linux-ext4 - Re: ext3/4: BUGS in changing journal mode in flight

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Date:	Mon, 14 Nov 2011 15:08:41 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"Ted Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: ext3/4: BUGS in changing journal mode in flight

On Fri, Nov 11, 2011 at 11:26 PM, Jan Kara <jack@...e.cz> wrote:
>  Hello,
>
> On Fri 11-11-11 16:20:21, Yongqiang Yang wrote:
>> Both ext3 and ext4 have an attribute 'j', thus users can change
>> journal mode of a file in flight.  However, neither ext3 or ext4
>> supports the feature rightly, specifically file systems will be
>> mounted RO due to I/O error in journal.  The error happens like this:
>  I suppose you have tested this, right?
>
>> when the journal mode of a file is changed from ordered to journaled,
>> then data blocks will be revoked in truncate, if the revoked block was
>> allocated as meta blocks last time, then its revoked flag is not
>> cleared, so revoke on a revoked buffer happens, then journal report an
>> I/O error.
>  So let me check whether I understand the problem: We have a buffer that
> has been previously used for metadata, then truncated (BH_Revoked set),
> allocated as data, switched the inode to journal mode, and then deletion of
> the inode causes error because we try to revoke a buffer with BH_Revoked
> set. Nasty.
>
>> Now revoked flag is cleared in get_access, actually it can be cleared
>> immediately after the transaction is closed(non-current transaction).
>  I'm not sure what you mean by closed but there is a difference between a
> BH_Revoked flag and the fact whether the buffer is recorded as revoked in
> the journal. If we concentrate on the flag, which is the problem here, then
> the flag seems to be used as "is buffer revoked in the running trasaction".
> Which means we can clear the flag as soon as we are done with adding
> buffers to the transation.
>
>> I can think of 3 solutions addressing the problem.
>>
>> 1. clear revoked flag on a buffer when allocating a block.
>>     This solution introduces overhead to ordered mode, because we have
>> to look up dev's buffer every time. So I think the solution can be
>> ignored.
>>
>> 2. clear revoked flag in commit_call_back function.
>>    This solution has some overhead too.  in ext3 commit_call_back is
>> not used at all, so changing to ext3 is not trivial.
>>
>> 3. clear revoked flag in write_revoke_table.
>>    IMHO, this solution is the best one.  We just need to add a field
>> in in-memory journal super block pointing to the super block of host
>> file system.
>>
>> What's your opinions?
>  Hum, maybe journal_switch_revoke_table() would be a better place than
> write_revoke_table() to do the clearing. Or we could just add a separate
> call just before journal_switch_revoke_table() which will also get a
> superblock as a parameter so that we don't have to store it in the journal
> structure.
Sorry for noise.  There is block_device(j_fs_dev) holding client fs in
journal_s, so we can do this easily.

Thanks,
Yongqiang.
>
>  Thanks for spotting the problem!
>
>                                                                Honza
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
>



-- 
Best Wishes
Yongqiang Yang
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