linux-kernel - Re: [PATCH] writeback: inode cgroup wb switch should skip inode with zero i

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Date:	Wed, 15 Jun 2016 10:55:13 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Tahsin Erdogan <tahsin@...gle.com>, Jens Axboe <axboe@...nel.dk>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] writeback: inode cgroup wb switch should skip inode with
 zero i_count

Hello, Jan.

On Wed, Jun 15, 2016 at 02:26:40PM +0200, Jan Kara wrote:
> Ugh, this looks ugly. Inode with i_count == 0 without I_FREEING set is
> sitting in inode LRU list. It may get reused at which point it would be
> actually good if it switched WB to the good one, no?

Yes, that'd be better but the switching is heuristics driven best
effort thing anyway, so occasionally failing to switch isn't critical.

> Since we actually hold i_lock and have checked the inode is not being
> freed, we can just use __iget() to grab the inode reference. That avoids
> the warning and fixes the race as well. Something like:
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 989a2ce..b44ede0 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -478,14 +478,15 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  		goto out_free;
>  	}
>  	inode->i_state |= I_WB_SWITCH;
> +	__iget(inode);
>  	spin_unlock(&inode->i_lock);
>  
> -	ihold(inode);
>  	isw->inode = inode;
>  
>  	atomic_inc(&isw_nr_in_flight);

That said, the above looks better.

Thanks!

-- 
tejun