[RFC PATCH 5/7] mm: add runtime toggle and sysctls for async teardown

Add the userspace interface that arms and tunes off-CPU exit teardown,
and the static key that gates it.

Introduce async_mm_teardown_key (static key, default false) and gate the
deferral path in mmput_exit() on it via static_branch_unlikely(); when
off, mmput_exit() runs __mmput() inline. The get_mm_rss() read moves
inside the static_branch_unlikely() block, alongside eligibility and
exit_aio(), so the default (key off) path costs nothing beyond the
branch itself -- no RSS scan for every exiting process on kernels or
boots that never enable the feature. Under CONFIG_SYSCTL, register
three knobs under vm. with register_sysctl_init():

  - async_mm_teardown: write 1/0 to enable/disable. A custom handler
    flips the static key, serialized by a mutex so the key state always
    matches the last value written.
  - async_mm_teardown_thresh_pages: RSS threshold (default ~64MB).
  - async_mm_teardown_max_pending_pages: backpressure cap.

The threshold defaults are set in the initcall outside any CONFIG_SYSCTL
guard, since eligible()/reserve() read them whenever the key is on; only
the toggle variable, its lock, the handler and the table depend on
CONFIG_SYSCTL. With CONFIG_SYSCTL=n there is no path to enable the key,
so the feature stays off and the thresholds are inert.

This patch is still dormant: nothing calls mmput_exit() yet, so flipping
async_mm_teardown on has no observable effect. exit_mm() is routed
through mmput_exit() later in the series; only then does enabling the key
defer teardown. The toggle and gate are introduced here so that, once
routing lands, the feature defaults off and an explicit opt-in is
required.

Signed-off-by: Aditya Sharma <[email protected]>
---
 Documentation/admin-guide/sysctl/vm.rst | 38 +++++++++++
 kernel/fork.c                           | 90 ++++++++++++++++++++-----
 mm/Kconfig                              |  3 +-
 3 files changed, 114 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 5b318d17a..ccda6c75f 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -22,6 +22,9 @@ the writeout of dirty data to disk.
 Currently, these files are in /proc/sys/vm:
 
 - admin_reserve_kbytes
+- async_mm_teardown
+- async_mm_teardown_max_pending_pages
+- async_mm_teardown_thresh_pages
 - compact_memory
 - compaction_proactiveness
 - compact_unevictable_allowed
@@ -108,6 +111,41 @@ On x86_64 this is about 128MB.
 Changing this takes effect whenever an application requests memory.
 
 
+async_mm_teardown
+=================
+
+Available only when CONFIG_ASYNC_MM_TEARDOWN is set. Controls whether the
+address-space teardown (exit_mmap()) of large exiting processes is deferred
+to the mm_reaper kernel thread instead of running on the exiting CPU.
+
+Writing 1 enables deferral, 0 disables it. Disabling makes mmput_exit() tear
+down inline again; it does not drain mms already queued. Defaults to 0.
+
+Deferred teardown consumes CPU in the mm_reaper kernel thread, which runs in
+the root cgroup: the teardown work of a task in a CPU-limited cgroup is not
+charged to that cgroup, similar to kswapd and the oom_reaper. Consider this
+before enabling the feature on systems that rely on strict per-cgroup CPU
+accounting.
+
+
+async_mm_teardown_max_pending_pages
+===================================
+
+Available only when CONFIG_ASYNC_MM_TEARDOWN is set. Backpressure cap, in
+pages, on the total RSS of mms queued for the mm_reaper but not yet torn
+down. An exit that would push the total over this cap tears down
+synchronously instead of queuing. Defaults to totalram_pages() / 4.
+
+
+async_mm_teardown_thresh_pages
+==============================
+
+Available only when CONFIG_ASYNC_MM_TEARDOWN is set. Minimum RSS, in pages,
+for an exiting mm to be eligible for deferral to the mm_reaper. Exiting mms
+below this threshold are always torn down synchronously. Defaults to 64MB
+worth of pages.
+
+
 compact_memory
 ==============
 
diff --git a/kernel/fork.c b/kernel/fork.c
index d45418e66..0976770db 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -3414,8 +3414,13 @@ subsys_initcall(init_fork_sysctl);
 static LLIST_HEAD(mm_reaper_list);
 static DECLARE_WAIT_QUEUE_HEAD(mm_reaper_wait);
 static atomic_long_t mm_reaper_pending_pages;
+static DEFINE_STATIC_KEY_FALSE(async_mm_teardown_key);
 static unsigned long sysctl_async_mm_teardown_thresh_pages;
 static unsigned long sysctl_async_mm_teardown_max_pending_pages;
+#ifdef CONFIG_SYSCTL
+static u8 sysctl_async_mm_teardown_enabled;
+static DEFINE_MUTEX(async_mm_teardown_lock);
+#endif
 
 static bool async_mm_teardown_eligible(struct mm_struct *mm, unsigned long rss)
 {
@@ -3485,8 +3490,6 @@ static int mm_reaper(void *unused)
 
 void mmput_exit(struct mm_struct *mm)
 {
-	unsigned long rss;
-
 	might_sleep();
 	/*
 	 * Fully ordered RMW: combined with exit_mm() (or exec_mmap(), the
@@ -3498,27 +3501,80 @@ void mmput_exit(struct mm_struct *mm)
 	if (!atomic_dec_and_test(&mm->mm_users))
 		return;
 
