mm: hide laptop_mode_wb_timer entirely behind the BDI API

Don't leak the detaіls of the timer into the block layer, instead
initialize the timer in bdi_alloc and delete it in bdi_unregister.
Note that this means the timer is initialized (but not armed) for
non-block queues as well now.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210809141744.1203023-2-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -394,10 +394,7 @@ void blk_cleanup_queue(struct request_queue *q)
 	/* for synchronous bio-based driver finish in-flight integrity i/o */
 	blk_flush_integrity();
 
-	/* @q won't process any more request, flush async actions */
-	del_timer_sync(&q->backing_dev_info->laptop_mode_wb_timer);
 	blk_sync_queue(q);
-
 	if (queue_is_mq(q))
 		blk_mq_exit_queue(q);
 
@@ -546,8 +543,6 @@ struct request_queue *blk_alloc_queue(int node_id)
 
 	atomic_set(&q->nr_active_requests_shared_sbitmap, 0);
 
-	timer_setup(&q->backing_dev_info->laptop_mode_wb_timer,
-		    laptop_mode_timer_fn, 0);
 	timer_setup(&q->timeout, blk_rq_timed_out_timer, 0);
 	INIT_WORK(&q->timeout_work, blk_timeout_work);
 	INIT_LIST_HEAD(&q->icq_list);

mm/backing-dev.c

@@ -807,6 +807,7 @@ struct backing_dev_info *bdi_alloc(int node_id)
 	bdi->capabilities = BDI_CAP_WRITEBACK | BDI_CAP_WRITEBACK_ACCT;
 	bdi->ra_pages = VM_READAHEAD_PAGES;
 	bdi->io_pages = VM_READAHEAD_PAGES;
+	timer_setup(&bdi->laptop_mode_wb_timer, laptop_mode_timer_fn, 0);
 	return bdi;
 }
 EXPORT_SYMBOL(bdi_alloc);
@@ -928,6 +929,8 @@ static void bdi_remove_from_list(struct backing_dev_info *bdi)
 
 void bdi_unregister(struct backing_dev_info *bdi)
 {
+	del_timer_sync(&bdi->laptop_mode_wb_timer);
+
 	/* make sure nobody finds us on the bdi_list anymore */
 	bdi_remove_from_list(bdi);
 	wb_shutdown(&bdi->wb);

mm/page-writeback.c

@@ -2010,7 +2010,6 @@ int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
 	return ret;
 }
 
-#ifdef CONFIG_BLOCK
 void laptop_mode_timer_fn(struct timer_list *t)
 {
 	struct backing_dev_info *backing_dev_info =
@@ -2045,7 +2044,6 @@ void laptop_sync_completion(void)
 
 	rcu_read_unlock();
 }
-#endif
 
 /*
  * If ratelimit_pages is too high then we can get into dirty-data overload