thp: change deferred_split_count() to return number of THP in queue

I've got meaning of shrinker::count_objects() wrong: it should return
number of potentially freeable objects, which is not necessary correlate
with freeable memory.

Returning 256 per THP in queue is not reasonable:
shrinker::scan_objects() never called with nr_to_scan > 128 in my setup.

Let's return 1 per THP and correct scan_object accordingly.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Jerome Marchand <[email protected]>
Cc: Sasha Levin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: kiryl

mm/huge_memory.c

@@ -3465,12 +3465,7 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
 		struct shrink_control *sc)
 {
 	struct pglist_data *pgdata = NODE_DATA(sc->nid);
-	/*
-	 * Split a page from split_queue will free up at least one page,
-	 * at most HPAGE_PMD_NR - 1. We don't track exact number.
-	 * Let's use HPAGE_PMD_NR / 2 as ballpark.
-	 */
-	return ACCESS_ONCE(pgdata->split_queue_len) * HPAGE_PMD_NR / 2;
+	return ACCESS_ONCE(pgdata->split_queue_len);
 }
 
 static unsigned long deferred_split_scan(struct shrinker *shrink,
@@ -3511,7 +3506,13 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 	list_splice_tail(&list, &pgdata->split_queue);
 	spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
 
-	return split * HPAGE_PMD_NR / 2;
+	/*
+	 * Stop shrinker if we didn't split any page, but the queue is empty.
+	 * This can happen if pages were freed under us.
+	 */
+	if (!split && list_empty(&pgdata->split_queue))
+		return SHRINK_STOP;
+	return split;
 }
 
 static struct shrinker deferred_split_shrinker = {