linux-ext4 - Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal

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Message-ID: <CAMBkX3dXrxRUSoRWWPzxaii-D0PF-1T2v9=RPhaApnCS2+x0FQ@mail.gmail.com>
Date:	Tue, 10 Jan 2012 13:55:17 -0800
From:	Surbhi Palande <csurbhi@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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

> > 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?

Regards,
Surbhi.
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