Revert "gfs2: stop using generic_writepages in gfs2_ail1_start_one"

Commit b2b0a5e978 switched from generic_writepages() to
filemap_fdatawrite_wbc() in gfs2_ail1_start_one() on the path to
replacing ->writepage() with ->writepages() and eventually eliminating
the former.  Function gfs2_ail1_start_one() is called from
gfs2_log_flush(), our main function for flushing the filesystem log.

Unfortunately, at least as implemented today, ->writepage() and
->writepages() are entirely different operations for journaled data
inodes: while the former creates and submits transactions covering the
data to be written, the latter flushes dirty buffers out to disk.

With gfs2_ail1_start_one() now calling ->writepages(), we end up
creating filesystem transactions while we are in the course of a log
flush, which immediately deadlocks on the sdp->sd_log_flush_lock
semaphore.

Work around that by going back to how things used to work before commit
b2b0a5e978 for now; figuring out a superior solution will take time we
don't have available right now.  However ...

Since the removal of generic_writepages() is imminent, open-code it
here.  We're already inside a blk_start_plug() ...  blk_finish_plug()
section here, so skip that part of the original generic_writepages().

This reverts commit b2b0a5e978.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>

fs/gfs2/log.c

@@ -80,6 +80,15 @@ void gfs2_remove_from_ail(struct gfs2_bufdata *bd)
 	brelse(bd->bd_bh);
 }
 
+static int __gfs2_writepage(struct page *page, struct writeback_control *wbc,
+		       void *data)
+{
+	struct address_space *mapping = data;
+	int ret = mapping->a_ops->writepage(page, wbc);
+	mapping_set_error(mapping, ret);
+	return ret;
+}
+
 /**
  * gfs2_ail1_start_one - Start I/O on a transaction
  * @sdp: The superblock
@@ -131,7 +140,7 @@ __acquires(&sdp->sd_ail_lock)
 		if (!mapping)
 			continue;
 		spin_unlock(&sdp->sd_ail_lock);
-		ret = filemap_fdatawrite_wbc(mapping, wbc);
+		ret = write_cache_pages(mapping, wbc, __gfs2_writepage, mapping);
 		if (need_resched()) {
 			blk_finish_plug(plug);
 			cond_resched();