mm/block/fs: remove laptop_mode

Laptop mode was introduced to save battery, by delaying and consolidating
writes and thereby maximize the time rotating hard drives wouldn't have to
spin.

Luckily, rotating hard drives, with their high spin-up times and power
draw, are a thing of the past for battery-powered devices.  Reclaim has
also since changed to not write single filesystem pages anymore, and
regular filesystem writeback is lumpy by design.

The juice doesn't appear worth the squeeze anymore.  The footprint of the
feature is small, but nevertheless it's a complicating factor in mm,
block, filesystems.  Developers don't think about it, and it likely hasn't
been tested with new reclaim and writeback changes in years.

Let's sunset it.  Keep the sysctl with a deprecation warning around for a
few more cycles, but remove all functionality behind it.

[akpm@linux-foundation.org: fix Documentation/admin-guide/laptops/index.rst]
Link: https://lkml.kernel.org/r/20251216185201.GH905277@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

3 files changed, 23 insertions, 84 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index c5740c6d37a2..a0e26d1b717f 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -1034,7 +1034,6 @@ struct backing_dev_info *bdi_alloc(int node_id)
 	bdi->capabilities = BDI_CAP_WRITEBACK;
 	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);
@@ -1156,8 +1155,6 @@ static void bdi_remove_from_list(struct backing_dev_info *bdi)
 
 void bdi_unregister(struct backing_dev_info *bdi)
 {
-	timer_delete_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);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ccdeb0e84d39..601a5e048d12 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -109,14 +109,6 @@ EXPORT_SYMBOL_GPL(dirty_writeback_interval);
  */
 unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
 
-/*
- * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
- * a full sync is triggered after this time elapses without any disk activity.
- */
-int laptop_mode;
-
-EXPORT_SYMBOL(laptop_mode);
-
 /* End of sysctl-exported parameters */
 
 struct wb_domain global_wb_domain;
@@ -1843,17 +1835,7 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 			balance_domain_limits(mdtc, strictlimit);
 		}
 
-		/*
-		 * In laptop mode, we wait until hitting the higher threshold
-		 * before starting background writeout, and then write out all
-		 * the way down to the lower threshold.  So slow writers cause
-		 * minimal disk activity.
-		 *
-		 * In normal mode, we start background writeout at the lower
-		 * background_thresh, to keep the amount of dirty memory low.
-		 */
-		if (!laptop_mode && nr_dirty > gdtc->bg_thresh &&
-		    !writeback_in_progress(wb))
+		if (nr_dirty > gdtc->bg_thresh && !writeback_in_progress(wb))
 			wb_start_background_writeback(wb);
 
 		/*
@@ -1876,10 +1858,6 @@ free_running:
 			break;
 		}
 
-		/* Start writeback even when in laptop mode */
-		if (unlikely(!writeback_in_progress(wb)))
-			wb_start_background_writeback(wb);
-
 		mem_cgroup_flush_foreign(wb);
 
 		/*
@@ -2198,41 +2176,6 @@ static int dirty_writeback_centisecs_handler(const struct ctl_table *table, int
 }
 #endif
 
-void laptop_mode_timer_fn(struct timer_list *t)
-{
-	struct backing_dev_info *backing_dev_info =
-		timer_container_of(backing_dev_info, t, laptop_mode_wb_timer);
-
-	wakeup_flusher_threads_bdi(backing_dev_info, WB_REASON_LAPTOP_TIMER);
-}
-
-/*
- * We've spun up the disk and we're in laptop mode: schedule writeback
- * of all dirty data a few seconds from now.  If the flush is already scheduled
- * then push it back - the user is still using the disk.
- */
-void laptop_io_completion(struct backing_dev_info *info)
-{
-	mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
-}
-
-/*
- * We're in laptop mode and we've just synced. The sync's writes will have
- * caused another writeback to be scheduled by laptop_io_completion.
- * Nothing needs to be written back anymore, so we unschedule the writeback.
- */
-void laptop_sync_completion(void)
-{
-	struct backing_dev_info *bdi;
-
-	rcu_read_lock();
-
-	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
-		timer_delete(&bdi->laptop_mode_wb_timer);
-
-	rcu_read_unlock();
-}
-
 /*
  * If ratelimit_pages is too high then we can get into dirty-data overload
  * if a large number of processes all perform writes at the same time.
@@ -2263,6 +2206,19 @@ static int page_writeback_cpu_online(unsigned int cpu)
 