-	rss = get_mm_rss(mm);
+	if (static_branch_unlikely(&async_mm_teardown_key)) {
+		unsigned long rss = get_mm_rss(mm);
 
-	if (async_mm_teardown_eligible(mm, rss)) {
-		/*
-		 * exit_aio() can block indefinitely. Run it here so a stuck
-		 * AIO only hangs this task (same as how it would happen in
-		 * case of synchronous mmput()) instead of stranding every
-		 * teardown queued behind this mm
-		 */
-		exit_aio(mm);
+		if (async_mm_teardown_eligible(mm, rss)) {
+			/*
+			 * exit_aio() can block indefinitely. Run it here so a stuck
+			 * AIO only hangs this task (same as how it would happen in
+			 * case of synchronous mmput()) instead of stranding every
+			 * teardown queued behind this mm
+			 */
+			exit_aio(mm);
 
-		if (async_mm_teardown_reserve(rss)) {
-			mm->async_reap_rss = rss;
-			async_mm_teardown_queue(mm);
-			return;
+			if (async_mm_teardown_reserve(rss)) {
+				mm->async_reap_rss = rss;
+				async_mm_teardown_queue(mm);
+				return;
+			}
 		}
 	}
 
 	__mmput(mm);
 }
 
+#ifdef CONFIG_SYSCTL
+static int async_mm_teardown_enabled_handler(const struct ctl_table *table,
+					     int write, void *buffer,
+					     size_t *lenp, loff_t *ppos)
+{
+	int ret;
+
+	/*
+	 * Serialize concurrent writers so the static key state always matches
+	 * the last value written to the variable.
+	 */
+	guard(mutex)(&async_mm_teardown_lock);
+
+	ret = proc_dou8vec_minmax(table, write, buffer, lenp, ppos);
+	if (ret || !write)
+		return ret;
+
+	if (sysctl_async_mm_teardown_enabled)
+		static_branch_enable(&async_mm_teardown_key);
+	else
+		static_branch_disable(&async_mm_teardown_key);
+	return 0;
+}
+
+static const struct ctl_table async_mm_teardown_table[] = {
+	{
+		.procname	= "async_mm_teardown",
+		.data		= &sysctl_async_mm_teardown_enabled,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= async_mm_teardown_enabled_handler,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
+	{
+		.procname	= "async_mm_teardown_thresh_pages",
+		.data		= &sysctl_async_mm_teardown_thresh_pages,
+		.maxlen		= sizeof(unsigned long),
+		.mode		= 0644,
+		.proc_handler	= proc_doulongvec_minmax,
+	},
+	{
+		.procname	= "async_mm_teardown_max_pending_pages",
+		.data		= &sysctl_async_mm_teardown_max_pending_pages,
+		.maxlen		= sizeof(unsigned long),
+		.mode		= 0644,
+		.proc_handler	= proc_doulongvec_minmax,
+	},
+};
+#endif /* CONFIG_SYSCTL */
+
 static int __init mm_reaper_init(void)
 {
 	struct task_struct *th;
@@ -3533,7 +3589,9 @@ static int __init mm_reaper_init(void)
 	sysctl_async_mm_teardown_thresh_pages      = SZ_64M >> PAGE_SHIFT;
 	sysctl_async_mm_teardown_max_pending_pages = totalram_pages() / 4;  /* TODO: placeholder */
 	wake_up_process(th);
-
+#ifdef CONFIG_SYSCTL
+	register_sysctl_init("vm", async_mm_teardown_table);
+#endif
 	return 0;
 }
 subsys_initcall(mm_reaper_init);
diff --git a/mm/Kconfig b/mm/Kconfig
index 296b16ad4..a9d925406 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1507,7 +1507,8 @@ config ASYNC_MM_TEARDOWN
 	help
 	  Defer the address space teardown (exit_mmap()) of large exiting processes
 	  to a kernel thread instead of running it on the exiting CPU. The feature
-	  is off by default.
+	  is off by default and is enabled at runtime via the vm.async_mm_teardown
+	  sysctl.
 
 	  If unsure, say N.
 
-- 
2.34.1