If the page is dirty (code)

from mm/vmscan.c
 		if (PageDirty(page) && is_page_cache_freeable(page) && page->mapping) {
 			/*
 			 * It is not critical here to write it only if
 			 * the page is unmapped because any direct writer
 			 * like O_DIRECT would set the PG_dirty bitflag
 			 * on the physical page after having successfully
 			 * pinned it and after the I/O to the page is finished,
 			 * so the direct writes to the page cannot get lost.
 			 */
 			int (*writepage)(struct page *);

 			writepage = page->mapping->a_ops->writepage;
 			if ((gfp_mask & __GFP_FS) && writepage) {
 				ClearPageDirty(page);
 				SetPageLaunder(page);
 				page_cache_get(page);
 				spin_unlock(&pagemap_lru_lock); 
 
 				writepage(page);
 				page_cache_release(page);

 				spin_lock(&pagemap_lru_lock);
 				continue;
 			}
 		}