linux-ext4 - Re: [powerpc][5.13.0-next-20210701] Kernel crash while running ltp(chdir01) tests

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Date:   Sat, 3 Jul 2021 11:05:07 +0800
From:   Zhang Yi <yi.zhang@...wei.com>
To:     Theodore Ts'o <tytso@....edu>
CC:     Jan Kara <jack@...e.cz>, <linuxppc-dev@...ts.ozlabs.org>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [powerpc][5.13.0-next-20210701] Kernel crash while running
 ltp(chdir01) tests

On 2021/7/3 0:11, Theodore Ts'o wrote:
> On Fri, Jul 02, 2021 at 09:52:13PM +0800, Zhang Yi wrote:
>>
>> Sorry about not catching this problem, this fix is not format corrected,
>> if you think this fix is OK, I can send a patch after test.
> 
> The issue I see with your approach, which removes the
> jbd2_journal_unregister_shrinker() call from jbd2_destsroy_journal(),
> is that means that *all* callers of jbd2_destroy_journal now have to
> be responsible for calling jbd2_journal_unregister_shrinker() --- and
> there a number of call sites to jbd2_journal_unregister_shrinker():
> 
> fs/ext4/super.c:		err = jbd2_journal_destroy(sbi->s_journal);
> fs/ext4/super.c:		jbd2_journal_destroy(sbi->s_journal);
> fs/ext4/super.c:	jbd2_journal_destroy(journal);
> fs/ext4/super.c:		jbd2_journal_destroy(journal);
> fs/ext4/super.c:	jbd2_journal_destroy(journal);
> fs/ocfs2/journal.c:	if (!jbd2_journal_destroy(journal->j_journal) && !status) {
> fs/ocfs2/journal.c:		jbd2_journal_destroy(journal);
> fs/ocfs2/journal.c:	jbd2_journal_destroy(journal);
> 

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.

If with my fix, there is no responsible for calling
jbd2_journal_unregister_shrinker() before every jbd2_journal_destroy(). There
are only two places that need to do this, one is the error path after
ext4_load_journal() because we have already register the shrinker, other one
is in ext4_put_super() before the final release of the journal.
jbd2_journal_unregister_shrinker() and jbd2_journal_destroy() do not have
the strong dependence.

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.

> So it probably makes more sense to keep jbd2_journal_unregister_shrinker()
> in jbd2_destroy_journal(), since arguably the fact that we are using a
> shrinker is an internal implementation detail, and the users of jbd2
> ideally shouldn't need to be expected to know they have unregister
> jbd2's shirnkers.
> 
> Similarly, perhaps we should be moving jbd2_journal_register_shirnker()
> into jbd2_journal_init_common().  We can un-export the register and
> unshrink register functions, and declare them as static functions internal
> to fs/jbd2/journal.c.
> 

Yeah, it's make sense and It's sound good to me if the shrinker doesn't have
side effects on osfs2.

Thanks,
Yi.