The patch below fixes it.
---
>From 28be8f9a9da95d54b7fce80332bf2c62abfcdf73 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@xxxxxx>
Date: Tue, 10 Aug 2021 14:39:24 +0200
Subject: writeback: make the laptop_mode prototypes available unconditionally
Fix the !CONFIG_BLOCK build after the recent cleanup.
Fixes: 5ed964f8e54e ("mm: hide laptop_mode_wb_timer entirely behind the BDI API")
Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
include/linux/writeback.h | 5 -----
1 file changed, 5 deletions(-)
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 667e86cfbdcf..270677dc4f36 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -336,14 +336,9 @@ static inline void cgroup_writeback_umount(void)
/*
* mm/page-writeback.c
*/
-#ifdef CONFIG_BLOCK
void laptop_io_completion(struct backing_dev_info *info);
void laptop_sync_completion(void);
-void laptop_mode_sync(struct work_struct *work);
void laptop_mode_timer_fn(struct timer_list *t);
-#else
-static inline void laptop_sync_completion(void) { }
-#endif
bool node_dirty_ok(struct pglist_data *pgdat);
int wb_domain_init(struct wb_domain *dom, gfp_t gfp);
#ifdef CONFIG_CGROUP_WRITEBACK
--
2.30.2
