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Subject: Re: [PATCH -v2] ext4: inline jbd2_journal_[un]register_shrinker()
To:     Theodore Ts'o <tytso@mit.edu>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>
CC:     Jan Kara <jack@suse.cz>, Sachin Sant <sachinp@linux.vnet.ibm.com>,
        Jon Hunter <jonathanh@nvidia.com>
References: <3acc3ee6-3a3d-3b26-7580-b20955270913@huawei.com>
 <20210705145025.3363130-1-tytso@mit.edu>
From:   Zhang Yi <yi.zhang@huawei.com>
Message-ID: <f7d41979-fe08-c765-16bd-6a497cb031ec@huawei.com>
Date:   Tue, 6 Jul 2021 09:38:31 +0800
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On 2021/7/5 22:50, Theodore Ts'o wrote:
> The function jbd2_journal_unregister_shrinker() was getting called
> twice when the file system was getting unmounted.  On Power and ARM
> platforms this was causing kernel crash when unmounting the file
> system, when a percpu_counter was destroyed twice.
> 
> Fix this by removing jbd2_journal_[un]register_shrinker() functions,
> and inlining the shrinker setup and teardown into
> journal_init_common() and jbd2_journal_destroy().  This means that
> ext4 and ocfs2 now no longer need to know about registering and
> unregistering jbd2's shrinker.
> 
> Also, while we're at it, rename the percpu counter from
> j_jh_shrink_count to j_checkpoint_jh_count, since this makes it
> clearer what this counter is intended to track.
> 
> Fixes: 4ba3fcdde7e3 ("jbd2,ext4: add a shrinker to release checkpointed buffers")
> Reported-by: Jon Hunter <jonathanh@nvidia.com>
> Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
> Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>

This patch looks good to me.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

Thanks,
Yi.

> ---
>  fs/ext4/super.c      |   8 ---
>  fs/jbd2/checkpoint.c |   4 +-
>  fs/jbd2/journal.c    | 149 +++++++++++++++++--------------------------
>  include/linux/jbd2.h |   6 +-
>  4 files changed, 64 insertions(+), 103 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index b8ff0399e171..dfa09a277b56 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1184,7 +1184,6 @@ static void ext4_put_super(struct super_block *sb)
>  	ext4_unregister_sysfs(sb);
>  
>  	if (sbi->s_journal) {
> -		jbd2_journal_unregister_shrinker(sbi->s_journal);
>  		aborted = is_journal_aborted(sbi->s_journal);
>  		err = jbd2_journal_destroy(sbi->s_journal);
>  		sbi->s_journal = NULL;
> @@ -5176,7 +5175,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>  	sbi->s_ea_block_cache = NULL;
>  
>  	if (sbi->s_journal) {
> -		jbd2_journal_unregister_shrinker(sbi->s_journal);
>  		jbd2_journal_destroy(sbi->s_journal);
>  		sbi->s_journal = NULL;
>  	}
> @@ -5502,12 +5500,6 @@ static int ext4_load_journal(struct super_block *sb,
>  		ext4_commit_super(sb);
>  	}
>  
> -	err = jbd2_journal_register_shrinker(journal);
> -	if (err) {
> -		EXT4_SB(sb)->s_journal = NULL;
> -		goto err_out;
> -	}
> -
>  	return 0;
>  
>  err_out:
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 51d1eb2ffeb9..746132998c57 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -701,7 +701,7 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
>  
>  	__buffer_unlink(jh);
>  	jh->b_cp_transaction = NULL;
> -	percpu_counter_dec(&journal->j_jh_shrink_count);
> +	percpu_counter_dec(&journal->j_checkpoint_jh_count);
>  	jbd2_journal_put_journal_head(jh);
>  
>  	/* Is this transaction empty? */
> @@ -764,7 +764,7 @@ void __jbd2_journal_insert_checkpoint(struct journal_head *jh,
>  		jh->b_cpnext->b_cpprev = jh;
>  	}
>  	transaction->t_checkpoint_list = jh;
> -	percpu_counter_inc(&transaction->t_journal->j_jh_shrink_count);
> +	percpu_counter_inc(&transaction->t_journal->j_checkpoint_jh_count);
>  }
>  
>  /*
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 152880c298ca..35302bc192eb 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1283,6 +1283,48 @@ static int jbd2_min_tag_size(void)
>  	return sizeof(journal_block_tag_t) - 4;
>  }
>  
> +/**
> + * jbd2_journal_shrink_scan()
> + *
> + * Scan the checkpointed buffer on the checkpoint list and release the
> + * journal_head.
