jbd2: limit number of reserved credits

Currently there is no limitation on number of reserved credits we can
ask for. If we ask for more reserved credits than 1/2 of maximum
transaction size, or if total number of credits exceeds the maximum
transaction size per operation (which is currently only possible with
the former) we will spin forever in start_this_handle().

Fix this by adding this limitation at the start of start_this_handle().

This patch also removes the credit limitation 1/2 of maximum transaction
size, since we really only want to limit the number of reserved credits.
There is not much point to limit the credits if there is still space in
the journal.

This accidentally also fixes the online resize, where due to the
limitation of the journal credits we're unable to grow file systems with
1k block size and size between 16M and 32M. It has been partially fixed
by 2c869b262a10ca99cb866d04087d75311587a30c, but not entirely.

Thanks Jan Kara for helping me getting the correct fix.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/transaction.c

@@ -204,6 +204,20 @@ static int add_transaction_credits(journal_t *journal, int blocks,
 		 * attach this handle to a new transaction.
 		 */
 		atomic_sub(total, &t->t_outstanding_credits);
+
+		/*
+		 * Is the number of reserved credits in the current transaction too
+		 * big to fit this handle? Wait until reserved credits are freed.
+		 */
+		if (atomic_read(&journal->j_reserved_credits) + total >
+		    journal->j_max_transaction_buffers) {
+			read_unlock(&journal->j_state_lock);
+			wait_event(journal->j_wait_reserved,
+				   atomic_read(&journal->j_reserved_credits) + total <=
+				   journal->j_max_transaction_buffers);
+			return 1;
+		}
+
 		wait_transaction_locked(journal);
 		return 1;
 	}
@@ -262,20 +276,24 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
 	int		rsv_blocks = 0;
 	unsigned long ts = jiffies;
 
+	if (handle->h_rsv_handle)
+		rsv_blocks = handle->h_rsv_handle->h_buffer_credits;
+
 	/*
-	 * 1/2 of transaction can be reserved so we can practically handle
-	 * only 1/2 of maximum transaction size per operation
+	 * Limit the number of reserved credits to 1/2 of maximum transaction
+	 * size and limit the number of total credits to not exceed maximum
+	 * transaction size per operation.
 	 */
-	if (WARN_ON(blocks > journal->j_max_transaction_buffers / 2)) {
-		printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n",
-		       current->comm, blocks,
-		       journal->j_max_transaction_buffers / 2);
+	if ((rsv_blocks > journal->j_max_transaction_buffers / 2) ||
+	    (rsv_blocks + blocks > journal->j_max_transaction_buffers)) {
+		printk(KERN_ERR "JBD2: %s wants too many credits "
+		       "credits:%d rsv_credits:%d max:%d\n",
+		       current->comm, blocks, rsv_blocks,
+		       journal->j_max_transaction_buffers);
+		WARN_ON(1);
 		return -ENOSPC;
 	}
 
-	if (handle->h_rsv_handle)
-		rsv_blocks = handle->h_rsv_handle->h_buffer_credits;
-
 alloc_transaction:
 	if (!journal->j_running_transaction) {
 		/*