linux-ext4 - Re: ext4: Can we talk about bforget() and metadata blocks

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Message-ID: <20090910065401.GB8690@skywalker.linux.vnet.ibm.com>
Date:	Thu, 10 Sep 2009 12:24:01 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Curt Wohlgemuth <curtw@...gle.com>, linux-ext4@...r.kernel.org
Subject: Re: ext4: Can we talk about bforget() and metadata blocks

On Wed, Sep 09, 2009 at 09:35:40PM -0400, Theodore Tso wrote:
> On Wed, Sep 09, 2009 at 05:07:28PM -0700, Curt Wohlgemuth wrote:
> > 
> > First, ext4_journal_forget() is called from ext4_forget() only when
> > we're journalling; without a journal, ext4_journal_forget() is only
> > called for various non-extent paths.  ext4_forget() could be changed,
> > of course...
> 
> Ext4_forget() calls either ext4_journal_forget() or
> ext4_journal_revoke().  So we need to fix up both functions.
> 
> 			      	      	  - Ted
> 
> commit 4afdf0958f6f7b878e6d85cb4e0c0c12a0bd74e2
> Author: Theodore Ts'o <tytso@....edu>
> Date:   Wed Sep 9 21:32:41 2009 -0400
> 
>     ext4: Use bforget() in no journal mode for ext4_journal_{forget,revoke}()
>     
>     When ext4 is using a journal, a metadata block which is deallocated
>     must be passed into the journal layer so it can be dropped from the
>     current transaction and/or revoked.  This is done by calling the
>     functions ext4_journal_forget() and ext4_journal_revoke(), which call
>     jbd2_journal_forget(), and jbd2_journal_revoke(), respectively.
>     
>     Since the jbd2_journal_forget() and jbd2_journal_revoke() call
>     bforget(), if ext4 is not using a journal, ext4_journal_forget() and
>     ext4_journal_revoke() must call bforget() to avoid a dirty metadata
>     block overwriting a block after it has been reallocated and reused for
>     another inode's data block.
>     

I am sure i am missing something. But where are we adding the buffer_head
to the mapping->private_list ?. For ext2 when we allocate meta data blocks
we do mark_buffer_dirty_inode which add the buffer_head to the inodes
private_list. Shouldn't we do something similar with Ext4 without journal ?

-aneesh
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