path: root/patches/0001-mm-memcg-Disable-threshold-event-handlers-on-PREEMPT.patch

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Tue, 18 Jan 2022 17:28:07 +0100
Subject: [PATCH 1/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT
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During the integration of PREEMPT_RT support, the code flow around
memcg_check_events() resulted in `twisted code'. Moving the code around
and avoiding then would then lead to an additional local-irq-save
section within memcg_check_events(). While looking better, it adds a
local-irq-save section to code flow which is usually within an
local-irq-off block on non-PREEMPT_RT configurations.

The threshold event handler is a deprecated memcg v1 feature. Instead of
trying to get it to work under PREEMPT_RT just disable it. There should
be no users on PREEMPT_RT. From that perspective it makes even less
sense to get it to work under PREEMPT_RT while having zero users.

Make memory.soft_limit_in_bytes and cgroup.event_control return
-EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
Document that the two knobs are disabled on PREEMPT_RT. Shuffle the code around
so that all unused function are in on #ifdef block.

Suggested-by: Michal Hocko <mhocko@kernel.org>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 Documentation/admin-guide/cgroup-v1/memory.rst |    2 
 mm/memcontrol.c                                |  728 ++++++++++++-------------
 2 files changed, 374 insertions(+), 356 deletions(-)

--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -64,6 +64,7 @@ Brief summary of control files.
 				     threads
  cgroup.procs			     show list of processes
  cgroup.event_control		     an interface for event_fd()
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.usage_in_bytes		     show current usage for memory
 				     (See 5.5 for details)
  memory.memsw.usage_in_bytes	     show current usage for memory+Swap
@@ -75,6 +76,7 @@ Brief summary of control files.
  memory.max_usage_in_bytes	     show max memory usage recorded
  memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
  memory.soft_limit_in_bytes	     set/show soft limit of memory usage
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.stat			     show various statistics
  memory.use_hierarchy		     set/show hierarchical account enabled
                                      This knob is deprecated and shouldn't be
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -169,7 +169,6 @@ struct mem_cgroup_event {
 	struct work_struct remove;
 };
 
-static void mem_cgroup_threshold(struct mem_cgroup *memcg);
 static void mem_cgroup_oom_notify(struct mem_cgroup *memcg);
 
 /* Stuffs for move charges at task migration. */
@@ -521,43 +520,6 @@ static unsigned long soft_limit_excess(s
 	return excess;
 }
 
-static void mem_cgroup_update_tree(struct mem_cgroup *memcg, int nid)
-{
-	unsigned long excess;
-	struct mem_cgroup_per_node *mz;
-	struct mem_cgroup_tree_per_node *mctz;
-
-	mctz = soft_limit_tree.rb_tree_per_node[nid];
-	if (!mctz)
-		return;
-	/*
-	 * Necessary to update all ancestors when hierarchy is used.
-	 * because their event counter is not touched.
-	 */
-	for (; memcg; memcg = parent_mem_cgroup(memcg)) {
-		mz = memcg->nodeinfo[nid];
-		excess = soft_limit_excess(memcg);
-		/*
-		 * We have to update the tree if mz is on RB-tree or
-		 * mem is over its softlimit.
-		 */
-		if (excess || mz->on_tree) {
-			unsigned long flags;
-
-			spin_lock_irqsave(&mctz->lock, flags);
-			/* if on-tree, remove it */
-			if (mz->on_tree)
-				__mem_cgroup_remove_exceeded(mz, mctz);
-			/*
-			 * Insert again. mz->usage_in_excess will be updated.
-			 * If excess is 0, no tree ops.
-			 */
-			__mem_cgroup_insert_exceeded(mz, mctz, excess);
-			spin_unlock_irqrestore(&mctz->lock, flags);
-		}
-	}
-}
-
 static void mem_cgroup_remove_from_trees(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup_tree_per_node *mctz;
@@ -827,50 +789,6 @@ static void mem_cgroup_charge_statistics
 	__this_cpu_add(memcg->vmstats_percpu->nr_page_events, nr_pages);
 }
 
