mm: compaction: introduce isolate_migratepages_range()

This commit introduces isolate_migratepages_range() function which
extracts functionality from isolate_migratepages() so that it can be
used on arbitrary PFN ranges.

isolate_migratepages() function is implemented as a simple wrapper
around isolate_migratepages_range().

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Tested-by: Rob Clark <rob.clark@linaro.org>
Tested-by: Ohad Ben-Cohen <ohad@wizery.com>
Tested-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Tested-by: Robert Nelson <robertcnelson@gmail.com>
Tested-by: Barry Song <Baohua.Song@csr.com>

mm/compaction.c

@@ -250,31 +250,34 @@ typedef enum {
 	ISOLATE_SUCCESS,	/* Pages isolated, migrate */
 } isolate_migrate_t;
 
-/*
- * Isolate all pages that can be migrated from the block pointed to by
- * the migrate scanner within compact_control.
+/**
+ * isolate_migratepages_range() - isolate all migrate-able pages in range.
+ * @zone:	Zone pages are in.
+ * @cc:		Compaction control structure.
+ * @low_pfn:	The first PFN of the range.
+ * @end_pfn:	The one-past-the-last PFN of the range.
+ *
+ * Isolate all pages that can be migrated from the range specified by
+ * [low_pfn, end_pfn).  Returns zero if there is a fatal signal
+ * pending), otherwise PFN of the first page that was not scanned
+ * (which may be both less, equal to or more then end_pfn).
+ *
+ * Assumes that cc->migratepages is empty and cc->nr_migratepages is
+ * zero.
+ *
+ * Apart from cc->migratepages and cc->nr_migratetypes this function
+ * does not modify any cc's fields, in particular it does not modify
+ * (or read for that matter) cc->migrate_pfn.
  */
-static isolate_migrate_t isolate_migratepages(struct zone *zone,
-					struct compact_control *cc)
+static unsigned long
+isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
+			   unsigned long low_pfn, unsigned long end_pfn)
 {
-	unsigned long low_pfn, end_pfn;
 	unsigned long last_pageblock_nr = 0, pageblock_nr;
 	unsigned long nr_scanned = 0, nr_isolated = 0;
 	struct list_head *migratelist = &cc->migratepages;
 	isolate_mode_t mode = ISOLATE_ACTIVE|ISOLATE_INACTIVE;
 
-	/* Do not scan outside zone boundaries */
-	low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
-
-	/* Only scan within a pageblock boundary */
-	end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);
-
-	/* Do not cross the free scanner or scan within a memory hole */
-	if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
-		cc->migrate_pfn = end_pfn;
-		return ISOLATE_NONE;
-	}
-
 	/*
 	 * Ensure that there are not too many pages isolated from the LRU
 	 * list by either parallel reclaimers or compaction. If there are,
@@ -283,12 +286,12 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
 	while (unlikely(too_many_isolated(zone))) {
 		/* async migration should just abort */
 		if (!cc->sync)
-			return ISOLATE_ABORT;
+			return 0;
 
 		congestion_wait(BLK_RW_ASYNC, HZ/10);
 
 		if (fatal_signal_pending(current))
-			return ISOLATE_ABORT;
+			return 0;
 	}
 
 	/* Time to isolate some pages for migration */
@@ -396,10 +399,40 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
 	acct_isolated(zone, cc);
 
 	spin_unlock_irq(&zone->lru_lock);
-	cc->migrate_pfn = low_pfn;
 
 	trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
 
+	return low_pfn;
+}
+
+/*
+ * Isolate all pages that can be migrated from the block pointed to by
+ * the migrate scanner within compact_control.
+ */
+static isolate_migrate_t isolate_migratepages(struct zone *zone,
+					struct compact_control *cc)
+{
+	unsigned long low_pfn, end_pfn;
+
+	/* Do not scan outside zone boundaries */
+	low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
+
+	/* Only scan within a pageblock boundary */
+	end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);
+
+	/* Do not cross the free scanner or scan within a memory hole */
+	if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
+		cc->migrate_pfn = end_pfn;
+		return ISOLATE_NONE;
+	}
+
+	/* Perform the isolation */
+	low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn);
+	if (!low_pfn)
+		return ISOLATE_ABORT;
+
+	cc->migrate_pfn = low_pfn;
+
 	return ISOLATE_SUCCESS;
 }