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

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Message-ID: <20140516001156.GI5421@dastard>
Date:	Fri, 16 May 2014 10:11:56 +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 Fri, May 16, 2014 at 01:19:09AM +0200, Mateusz Guzik wrote:
> On Fri, May 16, 2014 at 08:51:41AM +1000, Dave Chinner wrote:
> > On Fri, May 16, 2014 at 12:34:40AM +0200, Mateusz Guzik wrote:
> > > On Fri, May 16, 2014 at 08:21:35AM +1000, Dave Chinner wrote:
> > > > > 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)...
> > > 
> > > There are deamons which perform freezing and unfreezing on their own.
> > > Thus storing the name along with pid helps to determine whether someone
> > > went behind such daemon's back, or maybe it's the daemon which "forgot" to
> > > unfreeze after all.
> > 
> > Such a daemon should be logging the fact that it's freezing and
> > thawing the filesystem. The kernel is not the place to track what
> > buggy userspace applications are doing wrong.
> > 
> 
> Except there is no log entry if /var got frozen (and this is not an
> imaginary example).

Freezing the filesystem that the freezing daemon logs to is, well, a
major application architecture fail. Sorry, catering for the lowest
common denominator (i.e. stupidity) is not an valid argument for
adding stuff to the kernel....

> Grabbig a debugger to inspect daemon's state is not
> exactly what your typical support associate can or should do.

No, but they can read /proc/self/mountinfo, and grab sysrq-w output.
And they should be able to read that and tell that there is a freeze
hang from that info. This "filesystem hang triage 101" stuff....

> But this was a side request, I'm not going to argue about including
> this since turns out there is a better way.
> 
> Somewhere in the thread an idea to log long-standing freezes was
> mentioned which would provide sufficient information as far as

You've already got the hung task timer firing when a fs is frozen
for too long. You'll see processes hung in sb_write_wait(), and that
tells you the filesystem is frozen. Then look at
/proc/self/mountinfo to find which fs is frozen....

Cheers,

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