Re: [PATCH 2/2] fs: print a message when freezing/unfreezing filesystems

["Lukáš Czerner" <lczerner@redhat.com>]

On Thu, 15 May 2014, Jan Kara wrote:

> Date: Thu, 15 May 2014 12:01:57 +0200
> From: Jan Kara <jack@suse.cz>
> To: Mateusz Guzik <mguzik@redhat.com>
> Cc: Dave Chinner <david@fromorbit.com>, linux-kernel@vger.kernel.org,
>     linux-fsdevel@vger.kernel.org, Josef Bacik <jbacik@fb.com>,
>     Jan Kara <jack@suse.cz>, Al Viro <viro@ZenIV.linux.org.uk>,
>     Eric Sandeen <esandeen@redhat.com>
> Subject: Re: [PATCH 2/2] fs: print a message when freezing/unfreezing
>     filesystems
> 
> On Thu 15-05-14 11:42:37, Mateusz Guzik wrote:
> > On Thu, May 15, 2014 at 07:54:57AM +1000, Dave Chinner wrote:
> > > On Tue, May 13, 2014 at 08:31:02PM +0200, Mateusz Guzik wrote:
> > > > This helps hang troubleshooting efforts when only dmesg is available.
> > > 
> > > I really don't think that spamming dmesg every time a filesystem is
> > > frozen or thawed is a good idea. This happens a *lot* when systems
> > > are using snapshots, and for the most part nobody cares about
> > > freeze/thaw cycles because they almost always work just fine.
> > > 
> > 
> > I agree it may get noisy.
> > 
> > > I'd think that /proc/self/mounts would be a much better place to
> > > indicate that the fs is frozen. After all, that's where we tell
> > > people whether the filesystem is ro or rw, and frozen is just
> > > a temporary, non-invasive ro state...
> > > 
> > 
> > Except you can't inspect /proc/self/mounts when the only thing you got
> > is dmesg, so this does not really help my case.
> > 
> > That said, I'll try to come up with a different solution.
> > 
> > Poorly reported side-effects of frozen I/O are only a part of the real
> > problem which is hung task detector being able to typically report
> > backtraces of "victims" only. I came up with printks becuase these are
> > a cheap way and would help us out in a lot of cases.
>   I was tracking down a couple of times what the hell is freezing the
> filesystem (and not unfreezing it) and I agree with Mateusz it would be
> nice if we could tell after the fact who froze the fs. Maybe we could store
> that information in superblock and dump it during emergency thaw?

I like that idea and if we agree to create procfs file to list
frozen file system this might be one of the useful information to
report there.

-Lukas

> 
> 								Honza
>