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 <[email protected]>
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;
}
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 <[email protected]>
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 <[email protected]>
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 <[email protected]>
SUSE Labs, CR