ext4: warn if direct reclaim tries to writeback pages

Direct reclaim should never writeback pages.  Warn if an attempt is made.
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Alex Elder <aelder@sgi.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/inode.c

@@ -1811,8 +1811,12 @@ static int ext4_writepage(struct page *page,
 		 * We don't want to do block allocation, so redirty
 		 * the page and return.  We may reach here when we do
 		 * a journal commit via journal_submit_inode_data_buffers.
-		 * We can also reach here via shrink_page_list
+		 * We can also reach here via shrink_page_list but it
+		 * should never be for direct reclaim so warn if that
+		 * happens
 		 */
+		WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
+								PF_MEMALLOC);
 		goto redirty_page;
 	}
 	if (commit_write)