Patch "mm/compaction: fix invalid free_pfn and compact_cached_free_pfn" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

mm/compaction: fix invalid free_pfn and compact_cached_free_pfn

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-compaction-fix-invalid-free_pfn-and-compact_cached_free_pfn.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Joonsoo Kim <[email protected]>
Date: Tue, 15 Mar 2016 14:57:45 -0700
Subject: mm/compaction: fix invalid free_pfn and compact_cached_free_pfn

From: Joonsoo Kim <[email protected]>

commit 623446e4dc45b37740268165107cc63abb3022f0 upstream.

free_pfn and compact_cached_free_pfn are the pointer that remember restart position of freepage scanner. When they are reset or invalid, we set them to zone_end_pfn because freepage scanner works in reverse direction. But, because zone range is defined as [zone_start_pfn, zone_end_pfn), zone_end_pfn is invalid to access. Therefore, we should not store it to free_pfn and compact_cached_free_pfn. Instead, we need to store zone_end_pfn - 1 to them. There is one more thing we should consider. Freepage scanner scan reversely by pageblock unit. If free_pfn and compact_cached_free_pfn are set to middle of pageblock, it regards that sitiation as that it already scans front part of pageblock so we lose opportunity to scan there. To fix-up, this patch do round_down() to guarantee that reset position will be pageblock aligned.

Note that thanks to the current pageblock_pfn_to_page() implementation, actual access to zone_end_pfn doesn't happen until now. But, following patch will change pageblock_pfn_to_page() so this patch is needed from now on.

---
 mm/compaction.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

@@ -200,7 +200,8 @@ static void reset_cached_positions(struc
 {
 	zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
 	zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
-	zone->compact_cached_free_pfn = zone_end_pfn(zone);
+	zone->compact_cached_free_pfn =
+			round_down(zone_end_pfn(zone) - 1, pageblock_nr_pages);
 }
 
 /*

@@ -1358,11 +1359,11 @@ static int compact_zone(struct zone *zon
 	 */
 	cc->migrate_pfn = zone->compact_cached_migrate_pfn[sync];
 	cc->free_pfn = zone->compact_cached_free_pfn;
-	if (cc->free_pfn < start_pfn || cc->free_pfn > end_pfn) {
-		cc->free_pfn = end_pfn & ~(pageblock_nr_pages-1);
+	if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
+		cc->free_pfn = round_down(end_pfn - 1, pageblock_nr_pages);
 		zone->compact_cached_free_pfn = cc->free_pfn;
 	}
-	if (cc->migrate_pfn < start_pfn || cc->migrate_pfn > end_pfn) {
+	if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
 		cc->migrate_pfn = start_pfn;
 		zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
 		zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mm-compaction-pass-only-pageblock-aligned-range-to-pageblock_pfn_to_page.patch queue-4.4/mm-compaction-fix-invalid-free_pfn-and-compact_cached_free_pfn.patch