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Date: Mon, 3 Mar 2008 12:23:47 +0100
From: Jan Kara <jack@...e.cz>
To: Josef Bacik <jbacik@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] fix the way jbd/jbd2 clears the b_modified flag
On Wed 27-02-08 13:46:09, Josef Bacik wrote:
> Hello,
>
> Currently at the start of a journal commit we loop through all of the buffers on
> the committing transaction and clear the b_modified flag (the flag that is set
> when a transaction modifies the buffer) under the j_list_lock. The problem is
> that everywhere else this flag is modified only under the jbd lock buffer flag,
> so it will race with a running transaction who could potentially set it, and
> have it unset by the committing transaction. This is also a big waste, you can
> have several thousands of buffers that you are clearing the modified flag on
> when you may not need to. This patch removes this code and instead clears the
> b_modified flag upon entering do_get_write_access/journal_get_create_access, so
> if that transaction does indeed use the buffer then it will be accounted for
> properly, and if it does not then we know we didn't use it. That will be
> important for the next patch in this series. Tested thoroughly by myself using
> postmark/iozone/bonnie++. Thank you,
>
> Signed-off-by: Josef Bacik <jbacik@...hat.com>
Nice work. Andrew has already added the patch to his tree but anyway I've
reviewed the patch and you can add
Acked-by: Jan Kara <jack@...e.cz>
Honza
>
> Index: linux-2.6/fs/jbd/commit.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd/commit.c
> +++ linux-2.6/fs/jbd/commit.c
> @@ -407,22 +407,6 @@ void journal_commit_transaction(journal_
> jbd_debug (3, "JBD: commit phase 2\n");
>
> /*
> - * First, drop modified flag: all accesses to the buffers
> - * will be tracked for a new trasaction only -bzzz
> - */
> - spin_lock(&journal->j_list_lock);
> - if (commit_transaction->t_buffers) {
> - new_jh = jh = commit_transaction->t_buffers->b_tnext;
> - do {
> - J_ASSERT_JH(new_jh, new_jh->b_modified == 1 ||
> - new_jh->b_modified == 0);
> - new_jh->b_modified = 0;
> - new_jh = new_jh->b_tnext;
> - } while (new_jh != jh);
> - }
> - spin_unlock(&journal->j_list_lock);
> -
> - /*
> * Now start flushing things to disk, in the order they appear
> * on the transaction lists. Data blocks go first.
> */
> Index: linux-2.6/fs/jbd/transaction.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd/transaction.c
> +++ linux-2.6/fs/jbd/transaction.c
> @@ -609,6 +609,12 @@ repeat:
> goto done;
>
> /*
> + * this is the first time this transaction is touching this buffer,
> + * reset the modified flag
> + */
> + jh->b_modified = 0;
> +
> + /*
> * If there is already a copy-out version of this buffer, then we don't
> * need to make another one
> */
> @@ -820,9 +826,16 @@ int journal_get_create_access(handle_t *
>
> if (jh->b_transaction == NULL) {
> jh->b_transaction = transaction;
> +
> + /* first access by this transaction */
> + jh->b_modified = 0;
> +
> JBUFFER_TRACE(jh, "file as BJ_Reserved");
> __journal_file_buffer(jh, transaction, BJ_Reserved);
> } else if (jh->b_transaction == journal->j_committing_transaction) {
> + /* first access by this transaction */
> + jh->b_modified = 0;
> +
> JBUFFER_TRACE(jh, "set next transaction");
> jh->b_next_transaction = transaction;
> }
> Index: linux-2.6/fs/jbd2/commit.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd2/commit.c
> +++ linux-2.6/fs/jbd2/commit.c
> @@ -520,22 +520,6 @@ void jbd2_journal_commit_transaction(jou
> jbd_debug (3, "JBD: commit phase 2\n");
>
> /*
> - * First, drop modified flag: all accesses to the buffers
> - * will be tracked for a new trasaction only -bzzz
> - */
> - spin_lock(&journal->j_list_lock);
> - if (commit_transaction->t_buffers) {
> - new_jh = jh = commit_transaction->t_buffers->b_tnext;
> - do {
> - J_ASSERT_JH(new_jh, new_jh->b_modified == 1 ||
> - new_jh->b_modified == 0);
> - new_jh->b_modified = 0;
> - new_jh = new_jh->b_tnext;
> - } while (new_jh != jh);
> - }
> - spin_unlock(&journal->j_list_lock);
> -
> - /*
> * Now start flushing things to disk, in the order they appear
> * on the transaction lists. Data blocks go first.
> */
> Index: linux-2.6/fs/jbd2/transaction.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd2/transaction.c
> +++ linux-2.6/fs/jbd2/transaction.c
> @@ -618,6 +618,12 @@ repeat:
> goto done;
>
> /*
> + * this is the first time this transaction is touching this buffer,
> + * reset the modified flag
> + */
> + jh->b_modified = 0;
> +
> + /*
> * If there is already a copy-out version of this buffer, then we don't
> * need to make another one
> */
> @@ -829,9 +835,16 @@ int jbd2_journal_get_create_access(handl
>
> if (jh->b_transaction == NULL) {
> jh->b_transaction = transaction;
> +
> + /* first access by this transaction */
> + jh->b_modified = 0;
> +
> JBUFFER_TRACE(jh, "file as BJ_Reserved");
> __jbd2_journal_file_buffer(jh, transaction, BJ_Reserved);
> } else if (jh->b_transaction == journal->j_committing_transaction) {
> + /* first access by this transaction */
> + jh->b_modified = 0;
> +
> JBUFFER_TRACE(jh, "set next transaction");
> jh->b_next_transaction = transaction;
> }
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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