[PATCH v3 2/2] mm/mm_init: simplify ZONE_MOVABLE branch in zone_spanned_pages_in_node()

[Sang-Heon Jeon]

*zone_start_pfn and *zone_end_pfn are first assigned
clamp(node_start_pfn, zone_low, zone_high) and clamp(node_end_pfn,
zone_low, zone_high).

If movable_pfn != 0, the branches adjust them as follows.

1. If zone_type == ZONE_MOVABLE, *zone_start_pfn = movable_pfn, which
   can be outside the node, and *zone_end_pfn =
   min(node_end_pfn, ...), which is at most node_end_pfn.

2. Else If *zone_start_pfn < movable_pfn < *zone_end_pfn,
   *zone_end_pfn = movable_pfn.

3. Else If *zone_start_pfn >= movable_pfn, *zone_start_pfn =
   *zone_end_pfn.

4. Else *zone_start_pfn < movable_pfn && *zone_end_pfn <= movable_pfn,
   and nothing is assigned.

If node_end_pfn < zone_low or node_start_pfn > zone_high,
*zone_start_pfn and *zone_end_pfn are both zone_low or both
zone_high, outside the node.

Then case 2 cannot apply, in case 3 *zone_start_pfn = *zone_end_pfn,
which is that same pfn, and in case 4 nothing is assigned. So the
check against node_start_pfn and node_end_pfn returns 0 early.

Otherwise node_start_pfn <= *zone_start_pfn <= *zone_end_pfn <=
node_end_pfn.

Then in case 2 *zone_end_pfn = movable_pfn, in case 3
*zone_start_pfn = *zone_end_pfn, and in case 4 nothing is assigned.
So *zone_start_pfn and *zone_end_pfn stay within the node, the check
does not return 0, and min(*zone_end_pfn, node_end_pfn) and
max(*zone_start_pfn, node_start_pfn) change nothing.

So the check, min(*zone_end_pfn, node_end_pfn), and
max(*zone_start_pfn, node_start_pfn) only have an effect in case 1.

Also, cases 2 and 4 both have *zone_start_pfn < movable_pfn, and
min(*zone_end_pfn, movable_pfn) is movable_pfn in case 2 and
*zone_end_pfn in case 4. So the two cases can be merged into one
branch and case 3 becomes its else.

So remove *zone_end_pfn = min(*zone_end_pfn, node_end_pfn), move the
check and *zone_start_pfn = max(*zone_start_pfn, node_start_pfn)
inside the ZONE_MOVABLE branch, and merge cases 2 and 4 with
*zone_end_pfn = min(*zone_end_pfn, movable_pfn).

No functional change.

Signed-off-by: Sang-Heon Jeon <ekffu200098@xxxxxxxxx>
---
 mm/mm_init.c | 27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index df67d4e6f646..ab445111a846 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1208,25 +1208,24 @@ static unsigned long __init zone_spanned_pages_in_node(int nid,
 		*zone_end_pfn = min(node_end_pfn,
 			arch_zone_highest_possible_pfn[movable_zone]);
 
-	/* Adjust for ZONE_MOVABLE starting within this range */
-	} else if (*zone_start_pfn < movable_pfn &&
-		   *zone_end_pfn > movable_pfn) {
-		*zone_end_pfn = movable_pfn;
+		/* Check that this node has pages within the zone's required range */
+		if (*zone_end_pfn < node_start_pfn ||
+		    *zone_start_pfn > node_end_pfn)
+			return 0;
 
-	/* Check if this whole range is within ZONE_MOVABLE */
-	} else if (*zone_start_pfn >= movable_pfn) {
+		/* Move the zone start inside the node if necessary */
+		*zone_start_pfn = max(*zone_start_pfn, node_start_pfn);
+
+	/* This range starts below ZONE_MOVABLE */
+	} else if (*zone_start_pfn < movable_pfn) {
+		*zone_end_pfn = min(*zone_end_pfn, movable_pfn);
+
+	/* This whole range is within ZONE_MOVABLE */
+	} else {
 		*zone_start_pfn = *zone_end_pfn;
 	}
 
 out:
-	/* Check that this node has pages within the zone's required range */
-	if (*zone_end_pfn < node_start_pfn || *zone_start_pfn > node_end_pfn)
-		return 0;
-
-	/* Move the zone boundaries inside the node if necessary */
-	*zone_end_pfn = min(*zone_end_pfn, node_end_pfn);
-	*zone_start_pfn = max(*zone_start_pfn, node_start_pfn);
-
 	/* Return the spanned pages */
 	return *zone_end_pfn - *zone_start_pfn;
 }
-- 
2.43.0