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

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Date:	Fri, 16 May 2014 08:21:35 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	sandeen@...hat.com, Jan Kara <jack@...e.cz>,
	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 Thu, May 15, 2014 at 12:47:48PM +0200, Mateusz Guzik wrote:
> On Thu, May 15, 2014 at 12:40:19PM +0200, Lukáš Czerner wrote:
> > 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
> > ?
> > 
> 
> As the time goes on there will be more data to present about given
> mountpoint. So either mountinfo should be extendable to support this
> (can't see why not) or new generic file with that property should be
> provided. I vote for the former.
> 
> IOW, a new column in mountinfo. For frozen filesystems it would contain
> 'frozen_by=[%s]:[%d]' (escaped comm, pid).

I really don't see that the process that froze the filesystem is
particularly useful - it many cases that process is long gone (e.g.
fsfreeze is being used to allow a HW array to take a snapshot). Just
the fact it is in the process of freezing (if stuck, stack trace in
sysrq-w should be present) or frozen (freezing process may be long
gone, and is mostly irrelevant because you're now tracking down why
a thaw hasn't happened)...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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