linux-kernel - Re: [PATCH V2 2/2] fs: print a message when freezing/unfreezing filesystems

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Date:	Thu, 15 May 2014 12:40:19 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	sandeen@...hat.com
cc:	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Mateusz Guzik <mguzik@...hat.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Josef Bacik <jbacik@...com>, Al Viro <viro@...IV.linux.org.uk>,
	Joe Perches <joe@...ches.com>
Subject: Re: [PATCH V2 2/2] fs: print a message when freezing/unfreezing
 filesystems

On Wed, 14 May 2014, Eric Sandeen wrote:

> Date: Wed, 14 May 2014 17:40:22 -0500
> From: Eric Sandeen <esandeen@...hat.com>
> Reply-To: sandeen@...hat.com
> To: Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>
> Cc: Mateusz Guzik <mguzik@...hat.com>, linux-kernel@...r.kernel.org,
>     linux-fsdevel@...r.kernel.org, Josef Bacik <jbacik@...com>,
>     Al Viro <viro@...IV.linux.org.uk>, Joe Perches <joe@...ches.com>
> Subject: Re: [PATCH V2 2/2] fs: print a message when freezing/unfreezing
>     filesystems
> 
> On 5/14/14, 5:37 PM, Dave Chinner wrote:
> > On Thu, May 15, 2014 at 08:00:52AM +1000, Dave Chinner wrote:
> >> On Wed, May 14, 2014 at 01:39:45PM +0200, Jan Kara wrote:
> >>> On Wed 14-05-14 13:26:21, Mateusz Guzik wrote:
> >>>> On Wed, May 14, 2014 at 01:14:49PM +0200, Jan Kara wrote:
> >>>>> On Wed 14-05-14 00:04:43, Mateusz Guzik wrote:
> >>>>>> This helps hang troubleshooting efforts when only dmesg is available.
> >>>>>>
> >>>>>> While here remove code duplication with MS_RDONLY case and fix a
> >>>>>> whitespace nit.
> >>>>>   I'm somewhat undecided here I have to say. On one hand I don't like
> >>>>> printing to kernel log when everything is fine and kernel is operating
> >>>>> normally. On the other hand I've seen quite a few cases where people have
> >>>>> shot themselves in the foot with filesystem freezing so having some trace
> >>>>> of this in the log doesn't seem like a completely bad thing either. What do
> >>>>> other people think?
> >>>>>
> >>>>
> >>>> I would like to note that the kernel already prints messages when e.g.
> >>>> filesystems get mounted.
> >>>   Yeah, that's a fair point.
> >>
> >> But filesystems choose to output that info, not the VFS. When you do
> >> a remount,ro there is no output in syslog, because filesystems don't
> >> need to dump any output - the state change is reflected in
> >> /proc/self/mounts. IMO frozen should state should be communicated
> >> the same way so that it is silent when it just works, and the state
> >> can easily be determined when something goes wrong.
> > 
> > Say, like this:
> > 
> > $ grep /mnt/test /proc/mounts
> > /dev/vda /mnt/test xfs rw,relatime,attr2,inode64,noquota 0 0
> > $ sudo xfs_freeze -f /mnt/test
> > $ grep /mnt/test /proc/mounts
> > /dev/vda /mnt/test xfs rw,frozen,relatime,attr2,inode64,noquota 0 0
> > $ sudo xfs_freeze -u /mnt/test
> > $ grep /mnt/test /proc/mounts
> > /dev/vda /mnt/test xfs rw,relatime,attr2,inode64,noquota 0 0
> > $
> 
> I'm not totally convinced that including a non-mount option in what
> has always (?) been a list of mount options is a great idea.

I do not like it either. Mixing this together with other mount
options does not seem like a great idea, however we really need a
way to report this and I guess we can not just change the
/proc/self/mounts, or /proc/self/mountinfo format.

So what about crating a new file /proc/self/frozen with the list of
frozen file systems in the same format what mounts, or mountinfo has
?

> 
> (Granted, some options there are defaults, and weren't actually specified
> as a mount option, but if they had been, they'd have been accepted).
> 
> Maybe add a "mount -o remount,frozen" handler ?  ;)

That's the neat way to work around that :), but I would prefer a new
procfs file rather than this.

-Lukas

> 
> -Eric
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