[PATCH 1/2] fix the way jbd/jbd2 clears the b_modified flag

[Josef Bacik]

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@xxxxxxxxxx>

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;
 	}
--
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