Merge patch series "writeback: Avoid lockups when switching inodes"

Jan Kara <jack@suse.cz> says:

This patch series addresses lockups reported by users when systemd unit reading
lots of files from a filesystem mounted with lazytime mount option exits. See
patch 3 for more details about the reproducer.

There are two main issues why switching many inodes between wbs:

1) Multiple workers will be spawned to do the switching but they all contend
on the same wb->list_lock making all the parallelism pointless and just
wasting time.

2) Sorting of wb->b_dirty list by dirtied_time_when is inherently slow.

Patches 1-3 address these problems, patch 4 adds a tracepoint for better
observability of inode writeback switching.

* patches from https://lore.kernel.org/20250912103522.2935-1-jack@suse.cz:
  writeback: Add tracepoint to track pending inode switches
  writeback: Avoid excessively long inode switching times
  writeback: Avoid softlockup when switching many inodes
  writeback: Avoid contention on wb->list_lock when switching inodes

Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 783904d8c5ef..0beaca6bacf7 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -633,6 +633,7 @@ static void cgwb_release_workfn(struct work_struct *work)
 	wb_exit(wb);
 	bdi_put(bdi);
 	WARN_ON_ONCE(!list_empty(&wb->b_attached));
+	WARN_ON_ONCE(work_pending(&wb->switch_work));
 	call_rcu(&wb->rcu, cgwb_free_rcu);
 }
 
@@ -709,6 +710,8 @@ static int cgwb_create(struct backing_dev_info *bdi,
 	wb->memcg_css = memcg_css;
 	wb->blkcg_css = blkcg_css;
 	INIT_LIST_HEAD(&wb->b_attached);
+	INIT_WORK(&wb->switch_work, inode_switch_wbs_work_fn);
+	init_llist_head(&wb->switch_wbs_ctxs);
 	INIT_WORK(&wb->release_work, cgwb_release_workfn);
 	set_bit(WB_registered, &wb->state);
 	bdi_get(bdi);
@@ -839,6 +842,8 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
 	if (!ret) {
 		bdi->wb.memcg_css = &root_mem_cgroup->css;
 		bdi->wb.blkcg_css = blkcg_root_css;
+		INIT_WORK(&bdi->wb.switch_work, inode_switch_wbs_work_fn);
+		init_llist_head(&bdi->wb.switch_wbs_ctxs);
 	}
 	return ret;
 }