linux-ext4 - Re: Can a metadata buffer end up in journal_unmap

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Message-ID: <4E440068.6030403@redhat.com>
Date:	Thu, 11 Aug 2011 12:16:40 -0400
From:	Josef Bacik <josef@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: Can a metadata buffer end up in journal_unmap_buffer?

On 08/11/2011 11:28 AM, Jan Kara wrote:
>   Hello,
> 
> On Thu 11-08-11 09:32:22, Josef Bacik wrote:
>> I have this weird bug that has been plaguing me for a while where
>> t_outstanding_credits will end up less than t_nr_buffers.  I have done
>> all sorts of things to try and catch when it happens but nothing seems
>> to catch it.  At some point I had thought that we were screwing up in
>> journal_unmap_buffer.  If a buffer is not on a transaction but is still
>> a part of a checkpoint we will do a journal_file_buffer() onto the
>> current running transaction's forget list.  The thing is we can still
>> have b_modified set since we only clear it on
>> do_get_write_access/journal_get_create_access if it isn't a part of the
>> transaction yet.  So if we do the journal_file_buffer() before anybody
>> calls do_get_write_access/journal_get_create_access we will short
>> circuit these checks and b_modified will never be cleared and so when we
>> do journal_dirty_metadata we won't account for the new buffer and it
>> will end up inc'ing t_nr_buffers but not t_outstanding_credits.
>   Good spotting!
> 
>> I had thought this was the problem before and put in a jh->b_modified =
>> 0 in __dispose_buffer, but apparently the problem still happened.  But
>> that support person/customer were not entirely reliable so I'm back to
>> thinking this is what happened and they just didn't run with my patch.
>   Umm, I think there's one more way how buffer b_modified == 1 can get
> to other transaction's forget list. In journal_unmap_buffer(), transaction
> == journal->j_committing_transaction case we do set_buffer_freed() and
> set b_next_transaction to the running transaction. So when the currently
> committing transaction finishes, it refiles the buffer to BJ_Forget list
> of the running transaction. b_modified handling seems to be really fragile
> in this regard. I guess the rule is that whenever we are going to change
> b_transaction or b_next_transaction, we should clear b_modified.
> 

Well this is happening on RHEL5, where we have

set_buffer_freed();
if (jh->b_next_transaction)
	jh->b_next_transaction = NULL;

so the only way this happens if it goes through __dispose_buffer.

And the more I look at this I can't see how it would happen exactly.  I
can definitely get a modified buffer to show up on the forget list, but
I can't see how I would then re-modify the thing to get it to show up on
BJ_Metadata.  On data=journal mode I can definitely see how to do it,
but not with data=ordered mode.  The only way to go through
journal_unmap_buffer is to truncate the inode, and for a symlink the
only way to make that happen is to delete it.  So I don't see how I
could then make it get dirtied again?  Thanks,

Josef
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