Date:	Thu, 21 Feb 2008 13:58:55 -0500
From:	Josef Bacik <jbacik@...hat.com>
To:	linux-ext4@...r.kernel.org
Cc:	Jan Kara <jack@...e.cz>
Subject: [RFC][PATCH] fix journal overflow problem

Hello,

This is related to that jbd patch I sent a few weeks ago.  I originally found 
that the problem where t_nr_buffers would be > than t_outstanding_credits 
wouldn't happen upstream, but apparently I'm an idiot and I was just missing my 
messsages, and the problem does exist.  Now for the entirely too long of a 
description of whats going wrong.

Say we have a transaction dirty a bitmap buffer and go to flush it to the disk.  
Then ext3 goes to get write access to that buffer via 
journal_get_undo_access(), finds out it doesn't need it, and then subsequently 
does a journal_release_buffer() and then proceeds to never touch that buffer 
again.  Then the original committing transaction will go through and add its 
buffers to the checkpointing list, and refile the buffer.  Because we did a 
journal_get_undo_access(), the jh->b_next_transaction is set to our currently 
running transaction, and that buffer because it was set BH_JBDDirty by the 
committing transaction is filed onto the running transactions BJ_Metadata list, 
which increments our t_nr_buffers counter.  Because we never actually dirtied 
this buffer ourselves, we never accounted for the credit, and we end up with 
t_outstanding_credits being less than t_nr_buffers.

This is a problem because while we are writing out the metadata blocks to the 
journal, we do a t_outstanding_credits-- for each buffer.  If 
t_outstanding_credits is less than the number of buffers we have then 
t_outstanding_credits will eventually become negative, which means that 
jbd_space_needed will eventually start saying it needs way less credits than it 
actually does, and will allow transactions to grow huge and eventually we'll 
overflow the journal (albeit this is a bitch to try and reproduce).

So my approach is to have a counter that is incremented each time the 
transaction calls do_get_write_access (or journal_get_create_access) so we can 
keep track of how many people are currently trying to modify that buffer.  So 
in the case where we do a journal_get_undo_access()+journal_release_buffer() 
and nobody else ever touches the buffer, we can then set jh->b_next_transaction 
to NULL in journal_release_buffer() and avoid having the buffer filed onto our 
transaction.  If somebody else is modifying the journal head then we know to 
leave it alone because chances are it will be dirtied and the credit will be 
accounted for.

There is also a slight change to how we reset b_modified.  I originally reset 
b_nr_access (my access counter) in the same way b_modified was reset, but I 
didn't really like this because we were only taking the j_list_lock instead of 
the jbd_buffer lock, so we could race and still end up in the same situation 
(which is in fact what happened).  So I've changed how we reset b_modified.  
Instead of looping through all of the buffers for the transaction, which is a 
little innefficient anyway, we reset it in do_get_write_access in the cases 
where we know that this is the first time that this transaction has accessed 
the buffer (ie when b_next_transaction != transaction && b_transaction != 
transaction).  I reset b_nr_access in the same way.  I ran tests with this 
patch and verified that we no longer got into the situation where 
t_outstanding_credits was less than t_nr_buffers.

This is just my patch that I was using, I plan on cleaning it up if this is an 
acceptable way to fix the problem.  I'd also like to put an ASSERT before we 
process the t_buffers list for the case where t_outstanding_credits is less 
than t_nr_buffers.  If my particular solution isn't acceptable I'm open to 
suggestions, however I still think that resetting b_modified should be changed 
the way I have it as its a potential race condition and innefficient.  Thanks 
much,

Josef

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index a38c718..6cc0a1e 100644
--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -407,22 +407,6 @@ void journal_commit_transaction(journal_t *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.
 	 */
@@ -490,6 +474,11 @@ void journal_commit_transaction(journal_t *journal)
 
 	descriptor = NULL;
 	bufs = 0;
+
+	if (commit_transaction->t_nr_buffers >
+	    commit_transaction->t_outstanding_credits)
+		printk(KERN_EMERG "nr buffers greater than outstanding credits\n");
+
 	while (commit_transaction->t_buffers) {
 
 		/* Find the next buffer to be journaled... */
diff --git a/fs/jbd/journal.c b/fs/jbd/journal.c
index 3943a89..5059b79 100644
--- a/fs/jbd/journal.c
+++ b/fs/jbd/journal.c
@@ -1800,6 +1800,7 @@ static void __journal_remove_journal_head(struct 
buffer_head *bh)
 				printk(KERN_WARNING "%s: freeing "
 						"b_committed_data\n",
 						__FUNCTION__);
+				dump_stack();
 				jbd_free(jh->b_committed_data, bh->b_size);
 			}
 			bh->b_private = NULL;
diff --git a/fs/jbd/transaction.c b/fs/jbd/transaction.c
index 038ed74..db00d39 100644
--- a/fs/jbd/transaction.c
+++ b/fs/jbd/transaction.c
@@ -609,6 +609,13 @@ repeat:
 		goto done;
 
