GFS2: Improve bug trap code in ->releasepage()

If the buffer is dirty or pinned, then as well as printing a
warning, we should also refuse to release the page in
question.

Currently this can occur if there is a race between mmap()ed
writers and O_DIRECT on the same file. With the addition of
->launder_page() in the future, we should be able to close
this gap.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/aops.c

@@ -1076,8 +1076,8 @@ int gfs2_releasepage(struct page *page, gfp_t gfp_mask)
 		bd = bh->b_private;
 		if (bd && bd->bd_ail)
 			goto cannot_release;
-		gfs2_assert_warn(sdp, !buffer_pinned(bh));
-		gfs2_assert_warn(sdp, !buffer_dirty(bh));
+		if (buffer_pinned(bh) || buffer_dirty(bh))
+			goto not_possible;
 		bh = bh->b_this_page;
 	} while(bh != head);
 	gfs2_log_unlock(sdp);
@@ -1107,6 +1107,10 @@ int gfs2_releasepage(struct page *page, gfp_t gfp_mask)
 	} while (bh != head);
 
 	return try_to_free_buffers(page);
+
+not_possible: /* Should never happen */
+	WARN_ON(buffer_dirty(bh));
+	WARN_ON(buffer_pinned(bh));
 cannot_release:
 	gfs2_log_unlock(sdp);
 	return 0;