mm: migrate: move migrate_page_lock_buffers()

buffer_migrate_page() is the only caller of migrate_page_lock_buffers()
move it close to it and also drop the now unused stub for !CONFIG_BLOCK.

Link: http://lkml.kernel.org/r/20181211172143.7358-4-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -374,56 +374,6 @@ void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd)
 }
 #endif
 
-#ifdef CONFIG_BLOCK
-/* Returns true if all buffers are successfully locked */
-static bool buffer_migrate_lock_buffers(struct buffer_head *head,
-							enum migrate_mode mode)
-{
-	struct buffer_head *bh = head;
-
-	/* Simple case, sync compaction */
-	if (mode != MIGRATE_ASYNC) {
-		do {
-			get_bh(bh);
-			lock_buffer(bh);
-			bh = bh->b_this_page;
-
-		} while (bh != head);
-
-		return true;
-	}
-
-	/* async case, we cannot block on lock_buffer so use trylock_buffer */
-	do {
-		get_bh(bh);
-		if (!trylock_buffer(bh)) {
-			/*
-			 * We failed to lock the buffer and cannot stall in
-			 * async migration. Release the taken locks
-			 */
-			struct buffer_head *failed_bh = bh;
-			put_bh(failed_bh);
-			bh = head;
-			while (bh != failed_bh) {
-				unlock_buffer(bh);
-				put_bh(bh);
-				bh = bh->b_this_page;
-			}
-			return false;
-		}
-
-		bh = bh->b_this_page;
-	} while (bh != head);
-	return true;
-}
-#else
-static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
-							enum migrate_mode mode)
-{
-	return true;
-}
-#endif /* CONFIG_BLOCK */
-
 static int expected_page_refs(struct page *page)
 {
 	int expected_count = 1;
@@ -751,6 +701,48 @@ int migrate_page(struct address_space *mapping,
 EXPORT_SYMBOL(migrate_page);
 
 #ifdef CONFIG_BLOCK
+/* Returns true if all buffers are successfully locked */
+static bool buffer_migrate_lock_buffers(struct buffer_head *head,
+							enum migrate_mode mode)
+{
+	struct buffer_head *bh = head;
+
+	/* Simple case, sync compaction */
+	if (mode != MIGRATE_ASYNC) {
+		do {
+			get_bh(bh);
+			lock_buffer(bh);
+			bh = bh->b_this_page;
+
+		} while (bh != head);
+
+		return true;
+	}
+
+	/* async case, we cannot block on lock_buffer so use trylock_buffer */
+	do {
+		get_bh(bh);
+		if (!trylock_buffer(bh)) {
+			/*
+			 * We failed to lock the buffer and cannot stall in
+			 * async migration. Release the taken locks
+			 */
+			struct buffer_head *failed_bh = bh;
+			put_bh(failed_bh);
+			bh = head;
+			while (bh != failed_bh) {
+				unlock_buffer(bh);
+				put_bh(bh);
+				bh = bh->b_this_page;
+			}
+			return false;
+		}
+
+		bh = bh->b_this_page;
+	} while (bh != head);
+	return true;
+}
+
 /*
  * Migration function for pages with buffers. This function can only be used
  * if the underlying filesystem guarantees that no other references to "page"