mm: do not start/end writeback for pages stored in zswap

Most of the work done in folio_start_writeback is reversed in
folio_end_writeback.  For e.g.  NR_WRITEBACK and NR_ZONE_WRITE_PENDING are
incremented in start_writeback and decremented in end_writeback.  Calling
end_writeback immediately after start_writeback (separated by
folio_unlock) cancels the affect of most of the work done in start hence
can be removed.

There is some extra work done in folio_end_writeback, however it is
incorrect/not applicable to zswap:
- folio_end_writeback incorrectly increments NR_WRITTEN counter,
  eventhough the pages aren't written to disk, hence this change
  corrects this behaviour.
- folio_end_writeback calls folio_rotate_reclaimable, but that only
  makes sense for async writeback pages, while for zswap pages are
  synchronously reclaimed.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Usama Arif <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Suggested-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Nhat Pham <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: uarif1

mm/page_io.c

@@ -196,9 +196,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
 		return ret;
 	}
 	if (zswap_store(folio)) {
-		folio_start_writeback(folio);
 		folio_unlock(folio);
-		folio_end_writeback(folio);
 		return 0;
 	}
 	if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) {