[PATCH] mm/vmscan.c: fix an implementation flaw in proportional scanning

[Chen Yucong]

Via https://lkml.org/lkml/2013/4/10/897, we can know that the relative design
idea is to keep

    scan_target[anon] : scan_target[file]
        == really_scanned_num[anon] : really_scanned_num[file]

But we can find the following snippet in shrink_lruvec():

    if (nr_file > nr_anon) {
        ...
    } else {
        ...
    }

However, the above code fragment broke the design idea. We can assume:

      nr[LRU_ACTIVE_FILE] = 30
      nr[LRU_INACTIVE_FILE] = 30
      nr[LRU_ACTIVE_ANON] = 0
      nr[LRU_INACTIVE_ANON] = 40

When the value of (nr_reclaimed < nr_to_reclaim) become false, there are
the following results:

      nr[LRU_ACTIVE_FILE] = 15
      nr[LRU_INACTIVE_FILE] = 15
      nr[LRU_ACTIVE_ANON] = 0
      nr[LRU_INACTIVE_ANON] = 25
      nr_file = 30
      nr_anon = 25
      file_percent = 30 / 60 = 0.5
      anon_percent = 25 / 40 = 0.65

According to the above design idea, we should scan some pages from ANON,
but in fact we execute the an error code path due to "if (nr_file > nr_anon)".
In this way, nr[lru] is likely to be a negative number. Luckily,
"nr[lru] -= min(nr[lru], nr_scanned)" can help us to filter this situation,
but it has rebelled against our design idea.

Signed-off-by: Chen Yucong <slaoub@xxxxxxxxx>
---
 mm/vmscan.c |   39 ++++++++++++++++++---------------------
 1 file changed, 18 insertions(+), 21 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a8ffe4e..2c35e34 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2057,8 +2057,7 @@ out:
 static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 {
 	unsigned long nr[NR_LRU_LISTS];
-	unsigned long targets[NR_LRU_LISTS];
-	unsigned long nr_to_scan;
+	unsigned long file_target, anon_target;
 	enum lru_list lru;
 	unsigned long nr_reclaimed = 0;
 	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
@@ -2067,8 +2066,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 
 	get_scan_count(lruvec, sc, nr);
 
-	/* Record the original scan target for proportional adjustments later */
-	memcpy(targets, nr, sizeof(nr));
+	file_target = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
+	anon_target = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
 
 	/*
 	 * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
@@ -2087,8 +2086,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	blk_start_plug(&plug);
 	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
 					nr[LRU_INACTIVE_FILE]) {
-		unsigned long nr_anon, nr_file, percentage;
-		unsigned long nr_scanned;
+		unsigned long nr_anon, nr_file, file_percent, anon_percent;
+		unsigned long nr_to_scan, nr_scanned, percentage;
 
 		for_each_evictable_lru(lru) {
 			if (nr[lru]) {
@@ -2122,16 +2121,19 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 		if (!nr_file || !nr_anon)
 			break;
 
-		if (nr_file > nr_anon) {
-			unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
-						targets[LRU_ACTIVE_ANON] + 1;
+		file_percent = nr_file * 100 / file_target;
+		anon_percent = nr_anon * 100 / anon_target;
+
+		if (file_percent > anon_percent) {
 			lru = LRU_BASE;
-			percentage = nr_anon * 100 / scan_target;
+			nr_scanned = file_target - nr_file;
+			nr_to_scan = file_target * (100 - anon_percent) / 100;
+			percentage = nr[LRU_FILE] * 100 / nr_file;
 		} else {
-			unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
-						targets[LRU_ACTIVE_FILE] + 1;
 			lru = LRU_FILE;
-			percentage = nr_file * 100 / scan_target;
+			nr_scanned = anon_target - nr_anon;
+			nr_to_scan = anon_target * (100 - file_percent) / 100;
+			percentage = nr[LRU_BASE] * 100 / nr_anon;
 		}
 
 		/* Stop scanning the smaller of the LRU */
@@ -2143,14 +2145,9 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 		 * scan target and the percentage scanning already complete
 		 */
 		lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
-		nr_scanned = targets[lru] - nr[lru];
-		nr[lru] = targets[lru] * (100 - percentage) / 100;
-		nr[lru] -= min(nr[lru], nr_scanned);
-
-		lru += LRU_ACTIVE;
-		nr_scanned = targets[lru] - nr[lru];
-		nr[lru] = targets[lru] * (100 - percentage) / 100;
-		nr[lru] -= min(nr[lru], nr_scanned);
+		nr_to_scan -= min(nr_to_scan, nr_scanned);
+		nr[lru] = nr_to_scan * percentage / 100;
+		nr[lru + LRU_ACTIVE] = nr_to_scan - nr[lru];
 
 		scan_adjusted = true;
 	}
-- 
1.7.10.4

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