 	/*
+	 * This is the first time this transaction has touched this buffer
+	 * reset b_modified and b_nr_access
+	 */
+	jh->b_modified = 0;
+	jh->b_nr_access = 0;
+
+	/*
 	 * If there is already a copy-out version of this buffer, then we don't
 	 * need to make another one
 	 */
@@ -713,6 +720,11 @@ repeat:
 	}
 
 done:
+	/*
+	 * Ok we got write access, update the access counter
+	 */
+	jh->b_nr_access++;
+
 	if (need_copy) {
 		struct page *page;
 		int offset;
@@ -819,13 +831,33 @@ int journal_get_create_access(handle_t *handle, struct 
buffer_head *bh)
 	J_ASSERT_JH(jh, buffer_locked(jh2bh(jh)));
 
 	if (jh->b_transaction == NULL) {
+		/*
+		 * first time this transaction has touched this buffer,
+		 * reset b_modified
+		 */
+		jh->b_modified = 0;
+
 		jh->b_transaction = transaction;
 		JBUFFER_TRACE(jh, "file as BJ_Reserved");
 		__journal_file_buffer(jh, transaction, BJ_Reserved);
 	} else if (jh->b_transaction == journal->j_committing_transaction) {
 		JBUFFER_TRACE(jh, "set next transaction");
+
+		if (jh->b_next_transaction != transaction) {
+			/* reset b_modified */
+			jh->b_modified = 0;
+		}
+
 		jh->b_next_transaction = transaction;
 	}
+
+	/*
+	 * Nobody who calls journal_get_create_access calls
+	 * journal_release_buffer, but if at some point somebody decides to
+	 * we'll need this
+	 */
+	jh->b_nr_access++;
+
 	spin_unlock(&journal->j_list_lock);
 	jbd_unlock_bh_state(bh);
 
@@ -1190,11 +1222,41 @@ out:
  * journal_release_buffer: undo a get_write_access without any buffer
  * updates, if the update decided in the end that it didn't need access.
  *
+ * We decrement the b_nr_access as this person no longer requires write
+ * access to the buffer, and if nobody else requires write access then we want
+ * to set b_next_transaction to NULL in case this buffer is on the committing
+ * transaction, as it will get refiled onto this transaction without it being
+ * counted.  We don't worry about the case where this is the first time this
+ * jh has been used (ie jh->b_transaction == handle->h_transaction) since the
+ * beginning of the committing of this transaction will drop the jh.
  */
 void
 journal_release_buffer(handle_t *handle, struct buffer_head *bh)
 {
+	struct journal_head *jh = bh2jh(bh);
 	BUFFER_TRACE(bh, "entry");
+
+	jbd_lock_bh_state(bh);
+	/*
+	 * if nobody else has requested write access to this jh then set
+	 * b_next_transaction to NULL to keep it from getting refiled onto
+	 * this transaction
+	 */
+	if (!(--jh->b_nr_access)) {
+
+		/*
+		 * A journal_get_undo_access()+journal_release_buffer() will
+		 * leave undo-committed data that we no longer need
+		 */
+		if (jh->b_next_transaction && jh->b_committed_data) {
+			jbd_free(jh->b_committed_data, bh->b_size);
+			jh->b_committed_data = NULL;
+		}
+
+		jh->b_next_transaction = NULL;
+	}
+
+	jbd_unlock_bh_state(bh);
 }
 
 /**
@@ -1245,6 +1307,11 @@ int journal_forget (handle_t *handle, struct buffer_head 
*bh)
 	 */
 	jh->b_modified = 0;
 
+	/*
+	 * drop one of our write access credits
+	 */
+	jh->b_nr_access--;
+
 	if (jh->b_transaction == handle->h_transaction) {
 		J_ASSERT_JH(jh, !jh->b_frozen_data);
 
diff --git a/include/linux/journal-head.h b/include/linux/journal-head.h
index 8a62d1e..160f1d1 100644
--- a/include/linux/journal-head.h
+++ b/include/linux/journal-head.h
@@ -39,6 +39,13 @@ struct journal_head {
 	unsigned b_modified;
 
 	/*
+	 * This is a counter of the number of things who have requested write
+	 * access to this buffer by the current transaction
+	 * [jbd_lock_bh_state()]
+	 */
+	unsigned b_nr_access;
+
+	/*
 	 * Copy of the buffer data frozen for writing to the log.
 	 * [jbd_lock_bh_state()]
 	 */
-
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