Date:   Wed, 15 Mar 2023 18:28:17 +0100
From:   Jan Kara <jack@suse.cz>
To:     Zhang Yi <yi.zhang@huawei.com>
Cc:     Jan Kara <jack@suse.cz>, Zhang Yi <yi.zhang@huaweicloud.com>,
        linux-ext4@vger.kernel.org, tytso@mit.edu,
        adilger.kernel@dilger.ca, yukuai3@huawei.com,
        ocfs2-devel@oss.oracle.com
Subject: Re: [PATCH v3 1/2] jbd2: continue to record log between each mount
Message-ID: <20230315172817.egezft3msc5z4omm@quack3>
References: <20230314140522.3266591-1-yi.zhang@huaweicloud.com>
 <20230314140522.3266591-2-yi.zhang@huaweicloud.com>
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 <8c4ff3ab-4af2-58ed-4d08-3050c044f445@huawei.com>
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On Wed 15-03-23 20:37:32, Zhang Yi wrote:
> On 2023/3/15 17:48, Jan Kara wrote:
> > On Tue 14-03-23 22:05:21, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@huawei.com>
> >>
> >> For a newly mounted file system, the journal committing thread always
> >> record new transactions from the start of the journal area, no matter
> >> whether the journal was clean or just has been recovered. So the logdump
> >> code in debugfs cannot dump continuous logs between each mount, it is
> >> disadvantageous to analysis corrupted file system image and locate the
> >> file system inconsistency bugs.
> >>
> >> If we get a corrupted file system in the running products and want to
> >> find out what has happened, besides lookup the system log, one effective
> >> way is to backtrack the journal log. But we may not always run e2fsck
> >> before each mount and the default fsck -a mode also cannot always
> >> checkout all inconsistencies, so it could left over some inconsistencies
> >> into the next mount until we detect it. Finally, transactions in the
> >> journal may probably discontinuous and some relatively new transactions
> >> has been covered, it becomes hard to analyse. If we could record
> >> transactions continuously between each mount, we could acquire more
> >> useful info from the journal. Like this:
> >>
> >>  |Previous mount checkpointed/recovered logs|Current mount logs         |
> >>  |{------}{---}{--------} ... {------}| ... |{======}{========}...000000|
> >>
> >> And yes the journal area is limited and cannot record everything, the
> >> problematic transaction may also be covered even if we do this, but
> >> this is still useful for fuzzy tests and short-running products.
> >>
> >> This patch save the head blocknr in the superblock after flushing the
> >> journal or unmounting the file system, let the next mount could continue
> >> to record new transaction behind it. This change is backward compatible
> >> because the old kernel does not care about the head blocknr of the
> >> journal. It is also fine if we mount a clean old image without valid
> >> head blocknr, we fail back to set it to s_first just like before.
> >> Finally, for the case of mount an unclean file system, we could also get
> >> the journal head easily after scanning/replaying the journal, it will
> >> continue to record new transaction after the recovered transactions.
> >>
> >> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> > 
> > I like this implementation! I even think we could perhaps make ext4 always
> > behave this way to not increase size of the test matrix. Or do you see any
> > downside to this option?
> > 
> 
> Thanks for your suggestion. Indeed, I don't find any side effect on this
> option both in theory and in the actual use tests on ext4, I added a new
> option was just from the safe point of view and let user could disable it if
> they don't want it. I also prefer to make ext4 always behave this way.:)
> 
> I would like to keep the JBD2_CYCLE_RECORD flag(ocfs2 also use jbd2, I don't
> want to disturb it until it needs), remove EXT4_MOUNT2_JOURNAL_CYCLE_RECORD
> and always set JBD2_CYCLE_RECORD on ext4 in patch 2 in the next iteration.

Yes, that makes sense.

								Honza

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