linux-ext4 - Why jbd2 might freeze in kjournald2()

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Date:   Wed, 23 Jan 2019 06:32:12 -0800 (PST)
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     <linux-ext4@...r.kernel.org>
Cc:     Joe Jin <joe.jin@...cle.com>, Junxiao Bi <junxiao.bi@...cle.com>
Subject: Why jbd2 might freeze in kjournald2()

Hi,

I am writing to ask why jbd2 might freeze in kjournald2()?

I am asking this because since blk-mq commit bc27c01b5c46 (blk-mq: Do not
invoke .queue_rq() for a stopped queue), the I/O request will be queued
when the queue is stopped, that is, when OS is frozen (e.g., during xen
live migration or "echo freeze > /sys/power/state" in kvm guest). I think
it should be fine to not freeze jbd2.

Why would kjournald2() freeze to avoid submitting I/O?

Is it because of what is mentioned in code that "The simpler the better",
or there is any other reason?

Thank you very much!

Dongli Zhang