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

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Message-ID: <20140515134610.GB660@quack.suse.cz>
Date:	Thu, 15 May 2014 15:46:10 +0200
From:	Jan Kara <jack@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>, Mateusz Guzik <mguzik@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Josef Bacik <jbacik@...com>, Al Viro <viro@...IV.linux.org.uk>,
	Eric Sandeen <esandeen@...hat.com>
Subject: Re: [PATCH 2/2] fs: print a message when freezing/unfreezing
 filesystems

On Thu 15-05-14 08:47:25, Ted Tso wrote:
> On Thu, May 15, 2014 at 12:01:57PM +0200, Jan Kara wrote:
> >   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?
> 
> Saving it in the superblock would require changing a bunch of file
> systems.  What if we store this information in memory, and print it
> out under certain conditions (i.e., after a soft lockup detection, or
> upon request of some magic sysrq request)?
  By 'superblock' I meant 'struct super_block' ;) So we are in agreement I
believe.

> Or we could create a tunable threshold and print a message after a
> file system has been frozen more than a particular specified duration,
> with that duration set conservatively to something like 60 or 120
> seconds by default.
  I was thinking about this as well but all these "warn after X seconds"
warnings tend to have quite a few false positives in practice so dumping
this in emergency-thaw sysrq handler or exposing the information somewhere
in proc (e.g. mountinfo) would look like a better option to me.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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