Date:   Fri, 17 Mar 2023 12:25:28 +0100
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, Zhang Yi <yi.zhang@...weicloud.com>,
        linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, yukuai3@...wei.com,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v3 1/2] jbd2: continue to record log between each mount

On Wed 15-03-23 18:28:17, Jan Kara wrote:
> 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@...wei.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@...wei.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.

FWIW yesterday I'v spoken with Ted and he also agrees that we don't need
ext4 mount option for this.

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

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