mm/compaction: rename 'start_pfn' to 'iteration_start_pfn' in compact_zone()

There are two 'start_pfn' declared in compact_zone() which have different
meanings.  Rename the second one to 'iteration_start_pfn' to prevent
confusion.

Also, remove an useless semicolon.

Link: https://lkml.kernel.org/r/20201019115044.1571-1-yanfei.xu@windriver.com
Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Yanfei Xu

mm/compaction.c

@@ -2275,7 +2275,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 
 	while ((ret = compact_finished(cc)) == COMPACT_CONTINUE) {
 		int err;
-		unsigned long start_pfn = cc->migrate_pfn;
+		unsigned long iteration_start_pfn = cc->migrate_pfn;
 
 		/*
 		 * Avoid multiple rescans which can happen if a page cannot be
@@ -2287,7 +2287,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 		 */
 		cc->rescan = false;
 		if (pageblock_start_pfn(last_migrated_pfn) ==
-		    pageblock_start_pfn(start_pfn)) {
+		    pageblock_start_pfn(iteration_start_pfn)) {
 			cc->rescan = true;
 		}
 
@@ -2311,8 +2311,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 			goto check_drain;
 		case ISOLATE_SUCCESS:
 			update_cached = false;
-			last_migrated_pfn = start_pfn;
-			;
+			last_migrated_pfn = iteration_start_pfn;
 		}
 
 		err = migrate_pages(&cc->migratepages, compaction_alloc,