linux-ext4 - Re: question about jbd2 checksum v2 and v3 flag

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Date:   Sun, 25 Aug 2019 21:05:36 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Shehbaz Jaffer <shehbazjaffer007@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: question about jbd2 checksum v2 and v3 flag

On Aug 16, 2019, at 5:55 PM, Shehbaz Jaffer <shehbazjaffer007@...il.com> wrote:
> 
> Hello,
> 
> I am trying to understand jbd2 checksumming procedure. I reboot ext4
> in the middle of a metadata intensive operation using echo b>
> /proc/sysrq-trigger. I see that the journal gets replayed on next
> mount using prinks in jbd2/recovery.c: do_one_pass() function.
> 
> I then corrupt intermediate metadata logged on jbd2 and I still see
> the journal being replayed without multiple error messages which
> should ideally get printed when one of the two following flags -
> JBD2_FEATURE_INCOMPAT_CSUM_V2 or JBD2_FEATURE_INCOMPAT_CSUM_V3 are
> set.
> 
> I have 2 questions:
> 
> 1. Are the two flags: JBD2_FEATURE_INCOMPAT_CSUM_V2 and
> JBD2_FEATURE_INCOMPAT_CSUM_V3 set by default? If not, how do we set
> them so that the journal will detect and respond to injected
> corruptions?

See set_journal_csum_feature_set() for details on how these flags are set.

> 2. this is very naive question, but what do compat and incompat
> options mean? If flag X in incompat is set, does this mean the feature
> does not exist?

See description of the compat/incompat/ro_compat fields at:

https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#The_Super_Block

Cheers, Andreas






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