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Date:   Fri, 2 Jul 2021 23:35:11 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Zhang Yi <yi.zhang@huawei.com>
Cc:     Jan Kara <jack@suse.cz>, linuxppc-dev@lists.ozlabs.org,
        Guoqing Jiang <guoqing.jiang@linux.dev>,
        Sachin Sant <sachinp@linux.vnet.ibm.com>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [powerpc][5.13.0-next-20210701] Kernel crash while running
 ltp(chdir01) tests
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On Sat, Jul 03, 2021 at 11:05:07AM +0800, Zhang Yi wrote:
> 
> Originally, I want to add this shrinker as a optional feature for jbd2 because
> only ext4 use it now and I'm not sure does ocfs2 needs this feature. So I export
> jbd2_journal_[un]register_shrinker(), ext4 could invoke them individually.

The reason why bdev_try_to_free_page() callback was needed for ext4
--- namely so there was a way to release checkpointed buffers under
memory pressure --- also exists for ocfs2.  It was probably true that
in most deployments of ocfs2, they weren't running with super-tight
memory availability, so it may not have been necessary the same way
that it might be necessary, say, if ext4 was being used on a Rasberry
Pi.  :-)

> And one more thing we to could do is rename the 'j_jh_shrink_count' to something
> like 'j_checkpoint_jh_count' because we always init it no matter we register the
> shrinker or not later.

That makes sense.

In fact, unless I'm mistaken, I don't think it's legal to call
percpu_counter_{inc,dec} if the shrinker isn't initialized.  So for
ocfs2, if we didn't initialize percpu_counter, when
__jbd2_journal_insert_checkpoint() tries to call percpu_counter_inc(),
I believe things would potentially go *boom* on some implementations
of the percpu counter (e.g., on Power and ARM).  So not only would it
not hurt to register the shrinker for ocfs2, I think it's required.

So yeah, let's rename it to something like j_checkpoint_jh_count, and
then let's inline jbd2_journal_[un]register_shrinker() in
journal_init_common() and jbd2_journal_unregister_shrinker().

What do you think?

					- Ted