Re: [PATCH 1/2] JBD2: Allow feature checks before journal recovery

[Jan Kara <jack@suse.cz>]

> Before we start accessing a huge (> 16 TiB) OCFS2 volume, we need to
> confirm that its journal supports 64-bit offsets.  So we need to check
> the journal's feature bits before recovering the journal.
> 
> This is not possible with JBD2 at present, because the journal
> superblock (where the feature bits reside) is not loaded from disk until
> the journal is recovered.
> 
> This patch loads the journal superblock in
> jbd2_journal_check_used_features() if it has not already been loaded,
> allowing us to check the feature bits before journal recovery.
> 
> Signed-off-by: Patrick LoPresti <lopresti@gmail.com>
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index bc2ff59..c5a864f 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1365,6 +1365,8 @@ int jbd2_journal_check_used_features (journal_t *journal, unsigned long compat,
>  
>  	if (!compat && !ro && !incompat)
>  		return 1;
> +	if (journal_get_superblock(journal))
> +		return 0;
>  	if (journal->j_format_version == 1)
>  		return 0;
  This looks OK in principle. It would be even nicer to avoid all the checks
journal_get_superblock() when the superblock is actually loaded so that we
don't do them each time jbd2_journal_check_used_features is called...

								Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs