mm: reinstate unconditional writeback start in balance_dirty_pages()

Commit 64dd89a ("mm/block/fs: remove laptop_mode") removed this
unconditional writeback start from balance_dirty_pages():

       if (unlikely(!writeback_in_progress(wb)))
	       wb_start_background_writeback(wb);

This logic needs to be reinstated to prevent performance regressions for
strictlimited BDIs and memcg setups.  The problem occurs because:

a) For strictlimited BDIs, throttling is calculated using per-wb
   thresholds.  The per-wb threshold can be exceeded even when the global
   dirty threshold was not exceeded (nr_dirty < gdtc->bg_thresh)

b) For memcg-based throttling, memcg uses its own dirty count /
   thresholds and can trigger throttling even when the global threshold
   isn't exceeded

Without the unconditional writeback start, IO is throttled as it waits for
dirty pages to be written back but there is no writeback running.  This
leads to severe stalls.  On fuse, buffered write performance dropped from
1400 MiB/s to 2000 KiB/s.

Reinstate the unconditional writeback start so that writeback is
guaranteed to be running whenever IO needs to be throttled.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 64dd89a ("mm/block/fs: remove laptop_mode")
Signed-off-by: Joanne Koong <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: joannekoong

mm/page-writeback.c

@@ -1858,6 +1858,27 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 			break;
 		}
 
+		/*
+		 * Unconditionally start background writeback if it's not
+		 * already in progress. We need to do this because the global
+		 * dirty threshold check above (nr_dirty > gdtc->bg_thresh)
+		 * doesn't account for these cases:
+		 *
+		 * a) strictlimit BDIs: throttling is calculated using per-wb
+		 * thresholds. The per-wb threshold can be exceeded even when
+		 * nr_dirty < gdtc->bg_thresh
+		 *
+		 * b) memcg-based throttling: memcg uses its own dirty count and
+		 * thresholds and can trigger throttling even when global
+		 * nr_dirty < gdtc->bg_thresh
+		 *
+		 * Writeback needs to be started else the writer stalls in the
+		 * throttle loop waiting for dirty pages to be written back
+		 * while no writeback is running.
+		 */
+		if (unlikely(!writeback_in_progress(wb)))
+			wb_start_background_writeback(wb);
+
 		mem_cgroup_flush_foreign(wb);
 
 		/*