-static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
-				       enum mem_cgroup_events_target target)
-{
-	unsigned long val, next;
-
-	val = __this_cpu_read(memcg->vmstats_percpu->nr_page_events);
-	next = __this_cpu_read(memcg->vmstats_percpu->targets[target]);
-	/* from time_after() in jiffies.h */
-	if ((long)(next - val) < 0) {
-		switch (target) {
-		case MEM_CGROUP_TARGET_THRESH:
-			next = val + THRESHOLDS_EVENTS_TARGET;
-			break;
-		case MEM_CGROUP_TARGET_SOFTLIMIT:
-			next = val + SOFTLIMIT_EVENTS_TARGET;
-			break;
-		default:
-			break;
-		}
-		__this_cpu_write(memcg->vmstats_percpu->targets[target], next);
-		return true;
-	}
-	return false;
-}
-
-/*
- * Check events in order.
- *
- */
-static void memcg_check_events(struct mem_cgroup *memcg, int nid)
-{
-	/* threshold event is triggered in finer grain than soft limit */
-	if (unlikely(mem_cgroup_event_ratelimit(memcg,
-						MEM_CGROUP_TARGET_THRESH))) {
-		bool do_softlimit;
-
-		do_softlimit = mem_cgroup_event_ratelimit(memcg,
-						MEM_CGROUP_TARGET_SOFTLIMIT);
-		mem_cgroup_threshold(memcg);
-		if (unlikely(do_softlimit))
-			mem_cgroup_update_tree(memcg, nid);
-	}
-}
-
 struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
 {
 	/*
@@ -3763,8 +3681,12 @@ static ssize_t mem_cgroup_write(struct k
 		}
 		break;
 	case RES_SOFT_LIMIT:
+#ifndef CONFIG_PREEMPT_RT
 		memcg->soft_limit = nr_pages;
 		ret = 0;
+#else
+		ret = -EOPNOTSUPP;
+#endif
 		break;
 	}
 	return ret ?: nbytes;
@@ -4069,6 +3991,343 @@ static int mem_cgroup_swappiness_write(s
 	return 0;
 }
 
