ext4: optimize ext4_writepage() for attempted 4k delalloc writes

In cases where the file system block size is the same as the page
size, and ext4_writepage() is asked to write out a page which is
either has the unwritten bit set in the extent tree, or which does not
yet have a block assigned due to delayed allocation, we can bail out
early and, unlocking the page earlier and avoiding a round trip
through ext4_bio_write_page() with the attendant calls to
set_page_writeback() and redirty_page_for_writeback().
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -1815,11 +1815,22 @@ static int ext4_writepage(struct page *page,
 	 * the page. But we may reach here when we do a journal commit via
 	 * journal_submit_inode_data_buffers() and in that case we must write
 	 * allocated buffers to achieve data=ordered mode guarantees.
+	 *
+	 * Also, if there is only one buffer per page (the fs block
+	 * size == the page size), if one buffer needs block
+	 * allocation or needs to modify the extent tree to clear the
+	 * unwritten flag, we know that the page can't be written at
+	 * all, so we might as well refuse the write immediately.
+	 * Unfortunately if the block size != page size, we can't as
+	 * easily detect this case using ext4_walk_page_buffers(), but
+	 * for the extremely common case, this is an optimization that
+	 * skips a useless round trip through ext4_bio_write_page().
 	 */
 	if (ext4_walk_page_buffers(NULL, page_bufs, 0, len, NULL,
 				   ext4_bh_delay_or_unwritten)) {
 		redirty_page_for_writepage(wbc, page);
-		if (current->flags & PF_MEMALLOC) {
+		if ((current->flags & PF_MEMALLOC) ||
+		    (inode->i_sb->s_blocksize == PAGE_CACHE_SIZE)) {
 			/*
 			 * For memory cleaning there's no point in writing only
 			 * some buffers. So just bail out. Warn if we came here