 #ifdef CONFIG_SYSCTL
 
+static int laptop_mode;
+static int laptop_mode_handler(const struct ctl_table *table, int write,
+			       void *buffer, size_t *lenp, loff_t *ppos)
+{
+	int ret = proc_dointvec_jiffies(table, write, buffer, lenp, ppos);
+
+	if (!ret && write)
+		pr_warn("%s: vm.laptop_mode is deprecated. Ignoring setting.\n",
+			current->comm);
+
+	return ret;
+}
+
 /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
 static const unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
 
@@ -2332,7 +2288,7 @@ static const struct ctl_table vm_page_writeback_sysctls[] = {
 		.data		= &laptop_mode,
 		.maxlen		= sizeof(laptop_mode),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec_jiffies,
+		.proc_handler	= laptop_mode_handler,
 	},
 };
 #endif
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1c87945fa761..fc5691afb998 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -104,13 +104,13 @@ struct scan_control {
 	unsigned int force_deactivate:1;
 	unsigned int skipped_deactivate:1;
 
-	/* Writepage batching in laptop mode; RECLAIM_WRITE */
+	/* zone_reclaim_mode, boost reclaim */
 	unsigned int may_writepage:1;
 
-	/* Can mapped folios be reclaimed? */
+	/* zone_reclaim_mode */
 	unsigned int may_unmap:1;
 
-	/* Can folios be swapped as part of reclaim? */
+	/* zome_reclaim_mode, boost reclaim, cgroup restrictions */
 	unsigned int may_swap:1;
 
 	/* Not allow cache_trim_mode to be turned on as part of reclaim? */
@@ -6365,13 +6365,6 @@ retry:
 
 		if (sc->compaction_ready)
 			break;
-
-		/*
-		 * If we're getting trouble reclaiming, start doing
-		 * writepage even in laptop mode.
-		 */
-		if (sc->priority < DEF_PRIORITY - 2)
-			sc->may_writepage = 1;
 	} while (--sc->priority >= 0);
 
 	last_pgdat = NULL;
@@ -6580,7 +6573,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 		.order = order,
 		.nodemask = nodemask,
 		.priority = DEF_PRIORITY,
-		.may_writepage = !laptop_mode,
+		.may_writepage = 1,
 		.may_unmap = 1,
 		.may_swap = 1,
 	};
@@ -6624,7 +6617,7 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
 	struct scan_control sc = {
 		.nr_to_reclaim = SWAP_CLUSTER_MAX,
 		.target_mem_cgroup = memcg,
-		.may_writepage = !laptop_mode,
+		.may_writepage = 1,
 		.may_unmap = 1,
 		.reclaim_idx = MAX_NR_ZONES - 1,
 		.may_swap = !noswap,
@@ -6670,7 +6663,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 		.reclaim_idx = MAX_NR_ZONES - 1,
 		.target_mem_cgroup = memcg,
 		.priority = DEF_PRIORITY,
-		.may_writepage = !laptop_mode,
+		.may_writepage = 1,
 		.may_unmap = 1,
 		.may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
 		.proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
@@ -7051,7 +7044,7 @@ restart:
 		 * from reclaim context. If no pages are reclaimed, the
 		 * reclaim will be aborted.
 		 */
-		sc.may_writepage = !laptop_mode && !nr_boost_reclaim;
+		sc.may_writepage = !nr_boost_reclaim;
 		sc.may_swap = !nr_boost_reclaim;
 
 		/*
@@ -7061,13 +7054,6 @@ restart:
 		 */
 		kswapd_age_node(pgdat, &sc);
 
-		/*
-		 * If we're getting trouble reclaiming, start doing writepage
-		 * even in laptop mode.
-		 */
-		if (sc.priority < DEF_PRIORITY - 2)
-			sc.may_writepage = 1;
-
 		/* Call soft limit reclaim before calling shrink_node. */
 		sc.nr_scanned = 0;
 		nr_soft_scanned = 0;
@@ -7799,7 +7785,7 @@ int user_proactive_reclaim(char *buf,
 				.reclaim_idx = gfp_zone(gfp_mask),
 				.proactive_swappiness = swappiness == -1 ? NULL : &swappiness,
 				.priority = DEF_PRIORITY,
-				.may_writepage = !laptop_mode,
+				.may_writepage = 1,
 				.nr_to_reclaim = max(batch_size, SWAP_CLUSTER_MAX),
 				.may_unmap = 1,
 				.may_swap = 1,