+static int mem_cgroup_oom_notify_cb(struct mem_cgroup *memcg)
+{
+	struct mem_cgroup_eventfd_list *ev;
+
+	spin_lock(&memcg_oom_lock);
+
+	list_for_each_entry(ev, &memcg->oom_notify, list)
+		eventfd_signal(ev->eventfd, 1);
+
+	spin_unlock(&memcg_oom_lock);
+	return 0;
+}
+
+static void mem_cgroup_oom_notify(struct mem_cgroup *memcg)
+{
+	struct mem_cgroup *iter;
+
+	for_each_mem_cgroup_tree(iter, memcg)
+		mem_cgroup_oom_notify_cb(iter);
+}
+
+static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
+
+	seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
+	seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
+	seq_printf(sf, "oom_kill %lu\n",
+		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
+	return 0;
+}
+
+static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
+	struct cftype *cft, u64 val)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+
+	/* cannot set to root cgroup and only 0 and 1 are allowed */
+	if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
+		return -EINVAL;
+
+	memcg->oom_kill_disable = val;
+	if (!val)
+		memcg_oom_recover(memcg);
+
+	return 0;
+}
+
+#ifdef CONFIG_CGROUP_WRITEBACK
+
+#include <trace/events/writeback.h>
+
+static int memcg_wb_domain_init(struct mem_cgroup *memcg, gfp_t gfp)
+{
+	return wb_domain_init(&memcg->cgwb_domain, gfp);
+}
+
+static void memcg_wb_domain_exit(struct mem_cgroup *memcg)
+{
+	wb_domain_exit(&memcg->cgwb_domain);
+}
+
+static void memcg_wb_domain_size_changed(struct mem_cgroup *memcg)
+{
+	wb_domain_size_changed(&memcg->cgwb_domain);
+}
+
+struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
+
+	if (!memcg->css.parent)
+		return NULL;
+
+	return &memcg->cgwb_domain;
+}
+
+/**
+ * mem_cgroup_wb_stats - retrieve writeback related stats from its memcg
+ * @wb: bdi_writeback in question
+ * @pfilepages: out parameter for number of file pages
+ * @pheadroom: out parameter for number of allocatable pages according to memcg
+ * @pdirty: out parameter for number of dirty pages
+ * @pwriteback: out parameter for number of pages under writeback
+ *
+ * Determine the numbers of file, headroom, dirty, and writeback pages in
+ * @wb's memcg.  File, dirty and writeback are self-explanatory.  Headroom
+ * is a bit more involved.
+ *
+ * A memcg's headroom is "min(max, high) - used".  In the hierarchy, the
+ * headroom is calculated as the lowest headroom of itself and the
+ * ancestors.  Note that this doesn't consider the actual amount of
+ * available memory in the system.  The caller should further cap
+ * *@pheadroom accordingly.
+ */
+void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
+			 unsigned long *pheadroom, unsigned long *pdirty,
+			 unsigned long *pwriteback)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
+	struct mem_cgroup *parent;
+
+	mem_cgroup_flush_stats();
+
+	*pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
+	*pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
+	*pfilepages = memcg_page_state(memcg, NR_INACTIVE_FILE) +
+			memcg_page_state(memcg, NR_ACTIVE_FILE);
+
+	*pheadroom = PAGE_COUNTER_MAX;
+	while ((parent = parent_mem_cgroup(memcg))) {
+		unsigned long ceiling = min(READ_ONCE(memcg->memory.max),
+					    READ_ONCE(memcg->memory.high));
+		unsigned long used = page_counter_read(&memcg->memory);
+
+		*pheadroom = min(*pheadroom, ceiling - min(ceiling, used));
+		memcg = parent;
+	}
+}
+
+/*
+ * Foreign dirty flushing
+ *
+ * There's an inherent mismatch between memcg and writeback.  The former
+ * tracks ownership per-page while the latter per-inode.  This was a
+ * deliberate design decision because honoring per-page ownership in the
+ * writeback path is complicated, may lead to higher CPU and IO overheads
+ * and deemed unnecessary given that write-sharing an inode across
+ * different cgroups isn't a common use-case.
+ *
+ * Combined with inode majority-writer ownership switching, this works well
+ * enough in most cases but there are some pathological cases.  For
+ * example, let's say there are two cgroups A and B which keep writing to
+ * different but confined parts of the same inode.  B owns the inode and
+ * A's memory is limited far below B's.  A's dirty ratio can rise enough to
+ * trigger balance_dirty_pages() sleeps but B's can be low enough to avoid
+ * triggering background writeback.  A will be slowed down without a way to
+ * make writeback of the dirty pages happen.
+ *
+ * Conditions like the above can lead to a cgroup getting repeatedly and
+ * severely throttled after making some progress after each
+ * dirty_expire_interval while the underlying IO device is almost
+ * completely idle.
+ *
+ * Solving this problem completely requires matching the ownership tracking
+ * granularities between memcg and writeback in either direction.  However,
+ * the more egregious behaviors can be avoided by simply remembering the
+ * most recent foreign dirtying events and initiating remote flushes on
+ * them when local writeback isn't enough to keep the memory clean enough.
+ *
+ * The following two functions implement such mechanism.  When a foreign
+ * page - a page whose memcg and writeback ownerships don't match - is
+ * dirtied, mem_cgroup_track_foreign_dirty() records the inode owning
+ * bdi_writeback on the page owning memcg.  When balance_dirty_pages()
+ * decides that the memcg needs to sleep due to high dirty ratio, it calls
+ * mem_cgroup_flush_foreign() which queues writeback on the recorded
