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Date:   Tue, 8 Mar 2022 13:30:20 +0100
From:   Jan Kara <jack@suse.cz>
To:     Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Cc:     linux-ext4@vger.kernel.org, riteshh@linux.ibm.com, jack@suse.cz,
        tytso@mit.edu
Subject: Re: [PATCH 3/5] ext4: for committing inode, make ext4_fc_track_inode
 wait
Message-ID: <20220308123020.u4357jwbtoqhy5xd@quack3.lan>
References: <20220308105112.404498-1-harshads@google.com>
 <20220308105112.404498-4-harshads@google.com>
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On Tue 08-03-22 02:51:10, Harshad Shirwadkar wrote:
> From: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> 
> If the inode that's being requested to track using ext4_fc_track_inode
> is being committed, then wait until the inode finishes the commit.
> 
> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>

Looks mostly good. Just some notes below.

> diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
> index 3477a16d08ae..7fa301b0a35a 100644
> --- a/fs/ext4/ext4_jbd2.c
> +++ b/fs/ext4/ext4_jbd2.c
> @@ -106,6 +106,18 @@ handle_t *__ext4_journal_start_sb(struct super_block *sb, unsigned int line,
>  				   GFP_NOFS, type, line);
>  }
>  
> +handle_t *__ext4_journal_start(struct inode *inode, unsigned int line,
> +				  int type, int blocks, int rsv_blocks,
> +				  int revoke_creds)
> +{
> +	handle_t *handle = __ext4_journal_start_sb(inode->i_sb, line,
> +						   type, blocks, rsv_blocks,
> +						   revoke_creds);
> +	if (ext4_handle_valid(handle) && !IS_ERR(handle))
> +		ext4_fc_track_inode(handle, inode);

Why do you need to call ext4_fc_track_inode() here? Calls in
ext4_map_blocks() and ext4_mark_iloc_dirty() should be enough, shouldn't
they?

> +	return handle;
> +}
> +
>  int __ext4_journal_stop(const char *where, unsigned int line, handle_t *handle)
>  {
>  	struct super_block *sb;

...

> @@ -519,6 +525,33 @@ void ext4_fc_track_inode(handle_t *handle, struct inode *inode)
>  		return;
>  	}
>  
> +	if (!test_opt2(inode->i_sb, JOURNAL_FAST_COMMIT) ||
> +	    (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY))
> +		return;
> +
> +	spin_lock(&ei->i_fc_lock);
> +	while (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING)) {
> +#if (BITS_PER_LONG < 64)
> +		DEFINE_WAIT_BIT(wait, &ei->i_state_flags,
> +				EXT4_STATE_FC_COMMITTING);
> +		wq = bit_waitqueue(&ei->i_state_flags,
> +				   EXT4_STATE_FC_COMMITTING);
> +#else
> +		DEFINE_WAIT_BIT(wait, &ei->i_flags,
> +				EXT4_STATE_FC_COMMITTING);
> +		wq = bit_waitqueue(&ei->i_flags,
> +				   EXT4_STATE_FC_COMMITTING);
> +#endif
> +
> +		prepare_to_wait(wq, &wait.wq_entry, TASK_UNINTERRUPTIBLE);
> +		spin_unlock(&ei->i_fc_lock);
> +
> +		schedule();
> +		finish_wait(wq, &wait.wq_entry);
> +		spin_lock(&ei->i_fc_lock);
> +	}
> +	spin_unlock(&ei->i_fc_lock);

Hum, we operate inode state with atomic bitops. So I think there's no real
need for ei->i_fc_lock here. You just need to be careful and check inode
state again after prepare_to_wait() call. 

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR