mm/page-writeback: raise wb_thresh to prevent write blocking with strictlimit

With the strictlimit flag, wb_thresh acts as a hard limit in
balance_dirty_pages() and wb_position_ratio().  When device write
operations are inactive, wb_thresh can drop to 0, causing writes to be
blocked.  The issue occasionally occurs in fuse fs, particularly with
network backends, the write thread is blocked frequently during a period.
To address it, this patch raises the minimum wb_thresh to a controllable
level, similar to the non-strictlimit case.

Link: https://lkml.kernel.org/r/20241023100032.62952-1-jimzhao.ai@gmail.com
Signed-off-by: Jim Zhao <jimzhao.ai@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index fcd4c1439cb9c3b89e8ec79a4404d728826cf586..1d7179aba8e3e4840e07a6897b0f7e18d2eaf816 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -917,7 +917,9 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
                                      unsigned long thresh)
 {
        struct wb_domain *dom = dtc_dom(dtc);
+       struct bdi_writeback *wb = dtc->wb;
        u64 wb_thresh;
+       u64 wb_max_thresh;
        unsigned long numerator, denominator;
        unsigned long wb_min_ratio, wb_max_ratio;
 
@@ -931,11 +933,28 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
        wb_thresh *= numerator;
        wb_thresh = div64_ul(wb_thresh, denominator);
 
-       wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
+       wb_min_max_ratio(wb, &wb_min_ratio, &wb_max_ratio);
 
        wb_thresh += (thresh * wb_min_ratio) / (100 * BDI_RATIO_SCALE);
-       if (wb_thresh > (thresh * wb_max_ratio) / (100 * BDI_RATIO_SCALE))
-               wb_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
+       wb_max_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
+       if (wb_thresh > wb_max_thresh)
+               wb_thresh = wb_max_thresh;
+
+       /*
+        * With strictlimit flag, the wb_thresh is treated as
+        * a hard limit in balance_dirty_pages() and wb_position_ratio().
+        * It's possible that wb_thresh is close to zero, not because
+        * the device is slow, but because it has been inactive.
+        * To prevent occasional writes from being blocked, we raise wb_thresh.
+        */
+       if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
+               unsigned long limit = hard_dirty_limit(dom, dtc->thresh);
+               u64 wb_scale_thresh = 0;
+
+               if (limit > dtc->dirty)
+                       wb_scale_thresh = (limit - dtc->dirty) / 100;
+               wb_thresh = max(wb_thresh, min(wb_scale_thresh, wb_max_thresh / 4));
+       }
 
        return wb_thresh;
 }