linux-kernel - Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal

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Date:	Wed, 11 Jan 2012 01:04:48 +0100
From:	Jan Kara <jack@...e.cz>
To:	Surbhi Palande <csurbhi@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Eric Sandeen <sandeen@...deen.net>,
	Kamal Mostafa <kamal@...onical.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Matthew Wilcox <matthew@....cx>,
	Randy Dunlap <rdunlap@...otime.net>,
	Theodore Tso <tytso@....edu>, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Valerie Aurora <val@...consulting.com>,
	Christopher Chaltain <christopher.chaltain@...onical.com>,
	"Peter M. Petrakis" <peter.petrakis@...onical.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Surbhi Palande <surbhi.palande@...onical.com>
Subject: Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal

On Tue 10-01-12 13:50:22, Surbhi Palande wrote:
> > > Hrm let me think through this a little more; we actually do:
> > >
> > > t16) ext4_journal_start()
> > >   t17) ext4_journal_start_sb()
> > >     t18) handle = ext4_journal_current_handle();
> > >     t19) if (!handle) vfs_check_frozen()
> > >     t20) ... jbd2_journal_start()
> >   Ah, right. I forgot.
> >
> > > So actually we *do* block new handles, but let *existing* ones
> > > continue (see commits 6b0310fbf087ad6e9e3b8392adca97cd77184084
> > > and be4f27d324e8ddd57cc0d4d604fe85ee0425cba9)
> > >
> > > So your assertion that a new handle is started is incorrect
> > > in general, isn't it?  So then does the fix seem necessary?
> > > Or, at least, in the fashion below - maybe we need to just make
> > > sure all started handles complete before the unlock_updates?
> > > Or am I missing something...?
> >   Well, the problem with running operations and freezing is more
> > fundamental I believe. See my email
> > http://marc.info/?l=linux-fsdevel&m=132585911925796&w=2
> >
> > So I believe we'll need some better exclusion mechanism already in VFS.
> >
> >                                                                Honza
> >
> 
> 
> 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
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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