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

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Message-ID: <20140515094236.GE10637@mguzik.redhat.com>
Date:	Thu, 15 May 2014 11:42:37 +0200
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Josef Bacik <jbacik@...com>, Jan Kara <jack@...e.cz>,
	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, 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.

So general idea is to support providing callbacks when setting tasks to
TASK_UNINTERRUPTIBLE which could either tell the user what's up directly
or would perform some heuristics. I'll post this in a separate thread
later, maybe with PoC.

Thanks,
-- 
Mateusz Guzik
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