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SubjectRe: [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@kernel.org>
> Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
> Signed-off-by: zhanchengbin <zhanchengbin1@huawei.com>
> Suggested-by: Jan Kara <jack@suse.cz>

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

Reviewed-by: Jan Kara <jack@suse.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@suse.com>
SUSE Labs, CR

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