Re: [PATCH 07/12] jbd2: add fast_commit space check

[Jan Kara <jack@suse.cz>]

On Mon 07-08-23 18:53:09, Zhang Yi wrote:
> On 2023/8/3 22:38, Jan Kara wrote:
> > On Tue 04-07-23 21:42:28, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@huawei.com>
> >>
> >> If JBD2_FEATURE_INCOMPAT_FAST_COMMIT bit is set, it means the journal
> >> have fast commit records need to recover, so the fast commit size
> >> should not be zero, and also the leftover normal journal size should
> >> never less than JBD2_MIN_JOURNAL_BLOCKS. Add a check into the
> >> journal_check_superblock() and drop the pointless branch when
> >> initializing in-memory fastcommit parameters.
> >>
> >> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> > 
> > Some comments below.
> > 
> > 
> >> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> >> index efdb8db3c06e..210b532a3673 100644
> >> --- a/fs/jbd2/journal.c
> >> +++ b/fs/jbd2/journal.c
> >> @@ -1392,6 +1392,18 @@ static int journal_check_superblock(journal_t *journal)
> >>  		return err;
> >>  	}
> >>  
> >> +	if (jbd2_has_feature_fast_commit(journal)) {
> >> +		int num_fc_blks = be32_to_cpu(sb->s_num_fc_blks);
> >> +
> >> +		if (!num_fc_blks ||
> >> +		    (be32_to_cpu(sb->s_maxlen) - num_fc_blks <
> >> +		     JBD2_MIN_JOURNAL_BLOCKS)) {
> >> +			printk(KERN_ERR "JBD2: Invalid fast commit size %d\n",
> >> +			       num_fc_blks);
> >> +			return err;
> >> +		}
> > 
> > This is wrong sb->s_num_fc_blks == 0 means that the fast-commit area should
> > have the default size of 256 blocks. At least that's how it behaves
> > currently and we should not change the behavior.
> 
> Thanks for the review and correcting me. I missed the fc_debug_force
> mount option, this option enable fast commit feature without init
> sb->s_num_fc_blks to disk, so it could left over an unclean image with
> fast_commit feature but sb->s_num_fc_blks is still zero. And the mke2fs
> could also set sb->s_num_fc_blks to 0.

Yes.

> > Similarly if the number of fastcommit blocks was too big (i.e. there was
> > not enough space left for ordinary journal), we effectively silently
> > disable fastcommit and you break this behavior in this patch.
> > 
> 
> If the fastcommit is too big, jbd2_journal_initialize_fast_commit()
> will detect this corruption and refuse to mount.
> 
> [ 1213.810719] JBD2: Cannot enable fast commits.
> [ 1213.812282] EXT4-fs (pmem1): Failed to set fast commit journal feature
> 
> It only silently disable fastcommit while recovering the journal, but it
> doesn't seem to make much sense, because the journal->j_last is likely to
> be wrong (not point to the correct end of normal journal range) and will
> probably lead to incorrect recovery. It seems better to report the error
> and exit as early as possible. So I suppose we could keep this "too big"
> check in journal_check_superblock(). How does that sound ?

Ah, you are right. So let's keep the "space for journal too small" check as
you suggest.

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