+ * foreign bdi_writebacks which haven't expired.  Both the numbers of
+ * recorded bdi_writebacks and concurrent in-flight foreign writebacks are
+ * limited to MEMCG_CGWB_FRN_CNT.
+ *
+ * The mechanism only remembers IDs and doesn't hold any object references.
+ * As being wrong occasionally doesn't matter, updates and accesses to the
+ * records are lockless and racy.
+ */
+void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
+					     struct bdi_writeback *wb)
+{
+	struct mem_cgroup *memcg = folio_memcg(folio);
+	struct memcg_cgwb_frn *frn;
+	u64 now = get_jiffies_64();
+	u64 oldest_at = now;
+	int oldest = -1;
+	int i;
+
+	trace_track_foreign_dirty(folio, wb);
+
+	/*
+	 * Pick the slot to use.  If there is already a slot for @wb, keep
+	 * using it.  If not replace the oldest one which isn't being
+	 * written out.
+	 */
+	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
+		frn = &memcg->cgwb_frn[i];
+		if (frn->bdi_id == wb->bdi->id &&
+		    frn->memcg_id == wb->memcg_css->id)
+			break;
+		if (time_before64(frn->at, oldest_at) &&
+		    atomic_read(&frn->done.cnt) == 1) {
+			oldest = i;
+			oldest_at = frn->at;
+		}
+	}
+
+	if (i < MEMCG_CGWB_FRN_CNT) {
+		/*
+		 * Re-using an existing one.  Update timestamp lazily to
+		 * avoid making the cacheline hot.  We want them to be
+		 * reasonably up-to-date and significantly shorter than
+		 * dirty_expire_interval as that's what expires the record.
+		 * Use the shorter of 1s and dirty_expire_interval / 8.
+		 */
+		unsigned long update_intv =
+			min_t(unsigned long, HZ,
+			      msecs_to_jiffies(dirty_expire_interval * 10) / 8);
+
+		if (time_before64(frn->at, now - update_intv))
+			frn->at = now;
+	} else if (oldest >= 0) {
+		/* replace the oldest free one */
+		frn = &memcg->cgwb_frn[oldest];
+		frn->bdi_id = wb->bdi->id;
+		frn->memcg_id = wb->memcg_css->id;
+		frn->at = now;
+	}
+}
+
+/* issue foreign writeback flushes for recorded foreign dirtying events */
+void mem_cgroup_flush_foreign(struct bdi_writeback *wb)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
+	unsigned long intv = msecs_to_jiffies(dirty_expire_interval * 10);
+	u64 now = jiffies_64;
+	int i;
+
+	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
+		struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i];
+
+		/*
+		 * If the record is older than dirty_expire_interval,
+		 * writeback on it has already started.  No need to kick it
+		 * off again.  Also, don't start a new one if there's
+		 * already one in flight.
+		 */
+		if (time_after64(frn->at, now - intv) &&
+		    atomic_read(&frn->done.cnt) == 1) {
+			frn->at = 0;
+			trace_flush_foreign(wb, frn->bdi_id, frn->memcg_id);
+			cgroup_writeback_by_id(frn->bdi_id, frn->memcg_id,
+					       WB_REASON_FOREIGN_FLUSH,
+					       &frn->done);
+		}
+	}
+}
+
+#else	/* CONFIG_CGROUP_WRITEBACK */
+
+static int memcg_wb_domain_init(struct mem_cgroup *memcg, gfp_t gfp)
+{
+	return 0;
+}
+
+static void memcg_wb_domain_exit(struct mem_cgroup *memcg)
+{
+}
+
+static void memcg_wb_domain_size_changed(struct mem_cgroup *memcg)
+{
+}
+
+#endif	/* CONFIG_CGROUP_WRITEBACK */
+
+#ifndef CONFIG_PREEMPT_RT
+/*
+ * DO NOT USE IN NEW FILES.
+ *
+ * "cgroup.event_control" implementation.
+ *
+ * This is way over-engineered.  It tries to support fully configurable
+ * events for each user.  Such level of flexibility is completely
+ * unnecessary especially in the light of the planned unified hierarchy.
+ *
+ * Please deprecate this and replace with something simpler if at all
+ * possible.
+ */
+
+static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
+				       enum mem_cgroup_events_target target)
+{
+	unsigned long val, next;
+
+	val = __this_cpu_read(memcg->vmstats_percpu->nr_page_events);
+	next = __this_cpu_read(memcg->vmstats_percpu->targets[target]);
+	/* from time_after() in jiffies.h */
+	if ((long)(next - val) < 0) {
+		switch (target) {
+		case MEM_CGROUP_TARGET_THRESH:
+			next = val + THRESHOLDS_EVENTS_TARGET;
+			break;
+		case MEM_CGROUP_TARGET_SOFTLIMIT:
+			next = val + SOFTLIMIT_EVENTS_TARGET;
+			break;
+		default:
+			break;
+		}
+		__this_cpu_write(memcg->vmstats_percpu->targets[target], next);
+		return true;
+	}
+	return false;
+}
+
+static void mem_cgroup_update_tree(struct mem_cgroup *memcg, int nid)
+{
+	unsigned long excess;
+	struct mem_cgroup_per_node *mz;
+	struct mem_cgroup_tree_per_node *mctz;
+
+	mctz = soft_limit_tree.rb_tree_per_node[nid];
+	if (!mctz)
+		return;
+	/*
+	 * Necessary to update all ancestors when hierarchy is used.
+	 * because their event counter is not touched.
+	 */
+	for (; memcg; memcg = parent_mem_cgroup(memcg)) {
+		mz = memcg->nodeinfo[nid];
+		excess = soft_limit_excess(memcg);
+		/*
+		 * We have to update the tree if mz is on RB-tree or
+		 * mem is over its softlimit.
+		 */
+		if (excess || mz->on_tree) {
+			unsigned long flags;
+
+			spin_lock_irqsave(&mctz->lock, flags);
+			/* if on-tree, remove it */
+			if (mz->on_tree)
+				__mem_cgroup_remove_exceeded(mz, mctz);
+			/*
+			 * Insert again. mz->usage_in_excess will be updated.
+			 * If excess is 0, no tree ops.
+			 */
+			__mem_cgroup_insert_exceeded(mz, mctz, excess);
+			spin_unlock_irqrestore(&mctz->lock, flags);
+		}
+	}
+}
+
 static void __mem_cgroup_threshold(struct mem_cgroup *memcg, bool swap)
 {
 	struct mem_cgroup_threshold_ary *t;
@@ -4131,6 +4390,25 @@ static void mem_cgroup_threshold(struct
 	}
 }
 
