linux-ext4 - help with bug

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Date:	Tue, 1 Jul 2014 06:44:45 -0000
From:	"Dolev Raviv" <draviv@...eaurora.org>
To:	linux-ext4@...r.kernel.org
Cc:	"Tanya Brokhman" <tlinder@...eaurora.org>,
	"Maya Erez" <merez@...eaurora.org>, kdorfman@...eaurora.org,
	lsusman@...eaurora.org
Subject: help with bug_on on ext4 mount

Hi All,

Im working on a crash originating from ext4 mount path. Im running with
3.10 based kernel.

Crash description:
I saw a BUG_ON assertion failure in function ext4_clear_journal_err(). The
assertion that fails is:  !EXT4_HAS_COMPAT_FEATURE(sb,
EXT4_FEATURE_COMPAT_HAS_JOURNAL).
The strange thing is, that the same BUG_ON assertion is called at the
start of the function that calls ext4_clear_journal_err(), which is
ext4_load_journal(). This means that the capability flag is changed in
ext4_load_journal, before the call for journal_err().

Im not too familiar with ext4 code unfortunately. From analyzing the
journal path I came to the below conclusions:
This scenario is possible, if during journal replay, the super_block is
restored or overridden from the journal.
I have noticed a case where the sb is marked as dirty and later, it is
evicted through the address_space_operations .writepage = ext4_writepage
cb. This cb is using the journal and can cause the dirty sb appear on the
journal. If during the journal write operation a power cut occurs, and the
sb copy in the journal is corrupted, it may cause the BUG_ON assertion
failure above.

Is the scenario described above even possible (or am I missing something)?
Has anyone encountered similar issues? Are there any known fixes for this?

Thanks,
Dolev
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