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Date: Wed, 11 Jan 2012 01:51:19 +0100
From: Jan Kara <jack@suse.cz>
To: Surbhi Palande <csurbhi@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Eric Sandeen <sandeen@sandeen.net>,
        Kamal Mostafa <kamal@canonical.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
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        Theodore Tso <tytso@mit.edu>, linux-doc@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
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        Christopher Chaltain <christopher.chaltain@canonical.com>,
        "Peter M. Petrakis" <peter.petrakis@canonical.com>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Surbhi Palande <surbhi.palande@canonical.com>
Subject: Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal
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On Tue 10-01-12 16:13:46, Surbhi Palande wrote:
> >> If all the write operations were journaled, then this patch would not allow
> >> ext4 filesystem to have any dirty data after its frozen.
> >> (as journal_start() would block).
> >>
> >>  I think the only one candidate that creates dirty data without calling
> >> ext4_journal_start() is mmapped?
> >  No, the problem is in any write path. The problem is with operations
> > that happen during the phase when s_frozen == SB_FREEZE_WRITE. These
> > operations dirty the filesystem but running sync may easily miss them.
> > During this phase journal is not frozen so that does not help you in any
> > way.
> >
> >                                                                Honza
> 
> Ok! No new transaction can really start after the journal is frozen.
> But we can have dirty data after SB_FREEZE_WRITE and before
> SB_FREEZE_TRANS.
> I agree with you. However, can this be fixed by adding a
> sync_filesystem() in freeze_super() after the sb->s_op->freeze_fs() is
> over?
  The problem with this is that to do writeback you need to start a
transaction. So you would deadlock.

> So then essentially, when freeze_super() returns, the page cache is clean?
> 
> I do definitely agree that the fix is to add a lock for mutual
> exclusion between freeze filesystem and writes to a frozen filesystem.
  Yes, that would be the cleanest solution but it's quite some work.

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