[PATCH] mm-compaction-cleanup-isolate_freepages-fix3

[Joonsoo Kim]

What I did here is taking end_pfn out of the loop and considering zone
boundary once. After then, we can just set previous pfn to end_pfn on
every iteration to move scanning window. With this change, we can remove
local variable, z_end_pfn.

Another things I did are removing max() operation and un-needed
assignment to isolate variable.

In addition, I change both the variable names, from pfn and
end_pfn to block_start_pfn and block_end_pfn, respectively.
They represent their meaning perfectly.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/compaction.c b/mm/compaction.c
index 1c992dc..ba80bea 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -671,10 +671,10 @@ static void isolate_freepages(struct zone *zone,
 				struct compact_control *cc)
 {
 	struct page *page;
-	unsigned long pfn;	     /* scanning cursor */
+	unsigned long block_start_pfn;	/* start of current pageblock */
+	unsigned long block_end_pfn;	/* end of current pageblock */
 	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
 	unsigned long next_free_pfn; /* start pfn for scaning at next round */
-	unsigned long z_end_pfn;     /* zone's end pfn */
 	int nr_freepages = cc->nr_freepages;
 	struct list_head *freelist = &cc->freepages;
 
@@ -682,31 +682,33 @@ static void isolate_freepages(struct zone *zone,
 	 * Initialise the free scanner. The starting point is where we last
 	 * successfully isolated from, zone-cached value, or the end of the
 	 * zone when isolating for the first time. We need this aligned to
-	 * the pageblock boundary, because we do pfn -= pageblock_nr_pages
-	 * in the for loop.
+	 * the pageblock boundary, because we do
+	 * block_start_pfn -= pageblock_nr_pages in the for loop.
+	 * For ending point, take care when isolating in last pageblock of a
+	 * a zone which ends in the middle of a pageblock.
 	 * The low boundary is the end of the pageblock the migration scanner
 	 * is using.
 	 */
-	pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
+	block_start_pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
+	block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
+						zone_end_pfn(zone));
 	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
 
 	/*
-	 * Seed the value for max(next_free_pfn, pfn) updates. If no pages are
-	 * isolated, the pfn < low_pfn check will kick in.
+	 * If no pages are isolated, the block_start_pfn < low_pfn check
+	 * will kick in.
 	 */
 	next_free_pfn = 0;
 
-	z_end_pfn = zone_end_pfn(zone);
-
 	/*
 	 * Isolate free pages until enough are available to migrate the
 	 * pages on cc->migratepages. We stop searching if the migrate
 	 * and free page scanners meet or enough free pages are isolated.
 	 */
-	for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
-					pfn -= pageblock_nr_pages) {
+	for (;block_start_pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
+				block_end_pfn = block_start_pfn,
+				block_start_pfn -= pageblock_nr_pages) {
 		unsigned long isolated;
-		unsigned long end_pfn;
 
 		/*
 		 * This can iterate a massively long zone without finding any
@@ -715,7 +717,7 @@ static void isolate_freepages(struct zone *zone,
 		 */
 		cond_resched();
 
-		if (!pfn_valid(pfn))
+		if (!pfn_valid(block_start_pfn))
 			continue;
 
 		/*
@@ -725,7 +727,7 @@ static void isolate_freepages(struct zone *zone,
 		 * i.e. it's possible that all pages within a zones range of
 		 * pages do not belong to a single zone.
 		 */
-		page = pfn_to_page(pfn);
+		page = pfn_to_page(block_start_pfn);
 		if (page_zone(page) != zone)
 			continue;
 
@@ -738,15 +740,8 @@ static void isolate_freepages(struct zone *zone,
 			continue;
 
 		/* Found a block suitable for isolating free pages from */
-		isolated = 0;
-
-		/*
-		 * Take care when isolating in last pageblock of a zone which
-		 * ends in the middle of a pageblock.
-		 */
-		end_pfn = min(pfn + pageblock_nr_pages, z_end_pfn);
-		isolated = isolate_freepages_block(cc, pfn, end_pfn,
-						   freelist, false);
+		isolated = isolate_freepages_block(cc, block_start_pfn,
+					block_end_pfn, freelist, false);
 		nr_freepages += isolated;
 
 		/*
@@ -754,9 +749,9 @@ static void isolate_freepages(struct zone *zone,
 		 * looking for free pages, the search will restart here as
 		 * page migration may have returned some pages to the allocator
 		 */
-		if (isolated) {
+		if (isolated && next_free_pfn == 0) {
 			cc->finished_update_free = true;
-			next_free_pfn = max(next_free_pfn, pfn);
+			next_free_pfn = block_start_pfn;
 		}
 	}
 
@@ -767,7 +762,7 @@ static void isolate_freepages(struct zone *zone,
 	 * If we crossed the migrate scanner, we want to keep it that way
 	 * so that compact_finished() may detect this
 	 */
-	if (pfn < low_pfn)
+	if (block_start_pfn < low_pfn)
 		next_free_pfn = cc->migrate_pfn;
 
 	cc->free_pfn = next_free_pfn;
-- 
1.7.9.5

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