Merge branch 'akpm' (patches from Andrew)

Merge some more updates from Andrew Morton:

 - various hotfixes and minor things

 - hch's use_mm/unuse_mm clearnups

Subsystems affected by this patch series: mm/hugetlb, scripts, kcov,
lib, nilfs, checkpatch, lib, mm/debug, ocfs2, lib, misc.

* emailed patches from Andrew Morton <akpm@linux-foundation.org>:
  kernel: set USER_DS in kthread_use_mm
  kernel: better document the use_mm/unuse_mm API contract
  kernel: move use_mm/unuse_mm to kthread.c
  kernel: move use_mm/unuse_mm to kthread.c
  stacktrace: cleanup inconsistent variable type
  lib: test get_count_order/long in test_bitops.c
  mm: add comments on pglist_data zones
  ocfs2: fix spelling mistake and grammar
  mm/debug_vm_pgtable: fix kernel crash by checking for THP support
  lib: fix bitmap_parse() on 64-bit big endian archs
  checkpatch: correct check for kernel parameters doc
  nilfs2: fix null pointer dereference at nilfs_segctor_do_construct()
  lib/lz4/lz4_decompress.c: document deliberate use of `&'
  kcov: check kcov_softirq in kcov_remote_stop()
  scripts/spelling: add a few more typos
  khugepaged: selftests: fix timeout condition in wait_for_scan()

kernel/kcov.c

@@ -427,7 +427,8 @@ void kcov_task_exit(struct task_struct *t)
 	 *        WARN_ON(!kcov->remote && kcov->t != t);
 	 *
 	 * For KCOV_REMOTE_ENABLE devices, the exiting task is either:
-	 * 2. A remote task between kcov_remote_start() and kcov_remote_stop().
+	 *
+	 * 1. A remote task between kcov_remote_start() and kcov_remote_stop().
 	 *    In this case we should print a warning right away, since a task
 	 *    shouldn't be exiting when it's in a kcov coverage collection
 	 *    section. Here t points to the task that is collecting remote
@@ -437,7 +438,7 @@ void kcov_task_exit(struct task_struct *t)
 	 *        WARN_ON(kcov->remote && kcov->t != t);
 	 *
 	 * 2. The task that created kcov exiting without calling KCOV_DISABLE,
-	 *    and then again we can make sure that t->kcov->t == t:
+	 *    and then again we make sure that t->kcov->t == t:
 	 *        WARN_ON(kcov->remote && kcov->t != t);
 	 *
 	 * By combining all three checks into one we get:
@@ -764,7 +765,7 @@ static const struct file_operations kcov_fops = {
  * Internally, kcov_remote_start() looks up the kcov device associated with the
  * provided handle, allocates an area for coverage collection, and saves the
  * pointers to kcov and area into the current task_struct to allow coverage to
- * be collected via __sanitizer_cov_trace_pc()
+ * be collected via __sanitizer_cov_trace_pc().
  * In turns kcov_remote_stop() clears those pointers from task_struct to stop
  * collecting coverage and copies all collected coverage into the kcov area.
  */
@@ -972,16 +973,25 @@ void kcov_remote_stop(void)
 		local_irq_restore(flags);
 		return;
 	}
-	kcov = t->kcov;
-	area = t->kcov_area;
-	size = t->kcov_size;
-	sequence = t->kcov_sequence;
-
+	/*
+	 * When in softirq, check if the corresponding kcov_remote_start()
+	 * actually found the remote handle and started collecting coverage.
+	 */
+	if (in_serving_softirq() && !t->kcov_softirq) {
+		local_irq_restore(flags);
+		return;
+	}
+	/* Make sure that kcov_softirq is only set when in softirq. */
 	if (WARN_ON(!in_serving_softirq() && t->kcov_softirq)) {
 		local_irq_restore(flags);
 		return;
 	}
 
+	kcov = t->kcov;
+	area = t->kcov_area;
+	size = t->kcov_size;
+	sequence = t->kcov_sequence;
+
 	kcov_stop(t);
 	if (in_serving_softirq()) {
 		t->kcov_softirq = 0;

kernel/kthread.c

@@ -1,13 +1,17 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /* Kernel thread helper functions.
  *   Copyright (C) 2004 IBM Corporation, Rusty Russell.
+ *   Copyright (C) 2009 Red Hat, Inc.
  *
  * Creation is done via kthreadd, so that we get a clean environment
  * even if we're invoked from userspace (think modprobe, hotplug cpu,
  * etc.).
  */
 #include <uapi/linux/sched/types.h>
+#include <linux/mm.h>
+#include <linux/mmu_context.h>
 #include <linux/sched.h>
+#include <linux/sched/mm.h>
 #include <linux/sched/task.h>
 #include <linux/kthread.h>
 #include <linux/completion.h>
@@ -25,6 +29,7 @@
 #include <linux/numa.h>
 #include <trace/events/sched.h>
 
+
 static DEFINE_SPINLOCK(kthread_create_lock);
 static LIST_HEAD(kthread_create_list);
 struct task_struct *kthreadd_task;
@@ -48,6 +53,7 @@ struct kthread {
 	unsigned int cpu;
 	int (*threadfn)(void *);
 	void *data;
+	mm_segment_t oldfs;
 	struct completion parked;
 	struct completion exited;
 #ifdef CONFIG_BLK_CGROUP
@@ -1220,6 +1226,61 @@ void kthread_destroy_worker(struct kthread_worker *worker)
 }
 EXPORT_SYMBOL(kthread_destroy_worker);
 
+/**
+ * kthread_use_mm - make the calling kthread operate on an address space
+ * @mm: address space to operate on
+ */
+void kthread_use_mm(struct mm_struct *mm)
+{
+	struct mm_struct *active_mm;
+	struct task_struct *tsk = current;
+
+	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
+	WARN_ON_ONCE(tsk->mm);
+
+	task_lock(tsk);
+	active_mm = tsk->active_mm;
+	if (active_mm != mm) {
+		mmgrab(mm);
+		tsk->active_mm = mm;
+	}
+	tsk->mm = mm;
+	switch_mm(active_mm, mm, tsk);
+	task_unlock(tsk);
+#ifdef finish_arch_post_lock_switch
+	finish_arch_post_lock_switch();
+#endif
+
+	if (active_mm != mm)
+		mmdrop(active_mm);
+
+	to_kthread(tsk)->oldfs = get_fs();
+	set_fs(USER_DS);
+}
+EXPORT_SYMBOL_GPL(kthread_use_mm);
+
+/**
+ * kthread_unuse_mm - reverse the effect of kthread_use_mm()
+ * @mm: address space to operate on
+ */
+void kthread_unuse_mm(struct mm_struct *mm)
+{
+	struct task_struct *tsk = current;
+
+	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
+	WARN_ON_ONCE(!tsk->mm);
+
+	set_fs(to_kthread(tsk)->oldfs);
+
+	task_lock(tsk);
+	sync_mm_rss(mm);
+	tsk->mm = NULL;
+	/* active_mm is still 'mm' */
+	enter_lazy_tlb(mm, tsk);
+	task_unlock(tsk);
+}
+EXPORT_SYMBOL_GPL(kthread_unuse_mm);
+
 #ifdef CONFIG_BLK_CGROUP
 /**
  * kthread_associate_blkcg - associate blkcg to current kthread