Re: NOCOW + compress-force = bug

From: Jeff Mahoney <jeffm@suse.de>
To: Chris Mason <chris.mason@oracle.com>,
	Roman Mamedov <rm@romanrm.ru>,
	dave@jikos.cz, dima <dolenin@parallels.com>,
	linux-btrfs@vger.kernel.org
Subject: Re: NOCOW + compress-force = bug
Date: Tue, 13 Mar 2012 14:11:05 -0400	[thread overview]
Message-ID: <4F5F8DB9.40208@suse.de> (raw)
In-Reply-To: <20120216175815.GM21896@shiny>

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On 02/16/2012 12:58 PM, Chris Mason wrote:
> On Thu, Feb 16, 2012 at 07:55:15PM +0600, Roman Mamedov wrote:
>> Hello,
>> 
>> Please be aware that there seems to be a possible problem with
>> using NOCOW flag on files situated on a filesystem mounted with
>> compress-force(=lzo, in my case).
>> 
>> Since experimenting with NOCOW, I started regularly hitting this
>> BUG at extent-tree.c:5813
>> 
>> 5813                 BUG_ON(!(flags &
>> BTRFS_BLOCK_FLAG_FULL_BACKREF));
>> 
>> I was unable to make netconsole work over a bridged interface, so
>> can only post screenshots of this OOPS: 
>> http://romanrm.ru/pics/2012/2012-02-16-btrfs-bug-1.jpg 
>> http://romanrm.ru/pics/2012/2012-02-16-btrfs-bug-2.jpg
>> 
>> This happened four times already, and always on snapshot creation
>> (but not every case). I have hourly snapshots in crontab, and
>> only one case out of about ten fails with this problem. Did not
>> try to deliberately reproduce it yet by manually making snapshots
>> very often, etc.
> 
> Interesting, NOCOW and compression don't really mix.  We always cow
> for compression.  I'll try to reproduce it.

I hit this one today without nocow or compression. The only thing
non-default was that I mounted with -ossd. The backing store was a 1GB
non-sparse loopback file on tmpfs.

I had kdump enabled and with 16GB, I wasn't waiting around for the
dump to complete. If it happens again, I'll have a full stack trace.
My test case was filling the disk while making snapshots.

- -Jeff

- -- 
Jeff Mahoney
SUSE Labs
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