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

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Message-ID: <6601abe90909100846x3f7f491cnabc1474056155767@mail.gmail.com>
Date:	Thu, 10 Sep 2009 08:46:41 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: ext4: Can we talk about bforget() and metadata blocks

On Wed, Sep 9, 2009 at 11:54 PM, Aneesh Kumar
K.V<aneesh.kumar@...ux.vnet.ibm.com> wrote:
> 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 ?

As Ted explained to me, all buffer heads pointing to metadata blocks
are attached to the block device inode.  So pdflush writes of these
pages go through the block device address space ops.  Explicit
sync_dirty_buffer() calls for the metadata buffer heads still work, of
course.

Curt
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