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

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Message-ID: <6601abe90909100855h6ef60d32vfbbf23fa15fc66aa@mail.gmail.com>
Date:	Thu, 10 Sep 2009 08:55:14 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	Theodore Tso <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: ext4: Can we talk about bforget() and metadata blocks

On Wed, Sep 9, 2009 at 6:35 PM, Theodore Tso<tytso@....edu> 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.
>
>    Signed-off-by: "Theodore Ts'o" <tytso@....edu>
>
> diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
> index eb27fd0..ecb9ca4 100644
> --- a/fs/ext4/ext4_jbd2.c
> +++ b/fs/ext4/ext4_jbd2.c
> @@ -44,7 +44,7 @@ int __ext4_journal_forget(const char *where, handle_t *handle,
>                                                  handle, err);
>        }
>        else
> -               brelse(bh);
> +               bforget(bh);
>        return err;
>  }
>
> @@ -60,7 +60,7 @@ int __ext4_journal_revoke(const char *where, handle_t *handle,
>                                                  handle, err);
>        }
>        else
> -               brelse(bh);
> +               bforget(bh);
>        return err;
>  }

This looks quite reasonable to me.  And of course, your patch fixes
the brelse() calls that I added back in July.

Of course, validating this patch on my end will be tough; I need at
least a week of no corruptions to convince myself that the problem is
fixed, to the same degree that my call to sync_dirty_buffer() works.

Thanks,
Curt
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