Message-ID: <20100603125312.GH24062@thunk.org>
Date:	Thu, 3 Jun 2010 08:53:12 -0400
From:	tytso@....edu
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Always journal quota file modifications

On Wed, Jun 02, 2010 at 04:23:13PM +0200, Jan Kara wrote:
> When journaled quota options are not specified, we do writes
> to quota files just in data=ordered mode. This actually causes
> warnings from JBD2 about dirty journaled buffer because ext4_getblk
> unconditionally treats a block allocated by it as metadata. Since
> quota actually is filesystem metadata, the easiest way to get rid
> of the warning is to always treat quota writes as metadata...
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

I'm worried about this patch in the short-term.  In the long-term I
think the quota file should become a special file much like the
journal, and then this makes a huge amount of sense.  But I worry
about what might happen if (a) someone tries writing to the quota file
directly from userspace, maybe right before quota is enabled (and
before delayed allocation writes complete, such that some writes are
happening via the journal in ext4_quota_write and some w/o going
through the journal in ext4_writepage), and (b) what happens if quota
is disabled, the quota file is deleted, and some blocks get reused ---
and then system crashes before a journal commit can happen.

All of these problems go away if the quota file isn't visible from
userspace, and it becomes a special file.  In the short term I think
we could make this change, but I think we would also have to (1) treat
the quota file as immutable while quotas are enabled (so it cannot be
opened for writing), (2) force an fsync of the quota file and a
journal commit before enabling quotas, and (3) force a journal commit
after disabling quotas.

The other thing we might try that might mostly fix things is to change
ext4_should_journal_data() in ext4_jbd2.h to return true if it's a
quota file --- but we don't know which files are the quota files when
quotas are disabled, so we would still need to do (2) and (3).  But
this would allow us to write to the quota file while quotas are
enabled, if we think this is necessary --- although I think it's a bad
idea, so I'd be in favor of simply not allowing quota files to be
writable from userspace while quotas are enabled.  Jan, is this going
to cause any problems with quotautils?

OTOH, I think we have similar races with journaled quotas, and no one
has complained (although the vast majority of the quota documentation
on various HOWTO pages still don't talk about journaled quotas, so I
don't know how many people are using journaled quotas.  :-/ )

>   Ted, this patch fixes some JBD2 warning for me when running XFSQA
> with quotas enabled. I think this is a move into a direction you are
> trying to achieve as well. Will you merge the patch or should I do it?

I'm happy to carry the patch, since I Have Plans to try to make quotas
be a first class supported filesystem feature (i.e., make the quota
file a special file, and make quota files be always journaled if they
are journaled, and make the !@#! magic quota options handling in
/proc/mounts go away) in the 2.6.36 timeframe.

So the question is should we try to merge something like this for
2.6.35 or 2.6.35.y, and if so, how much bullet-proofing do we feel is
necessary for some of these races that I've outlined above.

	      	      	    	       	    - Ted
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