> + */
> +static unsigned long jbd2_journal_shrink_scan(struct shrinker *shrink,
> +					      struct shrink_control *sc)
> +{
> +	journal_t *journal = container_of(shrink, journal_t, j_shrinker);
> +	unsigned long nr_to_scan = sc->nr_to_scan;
> +	unsigned long nr_shrunk;
> +	unsigned long count;
> +
> +	count = percpu_counter_read_positive(&journal->j_checkpoint_jh_count);
> +	trace_jbd2_shrink_scan_enter(journal, sc->nr_to_scan, count);
> +
> +	nr_shrunk = jbd2_journal_shrink_checkpoint_list(journal, &nr_to_scan);
> +
> +	count = percpu_counter_read_positive(&journal->j_checkpoint_jh_count);
> +	trace_jbd2_shrink_scan_exit(journal, nr_to_scan, nr_shrunk, count);
> +
> +	return nr_shrunk;
> +}
> +
> +/**
> + * jbd2_journal_shrink_count()
> + *
> + * Count the number of checkpoint buffers on the checkpoint list.
> + */
> +static unsigned long jbd2_journal_shrink_count(struct shrinker *shrink,
> +					       struct shrink_control *sc)
> +{
> +	journal_t *journal = container_of(shrink, journal_t, j_shrinker);
> +	unsigned long count;
> +
> +	count = percpu_counter_read_positive(&journal->j_checkpoint_jh_count);
> +	trace_jbd2_shrink_count(journal, sc->nr_to_scan, count);
> +
> +	return count;
> +}
> +
>  /*
>   * Management for journal control blocks: functions to create and
>   * destroy journal_t structures, and to initialise and read existing
> @@ -1361,9 +1403,23 @@ static journal_t *journal_init_common(struct block_device *bdev,
>  	journal->j_sb_buffer = bh;
>  	journal->j_superblock = (journal_superblock_t *)bh->b_data;
>  
> +	journal->j_shrink_transaction = NULL;
> +	journal->j_shrinker.scan_objects = jbd2_journal_shrink_scan;
> +	journal->j_shrinker.count_objects = jbd2_journal_shrink_count;
> +	journal->j_shrinker.seeks = DEFAULT_SEEKS;
> +	journal->j_shrinker.batch = journal->j_max_transaction_buffers;
> +
> +	if (percpu_counter_init(&journal->j_checkpoint_jh_count, 0, GFP_KERNEL))
> +		goto err_cleanup;
> +
> +	if (register_shrinker(&journal->j_shrinker)) {
> +		percpu_counter_destroy(&journal->j_checkpoint_jh_count);
> +		goto err_cleanup;
> +	}
>  	return journal;
>  
>  err_cleanup:
> +	brelse(journal->j_sb_buffer);
>  	kfree(journal->j_wbuf);
>  	jbd2_journal_destroy_revoke(journal);
>  	kfree(journal);
> @@ -2050,93 +2106,6 @@ int jbd2_journal_load(journal_t *journal)
>  	return -EIO;
>  }
>  
> -/**
> - * jbd2_journal_shrink_scan()
> - *
> - * Scan the checkpointed buffer on the checkpoint list and release the
> - * journal_head.
> - */
> -static unsigned long jbd2_journal_shrink_scan(struct shrinker *shrink,
> -					      struct shrink_control *sc)
> -{
> -	journal_t *journal = container_of(shrink, journal_t, j_shrinker);
> -	unsigned long nr_to_scan = sc->nr_to_scan;
> -	unsigned long nr_shrunk;
> -	unsigned long count;
> -
> -	count = percpu_counter_read_positive(&journal->j_jh_shrink_count);
> -	trace_jbd2_shrink_scan_enter(journal, sc->nr_to_scan, count);
> -
> -	nr_shrunk = jbd2_journal_shrink_checkpoint_list(journal, &nr_to_scan);
> -
> -	count = percpu_counter_read_positive(&journal->j_jh_shrink_count);
> -	trace_jbd2_shrink_scan_exit(journal, nr_to_scan, nr_shrunk, count);
> -
> -	return nr_shrunk;
> -}
> -
> -/**
> - * jbd2_journal_shrink_count()
> - *
> - * Count the number of checkpoint buffers on the checkpoint list.
