Re: [PATCH 03/11] fs: add frozen sb state helpers

From: Jan Kara <jack@suse.cz>
To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Jan Kara <jack@suse.cz>,
	viro@zeniv.linux.org.uk, bart.vanassche@wdc.com,
	ming.lei@redhat.com, tytso@mit.edu, darrick.wong@oracle.com,
	jikos@kernel.org, rjw@rjwysocki.net, pavel@ucw.cz,
	len.brown@intel.com, linux-fsdevel@vger.kernel.org,
	boris.ostrovsky@oracle.com, jgross@suse.com,
	todd.e.brandt@linux.intel.com, nborisov@suse.com,
	martin.petersen@oracle.com, ONeukum@suse.com,
	oleksandr@natalenko.name, oleg.b.antonyan@gmail.com,
	yu.chen.surf@gmail.com, dan.j.williams@intel.com,
	linux-pm@vger.kernel.org, linux-block@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org,
	jlayton@redhat.com
Subject: Re: [PATCH 03/11] fs: add frozen sb state helpers
Date: Sun, 22 Apr 2018 01:53:23 +0200	[thread overview]
Message-ID: <20180421235323.ndj3gx65n7kmv5ta@quack2.suse.cz> (raw)
In-Reply-To: <20180420184932.57as7jup75nx463v@garbanzo.do-not-panic.com>

On Fri 20-04-18 11:49:32, Luis R. Rodriguez wrote:
> On Tue, Apr 17, 2018 at 05:59:36PM -0700, Luis R. Rodriguez wrote:
> > On Thu, Dec 21, 2017 at 12:03:29PM +0100, Jan Kara wrote:
> > > Hello,
> > > 
> > > I think I owe you a reply here... Sorry that it took so long.
> > 
> > Took me just as long :)
> > 
> > > On Fri 01-12-17 22:13:27, Luis R. Rodriguez wrote:
> > > > 
> > > > I'll note that its still not perfectly clear if really the semantics behind
> > > > freeze_bdev() match what I described above fully. That still needs to be
> > > > vetted for. For instance, does thaw_bdev() keep a superblock frozen if we
> > > > an ioctl initiated freeze had occurred before? If so then great. Otherwise
> > > > I think we'll need to distinguish the ioctl interface. Worst possible case
> > > > is that bdev semantics and in-kernel semantics differ somehow, then that
> > > > will really create a holy fucking mess.
> > > 
> > > I believe nobody really thought about mixing those two interfaces to fs
> > > freezing and so the behavior is basically defined by the implementation.
> > > That is:
> > > 
> > > freeze_bdev() on sb frozen by ioctl_fsfreeze() -> EBUSY
> > > freeze_bdev() on sb frozen by freeze_bdev() -> success
> > > ioctl_fsfreeze() on sb frozen by freeze_bdev() -> EBUSY
> > > ioctl_fsfreeze() on sb frozen by ioctl_fsfreeze() -> EBUSY
> > > 
> > > thaw_bdev() on sb frozen by ioctl_fsfreeze() -> EINVAL
> > 
> > Phew, so this is what we want for the in-kernel freezing so we're good
> > and *can* combine these then.
> 
> I double checked, and I don't see where you get EINVAL for this case.
> We *do* keep the sb frozen though, which is good, and the worst fear
> I had was that we did not. However we return 0 if there was already
> a prior freeze_bdev() or ioctl_fsfreeze() other than the context that
> started the prior freeze (--bdev->bd_fsfreeze_count > 0).
> 
> The -EINVAL is only returned currently if there were no freezers.
> 
> int thaw_bdev(struct block_device *bdev, struct super_block *sb)
> {
> 	int error = -EINVAL;
> 
> 	mutex_lock(&bdev->bd_fsfreeze_mutex);
> 	if (!bdev->bd_fsfreeze_count)
> 		goto out;

But this is precisely where we'd bail if we freeze sb by ioctl_fsfreeze()
but try to thaw by thaw_bdev(). ioctl_fsfreeze() does not touch
bd_fsfreeze_count...

								Honza

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