[PATCH v2] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots()

[Wei Yang <richard.weiyang@xxxxxxxxx>]

After commit c60aa176c6de8 ("swapfile: swap allocation cycle if
nonrot"), swap allocation is cyclic. Current approach is done with two
separate loop on the upper and lower half. This looks a little
redundant.

>From another point of view, the loop iterates [lowest_bit, highest_bit]
range starting with (offset + 1) but except scan_base. So we can
simplify the loop with condition (next_offset() != scan_base) by
introducing next_offset() which makes sure offset fit in that range
with correct order.

Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
CC: Hugh Dickins <hughd@xxxxxxxxxx>
CC: "Huang, Ying" <ying.huang@xxxxxxxxx>

---
v2:
  * return scan_base if the lower part is eaten
  * only start over when iterating on the upper part
---
 mm/swapfile.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index f903e5a165d5..0005a4a1c1b4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -729,6 +729,19 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 	}
 }
 
+static unsigned long next_offset(struct swap_info_struct *si,
+				unsigned long *offset, unsigned long scan_base)
+{
+	/* only start over when iterating on the upper part */
+	if (++(*offset) > si->highest_bit && *offset > scan_base) {
+		*offset = si->lowest_bit;
+		/* someone has eaten the lower part */
+		if (si->lowest_bit >= scan_base)
+			return scan_base;
+	}
+	return *offset;
+}
+
 static int scan_swap_map_slots(struct swap_info_struct *si,
 			       unsigned char usage, int nr,
 			       swp_entry_t slots[])
@@ -876,22 +889,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 
 scan:
 	spin_unlock(&si->lock);
-	while (++offset <= si->highest_bit) {
-		if (!si->swap_map[offset]) {
-			spin_lock(&si->lock);
-			goto checks;
-		}
-		if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
-			spin_lock(&si->lock);
-			goto checks;
-		}
-		if (unlikely(--latency_ration < 0)) {
-			cond_resched();
-			latency_ration = LATENCY_LIMIT;
-		}
-	}
-	offset = si->lowest_bit;
-	while (offset < scan_base) {
+	while (next_offset(si, &offset, scan_base) != scan_base) {
 		if (!si->swap_map[offset]) {
 			spin_lock(&si->lock);
 			goto checks;
@@ -904,7 +902,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 			cond_resched();
 			latency_ration = LATENCY_LIMIT;
 		}
-		offset++;
 	}
 	spin_lock(&si->lock);
 
-- 
2.23.0