> - */
> -static unsigned long jbd2_journal_shrink_count(struct shrinker *shrink,
> -					       struct shrink_control *sc)
> -{
> -	journal_t *journal = container_of(shrink, journal_t, j_shrinker);
> -	unsigned long count;
> -
> -	count = percpu_counter_read_positive(&journal->j_jh_shrink_count);
> -	trace_jbd2_shrink_count(journal, sc->nr_to_scan, count);
> -
> -	return count;
> -}
> -
> -/**
> - * jbd2_journal_register_shrinker()
> - * @journal: Journal to act on.
> - *
> - * Init a percpu counter to record the checkpointed buffers on the checkpoint
> - * list and register a shrinker to release their journal_head.
> - */
> -int jbd2_journal_register_shrinker(journal_t *journal)
> -{
> -	int err;
> -
> -	journal->j_shrink_transaction = NULL;
> -
> -	err = percpu_counter_init(&journal->j_jh_shrink_count, 0, GFP_KERNEL);
> -	if (err)
> -		return err;
> -
> -	journal->j_shrinker.scan_objects = jbd2_journal_shrink_scan;
> -	journal->j_shrinker.count_objects = jbd2_journal_shrink_count;
> -	journal->j_shrinker.seeks = DEFAULT_SEEKS;
> -	journal->j_shrinker.batch = journal->j_max_transaction_buffers;
> -
> -	err = register_shrinker(&journal->j_shrinker);
> -	if (err) {
> -		percpu_counter_destroy(&journal->j_jh_shrink_count);
> -		return err;
> -	}
> -
> -	return 0;
> -}
> -EXPORT_SYMBOL(jbd2_journal_register_shrinker);
> -
> -/**
> - * jbd2_journal_unregister_shrinker()
> - * @journal: Journal to act on.
> - *
> - * Unregister the checkpointed buffer shrinker and destroy the percpu counter.
> - */
> -void jbd2_journal_unregister_shrinker(journal_t *journal)
> -{
> -	percpu_counter_destroy(&journal->j_jh_shrink_count);
> -	unregister_shrinker(&journal->j_shrinker);
> -}
> -EXPORT_SYMBOL(jbd2_journal_unregister_shrinker);
> -
>  /**
>   * jbd2_journal_destroy() - Release a journal_t structure.
>   * @journal: Journal to act on.
> @@ -2209,8 +2178,10 @@ int jbd2_journal_destroy(journal_t *journal)
>  		brelse(journal->j_sb_buffer);
>  	}
>  
> -	jbd2_journal_unregister_shrinker(journal);
> -
> +	if (journal->j_shrinker.flags & SHRINKER_REGISTERED) {
> +		percpu_counter_destroy(&journal->j_checkpoint_jh_count);
> +		unregister_shrinker(&journal->j_shrinker);
> +	}
>  	if (journal->j_proc_entry)
>  		jbd2_stats_proc_exit(journal);
>  	iput(journal->j_inode);
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 6cc035321562..fd933c45281a 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -918,11 +918,11 @@ struct journal_s
>  	struct shrinker		j_shrinker;
>  
>  	/**
> -	 * @j_jh_shrink_count:
> +	 * @j_checkpoint_jh_count:
>  	 *
>  	 * Number of journal buffers on the checkpoint list. [j_list_lock]
>  	 */
> -	struct percpu_counter	j_jh_shrink_count;
> +	struct percpu_counter	j_checkpoint_jh_count;
>  
>  	/**
>  	 * @j_shrink_transaction:
> @@ -1556,8 +1556,6 @@ extern int	   jbd2_journal_set_features
>  		   (journal_t *, unsigned long, unsigned long, unsigned long);
>  extern void	   jbd2_journal_clear_features
>  		   (journal_t *, unsigned long, unsigned long, unsigned long);
> -extern int	   jbd2_journal_register_shrinker(journal_t *journal);
> -extern void	   jbd2_journal_unregister_shrinker(journal_t *journal);
>  extern int	   jbd2_journal_load       (journal_t *journal);
>  extern int	   jbd2_journal_destroy    (journal_t *);
>  extern int	   jbd2_journal_recover    (journal_t *journal);
>