xfs: PF_FSTRANS should never be set in ->writepage

Now that we reject direct reclaim in addition to always using GFP_NOFS
allocation there's no chance we'll ever end up in ->writepage with
PF_FSTRANS set.  Add a WARN_ON if we hit this case, and stop checking
if we'd actually need to start a transaction.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NAlex Elder <aelder@sgi.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>

fs/xfs/linux-2.6/xfs_aops.c

@@ -894,11 +894,6 @@ xfs_aops_discard_page(
  * For unwritten space on the page we need to start the conversion to
  * regular allocated space.
  * For any other dirty buffer heads on the page we should flush them.
- *
- * If we detect that a transaction would be required to flush the page, we
- * have to check the process flags first, if we are already in a transaction
- * or disk I/O during allocations is off, we need to fail the writepage and
- * redirty the page.
  */
 STATIC int
 xfs_vm_writepage(
@@ -906,7 +901,6 @@ xfs_vm_writepage(
 	struct writeback_control *wbc)
 {
 	struct inode		*inode = page->mapping->host;
-	int			delalloc, unwritten;
 	struct buffer_head	*bh, *head;
 	struct xfs_bmbt_irec	imap;
 	xfs_ioend_t		*ioend = NULL, *iohead = NULL;
@@ -938,15 +932,10 @@ xfs_vm_writepage(
 		goto redirty;
 
 	/*
-	 * We need a transaction if there are delalloc or unwritten buffers
-	 * on the page.
-	 *
-	 * If we need a transaction and the process flags say we are already
-	 * in a transaction, or no IO is allowed then mark the page dirty
-	 * again and leave the page as is.
+	 * Given that we do not allow direct reclaim to call us, we should
+	 * never be called while in a filesystem transaction.
 	 */
-	xfs_count_page_state(page, &delalloc, &unwritten);
-	if ((current->flags & PF_FSTRANS) && (delalloc || unwritten))
+	if (WARN_ON(current->flags & PF_FSTRANS))
 		goto redirty;
 
 	/* Is this page beyond the end of the file? */