linux-kernel - Re: [PATCH block/for-linus] writeback: flush inode cgroup wb switches instead of pinning super

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Date:	Mon, 29 Feb 2016 15:58:37 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Tahsin Erdogan <tahsin@...gle.com>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
	Theodore Ts'o <tytso@....edu>,
	Nauman Rafique <nauman@...gle.com>,
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>
Subject: Re: [PATCH block/for-linus] writeback: flush inode cgroup wb
 switches instead of pinning super_block

On Mon, Feb 29, 2016 at 08:54:28PM +0000, Al Viro wrote:
> > This patch removes the problematic super_block pinning and instead
> > makes generic_shutdown_super() flush in-flight wb switches.  wb
> > switches are now executed on a dedicated isw_wq so that they can be
> > flushed and isw_nr_in_flight keeps track of the number of in-flight wb
> > switches so that flushing can be avoided in most cases.
> 
> Wait a bloody minute.  What's to prevent shrink_dcache_for_umount() from
> dirtying more inodes, triggering more of the same?

Hmmm?  The flushing is done after shrink_dcache_for_umount() and
sync_filesystems().  Aren't inodes supposed to stay clean after that?

> > -	if (!atomic_inc_not_zero(&inode->i_sb->s_active))
> > -		goto out_unlock;
> 
> This would've failed for inodes on superblock in the middle of shutdown;
> what's to do the same for the new variant?

I don't follow.  As long as no new writeback operations are initiated
after flushing, none can be in flight for the super_block.  Isn't that
enough?

Thanks.

-- 
tejun