linux-kernel - Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned during cgroup writeback association switches

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Message-ID: <20160218132045.GD4338@quack.suse.cz>
Date:	Thu, 18 Feb 2016 14:20:45 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, Tahsin Erdogan <tahsin@...gle.com>,
	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>,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned
 during cgroup writeback association switches

Hi Tejun,

On Thu 18-02-16 08:00:33, Tejun Heo wrote:
> On Thu, Feb 18, 2016 at 10:55:38AM +0100, Jan Kara wrote:
> > I'm not sure I understand the question. Do you mean why both s_active and
> > s_umount rwsem exist? s_active is a reference count keeping superblock
> 
> Yes.
> 
> > alive - e.g. if the filesystem is mounted in more places, we need a
> > reference for each mountpoint. s_umount is used when we want to block any
> 
> I could be mistaken but I *think* we used to reject umounts based on
> s_active and s_umount is the mechanism to delay umounts rather than
> failing them and probably with bind mounts the behavior changed.
> 
> > umount operation until we are done. For example sync(2) is using it to make
> > sure superblock doesn't disappear and so that we don't keep superblock
> > alive after admin called umount(2).
> 
> So, the question is why aren't we just using s_active and draining it
> on umount of the last mountpoint.  Because, right now, the behavior is
> weird in that we allow umounts to proceed but then let the superblock
> hang onto the block device till s_active is drained.  This really
> should be synchronous.

Hum, I'm not sure. I guess Al can give you more qualified answer than me.
Added to CC...

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