writeback: factor out balance_wb_limits to remove repeated code

Factor out balance_wb_limits to remove repeated code

[shikemeng@huaweicloud.com: add comment]
  Link: https://lkml.kernel.org/r/20240606033547.344376-1-shikemeng@huaweicloud.com
[akpm@linux-foundation.org: s/fileds/fields/ in comment]
Link: https://lkml.kernel.org/r/20240514125254.142203-9-shikemeng@huaweicloud.comSigned-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page-writeback.c

@@ -1783,6 +1783,21 @@ static inline void wb_dirty_exceeded(struct dirty_throttle_control *dtc,
 		((dtc->dirty > dtc->thresh) || strictlimit);
 }
 
+/*
+ * The limits fields dirty_exceeded and pos_ratio won't be updated if wb is
+ * in freerun state. Please don't use these invalid fields in freerun case.
+ */
+static void balance_wb_limits(struct dirty_throttle_control *dtc,
+			      bool strictlimit)
+{
+	wb_dirty_freerun(dtc, strictlimit);
+	if (dtc->freerun)
+		return;
+
+	wb_dirty_exceeded(dtc, strictlimit);
+	wb_position_ratio(dtc);
+}
+
 /*
  * balance_dirty_pages() must be called by processes which are generating dirty
  * data.  It looks at the number of dirty pages in the machine and will force
@@ -1869,12 +1884,9 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 		 * Calculate global domain's pos_ratio and select the
 		 * global dtc by default.
 		 */
-		wb_dirty_freerun(gdtc, strictlimit);
+		balance_wb_limits(gdtc, strictlimit);
 		if (gdtc->freerun)
 			goto free_running;
-
-		wb_dirty_exceeded(gdtc, strictlimit);
-		wb_position_ratio(gdtc);
 		sdtc = gdtc;
 
 		if (mdtc) {
@@ -1884,12 +1896,9 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 			 * both global and memcg domains.  Choose the one
 			 * w/ lower pos_ratio.
 			 */
-			wb_dirty_freerun(mdtc, strictlimit);
+			balance_wb_limits(mdtc, strictlimit);
 			if (mdtc->freerun)
 				goto free_running;
-
-			wb_dirty_exceeded(mdtc, strictlimit);
-			wb_position_ratio(mdtc);
 			if (mdtc->pos_ratio < gdtc->pos_ratio)
 				sdtc = mdtc;
 		}