+/*
+ * Check events in order.
+ *
+ */
+static void memcg_check_events(struct mem_cgroup *memcg, int nid)
+{
+	/* threshold event is triggered in finer grain than soft limit */
+	if (unlikely(mem_cgroup_event_ratelimit(memcg,
+						MEM_CGROUP_TARGET_THRESH))) {
+		bool do_softlimit;
+
+		do_softlimit = mem_cgroup_event_ratelimit(memcg,
+						MEM_CGROUP_TARGET_SOFTLIMIT);
+		mem_cgroup_threshold(memcg);
+		if (unlikely(do_softlimit))
+			mem_cgroup_update_tree(memcg, nid);
+	}
+}
+
 static int compare_thresholds(const void *a, const void *b)
 {
 	const struct mem_cgroup_threshold *_a = a;
@@ -4145,27 +4423,6 @@ static int compare_thresholds(const void
 	return 0;
 }
 
-static int mem_cgroup_oom_notify_cb(struct mem_cgroup *memcg)
-{
-	struct mem_cgroup_eventfd_list *ev;
-
-	spin_lock(&memcg_oom_lock);
-
-	list_for_each_entry(ev, &memcg->oom_notify, list)
-		eventfd_signal(ev->eventfd, 1);
-
-	spin_unlock(&memcg_oom_lock);
-	return 0;
-}
-
-static void mem_cgroup_oom_notify(struct mem_cgroup *memcg)
-{
-	struct mem_cgroup *iter;
-
-	for_each_mem_cgroup_tree(iter, memcg)
-		mem_cgroup_oom_notify_cb(iter);
-}
-
 static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
 	struct eventfd_ctx *eventfd, const char *args, enum res_type type)
 {
@@ -4394,259 +4651,6 @@ static void mem_cgroup_oom_unregister_ev
 	spin_unlock(&memcg_oom_lock);
 }
 
