[PATCH v1] mm: fix writeback regression for strictlimit BDIs

[Joanne Koong <joannelkoong@gmail.com>]

Commit 64dd89ae01f2 ("mm/block/fs: remove laptop_mode") removed this
unconditional writeback kick from balance_dirty_pages():

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

Earlier in balance_dirty_pages(), background writeback is started if the
global dirty count exceeds the global background threshold (nr_dirty >
gdtc->bg_thresh). However for BDIs with BDI_CAP_STRICTLIMIT set (eg
fuse), throttling is calculated using the per-wb threshold, not global
thresholds. This means the per-wb threshold can be exceeded while the
global nr_dirty is below gdtc->bg_thresh. This causes two problems:

a) background writeback is never proactively kicked off. The flusher
should be kicked off while the writer is still free-running, so that
dirty pages are drained before the writer hits the throttle threshold.
For strictlimit BDIs with global nr_dirty < gdtc->bg_thresh, this never
kicks off the flusher, so dirty pages pile up unchecked until the per-wb
freerun ceiling gets hit and IO is throttled

b) while IO is throttled, the flusher is still not started, which means
the writer basically sits in the throttle loop sleeping while waiting
for dirty pages to be cleaned but no writeback is running

This leads to severe stalls and degraded throughput. On fuse, buffered
write performance drops from 1400 MiB/s to 2000 KiB/s.

This fixes the issue by kicking off the flusher if wb_dirty exceeds
wb_bg_thresh for strictlimit BDIs. This restores performance back to its
original baseline.

Fixes: 64dd89ae01f2 ("mm/block/fs: remove laptop_mode")
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 mm/page-writeback.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 601a5e048d12..3f89b08b11b4 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1835,7 +1835,9 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 			balance_domain_limits(mdtc, strictlimit);
 		}
 
-		if (nr_dirty > gdtc->bg_thresh && !writeback_in_progress(wb))
+		if (!writeback_in_progress(wb) &&
+		    (nr_dirty > gdtc->bg_thresh ||
+		     (strictlimit && gdtc->wb_dirty > gdtc->wb_bg_thresh)))
 			wb_start_background_writeback(wb);
 
 		/*
-- 
2.52.0