Subject: Re: [PATCH v3 1/2] jbd2: continue to record log between each mount
To:     Jan Kara <jack@suse.cz>, Zhang Yi <yi.zhang@huaweicloud.com>
CC:     <linux-ext4@vger.kernel.org>, <tytso@mit.edu>,
        <adilger.kernel@dilger.ca>, <yukuai3@huawei.com>,
        <ocfs2-devel@oss.oracle.com>
References: <20230314140522.3266591-1-yi.zhang@huaweicloud.com>
 <20230314140522.3266591-2-yi.zhang@huaweicloud.com>
 <20230315094826.okdarxaapjyqmlhq@quack3>
From:   Zhang Yi <yi.zhang@huawei.com>
Message-ID: <8c4ff3ab-4af2-58ed-4d08-3050c044f445@huawei.com>
Date:   Wed, 15 Mar 2023 20:37:32 +0800
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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.

Thanks,
Yi.