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Date:	Wed, 11 Jan 2012 08:45:17 -0800
From:	Surbhi Palande <>
To:	Jan Kara <>
Cc:	Eric Sandeen <>,
	Kamal Mostafa <>,
	Andreas Dilger <>,
	Randy Dunlap <>,
	Theodore Tso <>,,
	Valerie Aurora <>,
	Christopher Chaltain <>,
	"Peter M. Petrakis" <>,
	Mikulas Patocka <>
Subject: Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal

Hi Jan,

Thanks a lot for your comments :)

Isn't dirty data flushed out in "ordered" mode? as
ext4_jbd2_file_inode() will get called for ordered writes. Thus this
inode's data is flushed at journal commit time through

However I do see that we will still have a dirty data problem for
"writeback" and "journalled" mode?


On Wed, Jan 11, 2012 at 4:10 AM, Jan Kara <> wrote:
> On Tue 10-01-12 21:38:29, Surbhi Palande wrote:
>> On second thoughts, I fail to see why there is still a race window
>> after this patch.
>> Here are the reasons why i fail to see how the data can be dirtied
>> when all the operations involve a journal:
>> ----------
>> So here is the problem that we see
>>       CPU1                                                     CPU2
>>        Task1 (write operation)                                          Task2
>> ---------------------------------------------------------------------------------------
>> t1    ext4_journal_start()
>> t2      ext4_journal_start_sb()
>> t3        vfs_check_frozen                            sb->frozen=SB_FREEZE_WRITE
>> t4            jbd2_journal_start()                    /* hence forth all processes calling
>> vfs_check_frozen will wait */
>  Note that we call vfs_check_frozen(sb, SB_FREEZE_TRANS) in
> ext4_journal_start_sb(). Thus we start blocking only when s_frozen ==
> SB_FREEZE_TRANS and we just ignore s_frozen == SB_FREEZE_WRITE.
>> Now, our aim is to stop Task1 from dirtying the page cache ie in
>> starting this transaction. However if it is successful in starting
>> this transaction, then we want to make sure that this transaction is
>> flushed out.
>> Correct?
>  Not quite. Flushing a journal will flush dirty metadata but we will still
> have dirty pages (dirty data is not part of any transaction). So in the
> scenarion I describe in
> all metadata changes will be flushed inside ->freeze_fs (at least for
> journalling filesystems) but pages will be left dirty. Is it clearer now?
> But your comment makes me realize that the situation is simpler than I
> thought by the fact that we only have to protect paths that create dirty
> data as dirty metadata can be handled by flushing a journal. And there are
> only a few places creating dirty data. So a reasonably clean solution
> shouldn't be that complicated after all. I'll tweak my patch and try it in
> a moment.
>                                                                Honza
> --
> Jan Kara <>
> SUSE Labs, CR
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