-static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
-{
-	struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
-
-	seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
-	seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
-	seq_printf(sf, "oom_kill %lu\n",
-		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
-	return 0;
-}
-
-static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
-	struct cftype *cft, u64 val)
-{
-	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-
-	/* cannot set to root cgroup and only 0 and 1 are allowed */
-	if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
-		return -EINVAL;
-
-	memcg->oom_kill_disable = val;
-	if (!val)
-		memcg_oom_recover(memcg);
-
-	return 0;
-}
-
-#ifdef CONFIG_CGROUP_WRITEBACK
-
-#include <trace/events/writeback.h>
-
-static int memcg_wb_domain_init(struct mem_cgroup *memcg, gfp_t gfp)
-{
-	return wb_domain_init(&memcg->cgwb_domain, gfp);
-}
-
-static void memcg_wb_domain_exit(struct mem_cgroup *memcg)
-{
-	wb_domain_exit(&memcg->cgwb_domain);
-}
-
-static void memcg_wb_domain_size_changed(struct mem_cgroup *memcg)
-{
-	wb_domain_size_changed(&memcg->cgwb_domain);
-}
-
-struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
-{
-	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
-
-	if (!memcg->css.parent)
-		return NULL;
-
-	return &memcg->cgwb_domain;
-}
-
-/**
- * mem_cgroup_wb_stats - retrieve writeback related stats from its memcg
- * @wb: bdi_writeback in question
- * @pfilepages: out parameter for number of file pages
- * @pheadroom: out parameter for number of allocatable pages according to memcg
- * @pdirty: out parameter for number of dirty pages
- * @pwriteback: out parameter for number of pages under writeback
- *
- * Determine the numbers of file, headroom, dirty, and writeback pages in
- * @wb's memcg.  File, dirty and writeback are self-explanatory.  Headroom
- * is a bit more involved.
- *
- * A memcg's headroom is "min(max, high) - used".  In the hierarchy, the
- * headroom is calculated as the lowest headroom of itself and the
- * ancestors.  Note that this doesn't consider the actual amount of
- * available memory in the system.  The caller should further cap
- * *@pheadroom accordingly.
- */
-void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
-			 unsigned long *pheadroom, unsigned long *pdirty,
-			 unsigned long *pwriteback)
-{
-	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
-	struct mem_cgroup *parent;
-
-	mem_cgroup_flush_stats();
-
-	*pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
-	*pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
-	*pfilepages = memcg_page_state(memcg, NR_INACTIVE_FILE) +
-			memcg_page_state(memcg, NR_ACTIVE_FILE);
-
-	*pheadroom = PAGE_COUNTER_MAX;
-	while ((parent = parent_mem_cgroup(memcg))) {
-		unsigned long ceiling = min(READ_ONCE(memcg->memory.max),
-					    READ_ONCE(memcg->memory.high));
-		unsigned long used = page_counter_read(&memcg->memory);
-
-		*pheadroom = min(*pheadroom, ceiling - min(ceiling, used));
-		memcg = parent;
-	}
-}
-
-/*
- * Foreign dirty flushing
- *
- * There's an inherent mismatch between memcg and writeback.  The former
- * tracks ownership per-page while the latter per-inode.  This was a
- * deliberate design decision because honoring per-page ownership in the
- * writeback path is complicated, may lead to higher CPU and IO overheads
- * and deemed unnecessary given that write-sharing an inode across
- * different cgroups isn't a common use-case.
- *
- * Combined with inode majority-writer ownership switching, this works well
- * enough in most cases but there are some pathological cases.  For
- * example, let's say there are two cgroups A and B which keep writing to
- * different but confined parts of the same inode.  B owns the inode and
- * A's memory is limited far below B's.  A's dirty ratio can rise enough to
- * trigger balance_dirty_pages() sleeps but B's can be low enough to avoid
- * triggering background writeback.  A will be slowed down without a way to
- * make writeback of the dirty pages happen.
- *
- * Conditions like the above can lead to a cgroup getting repeatedly and
- * severely throttled after making some progress after each
- * dirty_expire_interval while the underlying IO device is almost
- * completely idle.
- *
- * Solving this problem completely requires matching the ownership tracking
- * granularities between memcg and writeback in either direction.  However,
- * the more egregious behaviors can be avoided by simply remembering the
- * most recent foreign dirtying events and initiating remote flushes on
