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Date: Tue, 16 Feb 2010 20:33:05 +0100
From: Jan Kara <jack@...e.cz>
To: tytso@....edu
Cc: Jan Kara <jack@...e.cz>, dingdinghua <dingdinghua@...hpc.ac.cn>,
linux-ext4@...r.kernel.org
Subject: Re: [PATCH] Jbd2: delay discarding buffers in journal_unmap_buffer
On Mon 15-02-10 20:45:21, tytso@....edu wrote:
> On Mon, Feb 08, 2010 at 05:03:33PM +0100, Jan Kara wrote:
> > On Sat 06-02-10 12:54:23, dingdinghua wrote:
> > > Delay discarding buffers in journal_unmap_buffer until
> > > we know that "add to orphan" operation has definitely been
> > > committed, otherwise the log space of committing transation
> > > may be freed and reused before truncate get committed, updates
> > > may get lost if crash happens.
> > >
> > > Signed-off-by: dingdinghua <dingdinghua@...hpc.ac.cn>
> > Thanks for the patch. Just two remarks to the comments below...
>
> Here's the version I've applied to the ext4 patch queue, which
> includes Jan's suggested changes to the comments.
Thanks for summing it up. The patch looks good.
Acked-by: Jan Kara <jack@...e.cz>
I'm going to backport the patch to JBD now.
Honza
> commit 20ca286ad19b76483a79b44c5172feeaad02065b
> Author: dingdinghua <dingdinghua@...hpc.ac.cn>
> Date: Mon Feb 15 16:35:42 2010 -0500
>
> jbd2: delay discarding buffers in journal_unmap_buffer
>
> Delay discarding buffers in journal_unmap_buffer until
> we know that "add to orphan" operation has definitely been
> committed, otherwise the log space of committing transation
> may be freed and reused before truncate get committed, updates
> may get lost if crash happens.
>
> Signed-off-by: dingdinghua <dingdinghua@...hpc.ac.cn>
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
>
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 1bc74b6..3ee211e 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -930,12 +930,12 @@ restart_loop:
> /* A buffer which has been freed while still being
> * journaled by a previous transaction may end up still
> * being dirty here, but we want to avoid writing back
> - * that buffer in the future now that the last use has
> - * been committed. That's not only a performance gain,
> - * it also stops aliasing problems if the buffer is left
> - * behind for writeback and gets reallocated for another
> + * that buffer in the future after the "add to orphan"
> + * operation been committed, That's not only a performance
> + * gain, it also stops aliasing problems if the buffer is
> + * left behind for writeback and gets reallocated for another
> * use in a different page. */
> - if (buffer_freed(bh)) {
> + if (buffer_freed(bh) && !jh->b_next_transaction) {
> clear_buffer_freed(bh);
> clear_buffer_jbddirty(bh);
> }
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index a051270..bfc70f5 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -1727,6 +1727,21 @@ static int journal_unmap_buffer(journal_t *journal, struct buffer_head *bh)
> if (!jh)
> goto zap_buffer_no_jh;
>
> + /*
> + * We cannot remove the buffer from checkpoint lists until the
> + * transaction adding inode to orphan list (let's call it T)
> + * is committed. Otherwise if the transaction changing the
> + * buffer would be cleaned from the journal before T is
> + * committed, a crash will cause that the correct contents of
> + * the buffer will be lost. On the other hand we have to
> + * clear the buffer dirty bit at latest at the moment when the
> + * transaction marking the buffer as freed in the filesystem
> + * structures is committed because from that moment on the
> + * buffer can be reallocated and used by a different page.
> + * Since the block hasn't been freed yet but the inode has
> + * already been added to orphan list, it is safe for us to add
> + * the buffer to BJ_Forget list of the newest transaction.
> + */
> transaction = jh->b_transaction;
> if (transaction == NULL) {
> /* First case: not on any transaction. If it
> @@ -1783,16 +1798,15 @@ static int journal_unmap_buffer(journal_t *journal, struct buffer_head *bh)
> } else if (transaction == journal->j_committing_transaction) {
> JBUFFER_TRACE(jh, "on committing transaction");
> /*
> - * If it is committing, we simply cannot touch it. We
> - * can remove it's next_transaction pointer from the
> - * running transaction if that is set, but nothing
> - * else. */
> + * The buffer is committing, we simply cannot touch
> + * it. So we just set j_next_transaction to the
> + * running transaction (if there is one) and mark
> + * buffer as freed so that commit code knows it should
> + * clear dirty bits when it is done with the buffer.
> + */
> set_buffer_freed(bh);
> - if (jh->b_next_transaction) {
> - J_ASSERT(jh->b_next_transaction ==
> - journal->j_running_transaction);
> - jh->b_next_transaction = NULL;
> - }
> + if (journal->j_running_transaction && buffer_jbddirty(bh))
> + jh->b_next_transaction = journal->j_running_transaction;
> jbd2_journal_put_journal_head(jh);
> spin_unlock(&journal->j_list_lock);
> jbd_unlock_bh_state(bh);
> @@ -1969,7 +1983,7 @@ void jbd2_journal_file_buffer(struct journal_head *jh,
> */
> void __jbd2_journal_refile_buffer(struct journal_head *jh)
> {
> - int was_dirty;
> + int was_dirty, jlist;
> struct buffer_head *bh = jh2bh(jh);
>
> J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
> @@ -1991,8 +2005,13 @@ void __jbd2_journal_refile_buffer(struct journal_head *jh)
> __jbd2_journal_temp_unlink_buffer(jh);
> jh->b_transaction = jh->b_next_transaction;
> jh->b_next_transaction = NULL;
> - __jbd2_journal_file_buffer(jh, jh->b_transaction,
> - jh->b_modified ? BJ_Metadata : BJ_Reserved);
> + if (buffer_freed(bh))
> + jlist = BJ_Forget;
> + else if (jh->b_modified)
> + jlist = BJ_Metadata;
> + else
> + jlist = BJ_Reserved;
> + __jbd2_journal_file_buffer(jh, jh->b_transaction, jlist);
> J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);
>
> if (was_dirty)
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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