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

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Date:	Tue, 10 Jan 2012 16:13:46 -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

Hi Jan,

>>
>>
>> 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?


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.

Thanks!

Regards,
Surbhi.
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