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

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Date:	Wed, 11 Jan 2012 08:45:17 -0800
From:	Surbhi Palande <csurbhi@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Eric Sandeen <sandeen@...deen.net>,
	Kamal Mostafa <kamal@...onical.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Randy Dunlap <rdunlap@...otime.net>,
	Theodore Tso <tytso@....edu>, linux-ext4@...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>
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
journal_submit_data_buffers()?

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

Regards,
Surbhi.








On Wed, Jan 11, 2012 at 4:10 AM, Jan Kara <jack@...e.cz> 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
> http://marc.info/?l=linux-fsdevel&m=132585911925796&w=2
> 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 <jack@...e.cz>
> SUSE Labs, CR
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