- * them when local writeback isn't enough to keep the memory clean enough.
- *
- * The following two functions implement such mechanism.  When a foreign
- * page - a page whose memcg and writeback ownerships don't match - is
- * dirtied, mem_cgroup_track_foreign_dirty() records the inode owning
- * bdi_writeback on the page owning memcg.  When balance_dirty_pages()
- * decides that the memcg needs to sleep due to high dirty ratio, it calls
- * mem_cgroup_flush_foreign() which queues writeback on the recorded
- * foreign bdi_writebacks which haven't expired.  Both the numbers of
- * recorded bdi_writebacks and concurrent in-flight foreign writebacks are
- * limited to MEMCG_CGWB_FRN_CNT.
- *
- * The mechanism only remembers IDs and doesn't hold any object references.
- * As being wrong occasionally doesn't matter, updates and accesses to the
- * records are lockless and racy.
- */
-void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
-					     struct bdi_writeback *wb)
-{
-	struct mem_cgroup *memcg = folio_memcg(folio);
-	struct memcg_cgwb_frn *frn;
-	u64 now = get_jiffies_64();
-	u64 oldest_at = now;
-	int oldest = -1;
-	int i;
-
-	trace_track_foreign_dirty(folio, wb);
-
-	/*
-	 * Pick the slot to use.  If there is already a slot for @wb, keep
-	 * using it.  If not replace the oldest one which isn't being
-	 * written out.
-	 */
-	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
-		frn = &memcg->cgwb_frn[i];
-		if (frn->bdi_id == wb->bdi->id &&
-		    frn->memcg_id == wb->memcg_css->id)
-			break;
-		if (time_before64(frn->at, oldest_at) &&
-		    atomic_read(&frn->done.cnt) == 1) {
-			oldest = i;
-			oldest_at = frn->at;
-		}
-	}
-
-	if (i < MEMCG_CGWB_FRN_CNT) {
-		/*
-		 * Re-using an existing one.  Update timestamp lazily to
-		 * avoid making the cacheline hot.  We want them to be
-		 * reasonably up-to-date and significantly shorter than
-		 * dirty_expire_interval as that's what expires the record.
-		 * Use the shorter of 1s and dirty_expire_interval / 8.
-		 */
-		unsigned long update_intv =
-			min_t(unsigned long, HZ,
-			      msecs_to_jiffies(dirty_expire_interval * 10) / 8);
-
-		if (time_before64(frn->at, now - update_intv))
-			frn->at = now;
-	} else if (oldest >= 0) {
-		/* replace the oldest free one */
-		frn = &memcg->cgwb_frn[oldest];
-		frn->bdi_id = wb->bdi->id;
-		frn->memcg_id = wb->memcg_css->id;
-		frn->at = now;
-	}
-}
-
-/* issue foreign writeback flushes for recorded foreign dirtying events */
-void mem_cgroup_flush_foreign(struct bdi_writeback *wb)
-{
-	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
-	unsigned long intv = msecs_to_jiffies(dirty_expire_interval * 10);
-	u64 now = jiffies_64;
-	int i;
-
-	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
-		struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i];
-
-		/*
-		 * If the record is older than dirty_expire_interval,
-		 * writeback on it has already started.  No need to kick it
-		 * off again.  Also, don't start a new one if there's
-		 * already one in flight.
-		 */
-		if (time_after64(frn->at, now - intv) &&
-		    atomic_read(&frn->done.cnt) == 1) {
-			frn->at = 0;
-			trace_flush_foreign(wb, frn->bdi_id, frn->memcg_id);
-			cgroup_writeback_by_id(frn->bdi_id, frn->memcg_id,
-					       WB_REASON_FOREIGN_FLUSH,
-					       &frn->done);
-		}
-	}
-}
-
-#else	/* CONFIG_CGROUP_WRITEBACK */
-
-static int memcg_wb_domain_init(struct mem_cgroup *memcg, gfp_t gfp)
-{
-	return 0;
-}
-
-static void memcg_wb_domain_exit(struct mem_cgroup *memcg)
-{
-}
-
-static void memcg_wb_domain_size_changed(struct mem_cgroup *memcg)
-{
-}
-
-#endif	/* CONFIG_CGROUP_WRITEBACK */
-
-/*
- * DO NOT USE IN NEW FILES.
- *
- * "cgroup.event_control" implementation.
- *
- * This is way over-engineered.  It tries to support fully configurable
- * events for each user.  Such level of flexibility is completely
- * unnecessary especially in the light of the planned unified hierarchy.
- *
- * Please deprecate this and replace with something simpler if at all
- * possible.
- */
-
 /*
  * Unregister event and free resources.
  *
@@ -4857,6 +4861,18 @@ static ssize_t memcg_write_event_control
 	return ret;
 }
 
+#else
+
+static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
+					 char *buf, size_t nbytes, loff_t off)
+{
+	return -EOPNOTSUPP;
+}
+
+static void memcg_check_events(struct mem_cgroup *memcg, int nid) { }
+
+#endif
+
 #if defined(CONFIG_MEMCG_KMEM) && (defined(CONFIG_SLAB) || defined(CONFIG_SLUB_DEBUG))
 static int mem_cgroup_slab_show(struct seq_file *m, void *p)
 {