[RFC PATCH 2/2] tracing/user_events: Fixup enable faults asyncly

[Beau Belgrave <beaub@linux.microsoft.com>]

When events are enabled within the various tracing facilities, such as
ftrace/perf, the event_mutex is held. As events are enabled pages are
accessed. We do not want page faults to occur under this lock. Instead
queue the fault to a workqueue to be handled in a process context safe
way without the lock.

The enable address is disabled while the async fault-in occurs. This
ensures that we don't attempt fault-in more than is necessary. Once the
page has been faulted in, the address write is attempted again. If the
page couldn't fault-in, then we wait until the next time the event is
enabled to prevent any potential infinite loops.

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
---
 kernel/trace/trace_events_user.c | 125 ++++++++++++++++++++++++++++++-
 1 file changed, 121 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 633f24c2a1ac..f1eb8101e053 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -81,11 +81,22 @@ struct user_event_enabler {
 	struct list_head link;
 	struct mm_struct *mm;
 	struct file *file;
+	refcount_t refcnt;
 	unsigned long enable_addr;
 	unsigned int enable_bit: 5,
-		     __reserved: 27;
+		     __reserved: 26,
+		     disabled: 1;
 };
 
+/* Used for asynchronous faulting in of pages */
+struct user_event_enabler_fault {
+	struct work_struct work;
+	struct user_event_enabler *enabler;
+	struct user_event *event;
+};
+
+static struct kmem_cache *fault_cache;
+
 /*
  * Stores per-event properties, as users register events
  * within a file a user_event might be created if it does not
@@ -236,6 +247,19 @@ static void user_event_enabler_destroy(struct user_event_enabler *enabler)
 	kfree(enabler);
 }
 
+static __always_inline struct user_event_enabler
+*user_event_enabler_get(struct user_event_enabler *enabler)
+{
+	refcount_inc(&enabler->refcnt);
+	return enabler;
+}
+
+static void user_event_enabler_put(struct user_event_enabler *enabler)
+{
+	if (refcount_dec_and_test(&enabler->refcnt))
+		user_event_enabler_destroy(enabler);
+}
+
 static void user_event_enabler_remove(struct file *file,
 				      struct user_event *user)
 {
@@ -249,13 +273,93 @@ static void user_event_enabler_remove(struct file *file,
 		if (enabler->file != file)
 			continue;
 
+		enabler->disabled = 0;
 		list_del(&enabler->link);
-		user_event_enabler_destroy(enabler);
+		user_event_enabler_put(enabler);
 	}
 
 	mutex_unlock(&event_mutex);
 }
 
+static void user_event_enabler_write(struct user_event_enabler *enabler,
+				     struct user_event *user);
+
+static void user_event_enabler_fault_fixup(struct work_struct *work)
+{
+	struct user_event_enabler_fault *fault = container_of(
+		work, struct user_event_enabler_fault, work);
+	struct user_event_enabler *enabler = fault->enabler;
+	struct user_event *user = fault->event;
+	struct mm_struct *mm = enabler->mm;
+	unsigned long uaddr = enabler->enable_addr;
+	bool unlocked = false;
+	int ret;
+
+	might_sleep();
+
+	mmap_read_lock(mm);
+
+	ret = fixup_user_fault(mm, uaddr, FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
+			       &unlocked);
+
+	mmap_read_unlock(mm);
+
+	if (ret)
+		pr_warn("user_events: Fixup fault failed with %d "
+			"for mm: 0x%pK offset: 0x%llx event: %s\n", ret, mm,
+			(unsigned long long)uaddr, EVENT_NAME(user));
+
+	/* Prevent state changes from racing */
+	mutex_lock(&event_mutex);
+
+	/*
+	 * If we managed to get the page, re-issue the write. We do not
+	 * want to get into a possible infinite loop, which is why we only
+	 * attempt again directly if the page came in. If we couldn't get
+	 * the page here, then we will try again the next time the event is
+	 * enabled/disabled.
+	 */
+	enabler->disabled = 0;
+
+	if (!ret)
+		user_event_enabler_write(enabler, user);
+
+	mutex_unlock(&event_mutex);
+
+	refcount_dec(&user->refcnt);
+	user_event_enabler_put(enabler);
+	kmem_cache_free(fault_cache, fault);
+}
+
+static bool user_event_enabler_queue_fault(struct user_event_enabler *enabler,
+					   struct user_event *user)
+{
+	struct user_event_enabler_fault *fault;
+
+	fault = kmem_cache_zalloc(fault_cache, GFP_NOWAIT | __GFP_NOWARN);
+
+	if (!fault)
+		return false;
+
+	INIT_WORK(&fault->work, user_event_enabler_fault_fixup);
+	fault->enabler = user_event_enabler_get(enabler);
+	fault->event = user;
+
+	refcount_inc(&user->refcnt);
+	enabler->disabled = 1;
+
+	if (!schedule_work(&fault->work)) {
+		enabler->disabled = 0;
+		refcount_dec(&user->refcnt);
+		user_event_enabler_put(enabler);
+		kmem_cache_free(fault_cache, fault);
+
+		return false;
+	}
+
+	return true;
+}
+
 static void user_event_enabler_write(struct user_event_enabler *enabler,
 				     struct user_event *user)
 {
@@ -266,6 +370,11 @@ static void user_event_enabler_write(struct user_event_enabler *enabler,
 	void *kaddr;
 	int ret;
 
+	lockdep_assert_held(&event_mutex);
+
+	if (unlikely(enabler->disabled))
+		return;
+
 	mmap_read_lock(mm);
 
 	ret = pin_user_pages_remote(mm, uaddr, 1, FOLL_WRITE | FOLL_NOFAULT,
@@ -273,8 +382,10 @@ static void user_event_enabler_write(struct user_event_enabler *enabler,
 
 	mmap_read_unlock(mm);
 
-	if (ret <= 0) {
-		pr_warn("user_events: Enable write failed\n");
+	if (unlikely(ret <= 0)) {
+		if (!user_event_enabler_queue_fault(enabler, user))
+			pr_warn("user_events: Unable to queue fault handler\n");
+
 		return;
 	}
 
@@ -321,6 +432,7 @@ static struct user_event_enabler
 	enabler->file = file;
 	enabler->enable_addr = (unsigned long)reg->enable_addr;
 	enabler->enable_bit = reg->enable_bit;
+	refcount_set(&enabler->refcnt, 1);
 
 	/* Prevents state changes from racing with new enablers */
 	mutex_lock(&event_mutex);
@@ -1902,6 +2014,11 @@ static int __init trace_events_user_init(void)
 {
 	int ret;
 
+	fault_cache = KMEM_CACHE(user_event_enabler_fault, 0);
+
+	if (!fault_cache)
+		return -ENOMEM;
+
 	init_group = user_event_group_create(&init_user_ns);
 
 	if (!init_group)
-- 
2.25.1