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 22:21:19 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	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 29-02-16 16:08:00, Tejun Heo wrote:
> On Mon, Feb 29, 2016 at 09:06:15PM +0000, Al Viro wrote:
> > > Hmmm?  The flushing is done after shrink_dcache_for_umount() and
> > > sync_filesystems().  Aren't inodes supposed to stay clean after that?
> > 
> > s/shrink_dcache_for_umount/fsnotify_unmount_inodes/ - sorry.
> 
> Is that allowed to dirty indoes and initiate writebacks again, after
> sync_filesystems() is done?  That sounds weird but it's trivial to
> move cgroup_writeback_umount() below that if so.

Hardly, but generally it is true that filesystem may still dirty something
(e.g. from outstanding workqueue work, but most likely some special "system
inodes" may still become dirty) until ->put_super() is finished.

Anyway, to make this foolproof, I'd just avoid queueing any new switching
work after S_ACTIVE is cleared on the superblock and flush the workqueue just
before evict_inodes() call.

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