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

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Date:   Fri, 16 Aug 2019 19:55:54 -0400
From:   Shehbaz Jaffer <shehbazjaffer007@...il.com>
To:     linux-ext4@...r.kernel.org
Subject: question about jbd2 checksum v2 and v3 flag

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?

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?

Thank you,

-- 
Shehbaz Jaffer