   Jan Kara <jack@...e.cz>
To:     Zhihao Cheng <chengzhihao1@...wei.com>
Cc:     tytso@....edu, jack@...e.com, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        libaokun1@...wei.com, zhanchengbin1@...wei.com
Subject: Re: [PATCH -next v3] jbd2: Fix data missing when reusing bh which is
 ready to be checkpointed

On Mon 09-01-23 21:45:45, Zhihao Cheng wrote:
> Following process will make data lost and could lead to a filesystem
> corrupted problem:
> 
> 1. jh(bh) is inserted into T1->t_checkpoint_list, bh is dirty, and
>    jh->b_transaction = NULL
> 2. T1 is added into journal->j_checkpoint_transactions.
> 3. Get bh prepare to write while doing checkpoing:
>            PA				    PB
>    do_get_write_access             jbd2_log_do_checkpoint
>     spin_lock(&jh->b_state_lock)
>      if (buffer_dirty(bh))
>       clear_buffer_dirty(bh)   // clear buffer dirty
>        set_buffer_jbddirty(bh)
> 				    transaction =
> 				    journal->j_checkpoint_transactions
> 				    jh = transaction->t_checkpoint_list
> 				    if (!buffer_dirty(bh))
> 		                      __jbd2_journal_remove_checkpoint(jh)
> 				      // bh won't be flushed
> 		                    jbd2_cleanup_journal_tail
>     __jbd2_journal_file_buffer(jh, transaction, BJ_Reserved)
> 4. Aborting journal/Power-cut before writing latest bh on journal area.
> 
> In this way we get a corrupted filesystem with bh's data lost.
> 
> Fix it by moving the clearing of buffer_dirty bit just before the call
> to __jbd2_journal_file_buffer(), both bit clearing and jh->b_transaction
> assignment are under journal->j_list_lock locked, so that
> jbd2_log_do_checkpoint() will wait until jh's new transaction fininshed
> even bh is currently not dirty. And journal_shrink_one_cp_list() won't
> remove jh from checkpoint list if the buffer head is reused in
> do_get_write_access().
> 
> Fetch a reproducer in [Link].
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216898
> Cc: <stable@...nel.org>
> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com>
> Signed-off-by: zhanchengbin <zhanchengbin1@...wei.com>
> Suggested-by: Jan Kara <jack@...e.cz>

Great, the patch looks good. Feel free to add:

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

Just a suggestion for rephrasing of the comment below

> -		/*
> -		 * In any case we need to clean the dirty flag and we must
> -		 * do it under the buffer lock to be sure we don't race
> -		 * with running write-out.
> +		 * We need to clean the dirty flag and we must do it under the
> +		 * buffer lock to be sure we don't race with running write-out.
>  		 */
>  		JBUFFER_TRACE(jh, "Journalling dirty buffer");
>  		clear_buffer_dirty(bh);
> +		/*
> +		 * Setting jbddirty after clearing buffer dirty is necessary.
> +		 * Function jbd2_journal_restart() could keep buffer on
> +		 * BJ_Reserved list until the transaction committing, then the
> +		 * buffer won't be dirtied by jbd2_journal_refile_buffer()
> +		 * after committing, the buffer couldn't fall on disk even
> +		 * last checkpoint finished, which may corrupt filesystem.
> +		 */

As far as I understand you want to say:
		/*
		 * The buffer is going to be added to BJ_Reserved list now
		 * and nothing guarantees jbd2_journal_dirty_metadata()
		 * will be ever called for it. So we need to set jbddirty
		 * bit here to make sure the buffer is dirtied and written
		 * out when the journaling machinery is done with it.
		 */

>  		set_buffer_jbddirty(bh);
>  	}

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR