linux-ext4 - Re: [PATCH v2] jbd2: make sure dirty flag is cleared while revorking a buffer which belongs to older transaction

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Date:   Wed, 16 Jan 2019 15:36:45 +0100
From:   Jan Kara <jack@...e.cz>
To:     "zhangyi (F)" <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, miaoxie@...wei.com
Subject: Re: [PATCH v2] jbd2: make sure dirty flag is cleared while revorking
 a buffer which belongs to older transaction

On Wed 16-01-19 21:38:23, zhangyi (F) wrote:
> Now, we capture a data corruption problem on ext4 while we're truncating
> an extent index block. Imaging that if we are revoking a buffer which
> has been journaled by the committing transaction, the buffer's jbddirty
> flag will not be cleared in jbd2_journal_forget(), so the commit code
> will set the buffer dirty flag again after refile the buffer.
> 
> fsx                               kjournald2
>                                   jbd2_journal_commit_transaction
> jbd2_journal_revoke                commit phase 1~5...
>  jbd2_journal_forget
>    belongs to older transaction    commit phase 6
>    jbddirty not clear               __jbd2_journal_refile_buffer
>                                      __jbd2_journal_unfile_buffer
>                                       test_clear_buffer_jbddirty
>                                        mark_buffer_dirty
> 
> Finally, if the freed extent index block was allocated again as data
> block by some other files, it may corrupt the file data when writing
> cached pages later, such as during umount time.

Thanks for the patch! I'm sorry this didn't occur to me the first time when
I was reading your analysis but now there is one question I have: When the
freed extent index block gets reallocated as data block, we should call
clean_bdev_aliases() or clean_bdev_bh_alias() for it (it usually happens
shortly after block allocation either in ext4_block_write_begin() or
mpage_map_one_extent()). Which will clear the buffer dirty bit and thus
should avoid this kind of corruption. So how come this didn't work? Is it
that we for some reason didn't call clean_bdev_aliases() or that function
didn't work for some reason? Can you debug that with your reproducer?
Thanks a lot!

								Honza

> This patch mark buffer as freed and set j_next_transaction to the new
> transaction when it already belongs to the committing transaction in
> jbd2_journal_forget(), so that commit code knows it should clear dirty
> bits when it is done with the buffer.
> 
> This problem can be reproduced by xfstests generic/455 easily with
> seeds (3246 3247 3248 3249).
> 
> Signed-off-by: zhangyi (F) <yi.zhang@...wei.com>
> Cc: stable@...r.kernel.org
> ---
>  fs/jbd2/transaction.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index f07f006..f7f9647 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -1609,14 +1609,19 @@ int jbd2_journal_forget (handle_t *handle, struct buffer_head *bh)
>  		/* However, if the buffer is still owned by a prior
>  		 * (committing) transaction, we can't drop it yet... */
>  		JBUFFER_TRACE(jh, "belongs to older transaction");
> -		/* ... but we CAN drop it from the new transaction if we
> -		 * have also modified it since the original commit. */
> +		/* ... but we CAN drop it from the new transaction, mark
> +		 * buffer as freed and set j_next_transaction to the new
> +		 * transaction so that commit code knows it should clear
> +		 * dirty bits when it is done with the buffer. */
>  
> -		if (jh->b_next_transaction) {
> -			J_ASSERT(jh->b_next_transaction == transaction);
> +		set_buffer_freed(bh);
> +
> +		if (!jh->b_next_transaction) {
>  			spin_lock(&journal->j_list_lock);
> -			jh->b_next_transaction = NULL;
> +			jh->b_next_transaction = transaction;
>  			spin_unlock(&journal->j_list_lock);
> +		} else {
> +			J_ASSERT(jh->b_next_transaction == transaction);
>  
>  			/*
>  			 * only drop a reference if this transaction modified
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR