mm/mmu_notifier.c - kernel/msm-5.4 - Gitiles

 /*
  *  linux/mm/mmu_notifier.c
  *
  *  Copyright (C) 2008  Qumranet, Inc.
  *  Copyright (C) 2008  SGI
  *             Christoph Lameter <cl@linux.com>
  *
  *  This work is licensed under the terms of the GNU GPL, version 2. See
  *  the COPYING file in the top-level directory.
  */

 #include <linux/rculist.h>
 #include <linux/mmu_notifier.h>
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/err.h>
 #include <linux/srcu.h>
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
 #include <linux/slab.h>

 /* global SRCU for all MMs */
 DEFINE_STATIC_SRCU(srcu);

 /*
  * This function allows mmu_notifier::release callback to delay a call to
  * a function that will free appropriate resources. The function must be
  * quick and must not block.
  */
 void mmu_notifier_call_srcu(struct rcu_head *rcu,
 			    void (*func)(struct rcu_head *rcu))
 {
 	call_srcu(&srcu, rcu, func);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_call_srcu);

 void mmu_notifier_synchronize(void)
 {
 	/* Wait for any running method to finish. */
 	srcu_barrier(&srcu);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_synchronize);

 /*
  * This function can't run concurrently against mmu_notifier_register
  * because mm->mm_users > 0 during mmu_notifier_register and exit_mmap
  * runs with mm_users == 0. Other tasks may still invoke mmu notifiers
  * in parallel despite there being no task using this mm any more,
  * through the vmas outside of the exit_mmap context, such as with
  * vmtruncate. This serializes against mmu_notifier_unregister with
  * the mmu_notifier_mm->lock in addition to SRCU and it serializes
  * against the other mmu notifiers with SRCU. struct mmu_notifier_mm
  * can't go away from under us as exit_mmap holds an mm_count pin
  * itself.
  */
 void __mmu_notifier_release(struct mm_struct *mm)
 {
 	struct mmu_notifier *mn;
 	int id;

 	/*
 	 * SRCU here will block mmu_notifier_unregister until
 	 * ->release returns.
 	 */
 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist)
 		/*
 		 * If ->release runs before mmu_notifier_unregister it must be
 		 * handled, as it's the only way for the driver to flush all
 		 * existing sptes and stop the driver from establishing any more
 		 * sptes before all the pages in the mm are freed.
 		 */
 		if (mn->ops->release)
 			mn->ops->release(mn, mm);

 	spin_lock(&mm->mmu_notifier_mm->lock);
 	while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
 		mn = hlist_entry(mm->mmu_notifier_mm->list.first,
 				 struct mmu_notifier,
 				 hlist);
 		/*
 		 * We arrived before mmu_notifier_unregister so
 		 * mmu_notifier_unregister will do nothing other than to wait
 		 * for ->release to finish and for mmu_notifier_unregister to
 		 * return.
 		 */
 		hlist_del_init_rcu(&mn->hlist);
 	}
 	spin_unlock(&mm->mmu_notifier_mm->lock);
 	srcu_read_unlock(&srcu, id);

 	/*
 	 * synchronize_srcu here prevents mmu_notifier_release from returning to
 	 * exit_mmap (which would proceed with freeing all pages in the mm)
 	 * until the ->release method returns, if it was invoked by
 	 * mmu_notifier_unregister.
 	 *
 	 * The mmu_notifier_mm can't go away from under us because one mm_count
 	 * is held by exit_mmap.
 	 */
 	synchronize_srcu(&srcu);
 }

 /*
  * If no young bitflag is supported by the hardware, ->clear_flush_young can
  * unmap the address and return 1 or 0 depending if the mapping previously
  * existed or not.
  */
 int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
 					unsigned long start,
 					unsigned long end)
 {
 	struct mmu_notifier *mn;
 	int young = 0, id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->clear_flush_young)
 			young |= mn->ops->clear_flush_young(mn, mm, start, end);
 	}
 	srcu_read_unlock(&srcu, id);

 	return young;
 }

 int __mmu_notifier_clear_young(struct mm_struct *mm,
 			       unsigned long start,
 			       unsigned long end)
 {
 	struct mmu_notifier *mn;
 	int young = 0, id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->clear_young)
 			young |= mn->ops->clear_young(mn, mm, start, end);
 	}
 	srcu_read_unlock(&srcu, id);

 	return young;
 }

 int __mmu_notifier_test_young(struct mm_struct *mm,
 			      unsigned long address)
 {
 	struct mmu_notifier *mn;
 	int young = 0, id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->test_young) {
 			young = mn->ops->test_young(mn, mm, address);
 			if (young)
 				break;
 		}
 	}
 	srcu_read_unlock(&srcu, id);

 	return young;
 }

 void __mmu_notifier_change_pte(struct mm_struct *mm, unsigned long address,
 			       pte_t pte)
 {
 	struct mmu_notifier *mn;
 	int id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->change_pte)
 			mn->ops->change_pte(mn, mm, address, pte);
 	}
 	srcu_read_unlock(&srcu, id);
 }

 int __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
 				  unsigned long start, unsigned long end,
 				  bool blockable)
 {
 	struct mmu_notifier *mn;
 	int ret = 0;
 	int id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->invalidate_range_start) {
 			int _ret = mn->ops->invalidate_range_start(mn, mm, start, end, blockable);
 			if (_ret) {
 				pr_info("%pS callback failed with %d in %sblockable context.\n",
 						mn->ops->invalidate_range_start, _ret,
 						!blockable ? "non-" : "");
 				ret = _ret;
 			}
 		}
 	}
 	srcu_read_unlock(&srcu, id);

 	return ret;
 }
 EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_start);

 void __mmu_notifier_invalidate_range_end(struct mm_struct *mm,
 					 unsigned long start,
 					 unsigned long end,
 					 bool only_end)
 {
 	struct mmu_notifier *mn;
 	int id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		/*
 		 * Call invalidate_range here too to avoid the need for the
 		 * subsystem of having to register an invalidate_range_end
 		 * call-back when there is invalidate_range already. Usually a
 		 * subsystem registers either invalidate_range_start()/end() or
 		 * invalidate_range(), so this will be no additional overhead
 		 * (besides the pointer check).
 		 *
 		 * We skip call to invalidate_range() if we know it is safe ie
 		 * call site use mmu_notifier_invalidate_range_only_end() which
 		 * is safe to do when we know that a call to invalidate_range()
 		 * already happen under page table lock.
 		 */
 		if (!only_end && mn->ops->invalidate_range)
 			mn->ops->invalidate_range(mn, mm, start, end);
 		if (mn->ops->invalidate_range_end)
 			mn->ops->invalidate_range_end(mn, mm, start, end);
 	}
 	srcu_read_unlock(&srcu, id);
 }
 EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_end);

 void __mmu_notifier_invalidate_range(struct mm_struct *mm,
 				  unsigned long start, unsigned long end)
 {
 	struct mmu_notifier *mn;
 	int id;

 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->invalidate_range)
 			mn->ops->invalidate_range(mn, mm, start, end);
 	}
 	srcu_read_unlock(&srcu, id);
 }
 EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range);

 static int do_mmu_notifier_register(struct mmu_notifier *mn,
 				    struct mm_struct *mm,
 				    int take_mmap_sem)
 {
 	struct mmu_notifier_mm *mmu_notifier_mm;
 	int ret;

 	BUG_ON(atomic_read(&mm->mm_users) <= 0);

 	ret = -ENOMEM;
 	mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
 	if (unlikely(!mmu_notifier_mm))
 		goto out;

 	if (take_mmap_sem)
 		down_write(&mm->mmap_sem);
 	ret = mm_take_all_locks(mm);
 	if (unlikely(ret))
 		goto out_clean;

 	if (!mm_has_notifiers(mm)) {
 		INIT_HLIST_HEAD(&mmu_notifier_mm->list);
 		spin_lock_init(&mmu_notifier_mm->lock);

 		mm->mmu_notifier_mm = mmu_notifier_mm;
 		mmu_notifier_mm = NULL;
 	}
 	mmgrab(mm);

 	/*
 	 * Serialize the update against mmu_notifier_unregister. A
 	 * side note: mmu_notifier_release can't run concurrently with
 	 * us because we hold the mm_users pin (either implicitly as
 	 * current->mm or explicitly with get_task_mm() or similar).
 	 * We can't race against any other mmu notifier method either
 	 * thanks to mm_take_all_locks().
 	 */
 	spin_lock(&mm->mmu_notifier_mm->lock);
 	hlist_add_head(&mn->hlist, &mm->mmu_notifier_mm->list);
 	spin_unlock(&mm->mmu_notifier_mm->lock);

 	mm_drop_all_locks(mm);
 out_clean:
 	if (take_mmap_sem)
 		up_write(&mm->mmap_sem);
 	kfree(mmu_notifier_mm);
 out:
 	BUG_ON(atomic_read(&mm->mm_users) <= 0);
 	return ret;
 }

 /*
  * Must not hold mmap_sem nor any other VM related lock when calling
  * this registration function. Must also ensure mm_users can't go down
  * to zero while this runs to avoid races with mmu_notifier_release,
  * so mm has to be current->mm or the mm should be pinned safely such
  * as with get_task_mm(). If the mm is not current->mm, the mm_users
  * pin should be released by calling mmput after mmu_notifier_register
  * returns. mmu_notifier_unregister must be always called to
  * unregister the notifier. mm_count is automatically pinned to allow
  * mmu_notifier_unregister to safely run at any time later, before or
  * after exit_mmap. ->release will always be called before exit_mmap
  * frees the pages.
  */
 int mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
 {
 	return do_mmu_notifier_register(mn, mm, 1);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_register);

 /*
  * Same as mmu_notifier_register but here the caller must hold the
  * mmap_sem in write mode.
  */
 int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
 {
 	return do_mmu_notifier_register(mn, mm, 0);
 }
 EXPORT_SYMBOL_GPL(__mmu_notifier_register);

 /* this is called after the last mmu_notifier_unregister() returned */
 void __mmu_notifier_mm_destroy(struct mm_struct *mm)
 {
 	BUG_ON(!hlist_empty(&mm->mmu_notifier_mm->list));
 	kfree(mm->mmu_notifier_mm);
 	mm->mmu_notifier_mm = LIST_POISON1; /* debug */
 }

 /*
  * This releases the mm_count pin automatically and frees the mm
  * structure if it was the last user of it. It serializes against
  * running mmu notifiers with SRCU and against mmu_notifier_unregister
  * with the unregister lock + SRCU. All sptes must be dropped before
  * calling mmu_notifier_unregister. ->release or any other notifier
  * method may be invoked concurrently with mmu_notifier_unregister,
  * and only after mmu_notifier_unregister returned we're guaranteed
  * that ->release or any other method can't run anymore.
  */
 void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm)
 {
 	BUG_ON(atomic_read(&mm->mm_count) <= 0);

 	if (!hlist_unhashed(&mn->hlist)) {
 		/*
 		 * SRCU here will force exit_mmap to wait for ->release to
 		 * finish before freeing the pages.
 		 */
 		int id;

 		id = srcu_read_lock(&srcu);
 		/*
 		 * exit_mmap will block in mmu_notifier_release to guarantee
 		 * that ->release is called before freeing the pages.
 		 */
 		if (mn->ops->release)
 			mn->ops->release(mn, mm);
 		srcu_read_unlock(&srcu, id);

 		spin_lock(&mm->mmu_notifier_mm->lock);
 		/*
 		 * Can not use list_del_rcu() since __mmu_notifier_release
 		 * can delete it before we hold the lock.
 		 */
 		hlist_del_init_rcu(&mn->hlist);
 		spin_unlock(&mm->mmu_notifier_mm->lock);
 	}

 	/*
 	 * Wait for any running method to finish, of course including
 	 * ->release if it was run by mmu_notifier_release instead of us.
 	 */
 	synchronize_srcu(&srcu);

 	BUG_ON(atomic_read(&mm->mm_count) <= 0);

 	mmdrop(mm);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_unregister);

 /*
  * Same as mmu_notifier_unregister but no callback and no srcu synchronization.
  */
 void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
 					struct mm_struct *mm)
 {
 	spin_lock(&mm->mmu_notifier_mm->lock);
 	/*
 	 * Can not use list_del_rcu() since __mmu_notifier_release
 	 * can delete it before we hold the lock.
 	 */
 	hlist_del_init_rcu(&mn->hlist);
 	spin_unlock(&mm->mmu_notifier_mm->lock);

 	BUG_ON(atomic_read(&mm->mm_count) <= 0);
 	mmdrop(mm);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release);
	/*
	* linux/mm/mmu_notifier.c
	*
	* Copyright (C) 2008 Qumranet, Inc.
	* Copyright (C) 2008 SGI
	* Christoph Lameter <cl@linux.com>
	*
	* This work is licensed under the terms of the GNU GPL, version 2. See
	* the COPYING file in the top-level directory.
	*/

	#include <linux/rculist.h>
	#include <linux/mmu_notifier.h>
	#include <linux/export.h>
	#include <linux/mm.h>
	#include <linux/err.h>
	#include <linux/srcu.h>
	#include <linux/rcupdate.h>
	#include <linux/sched.h>
	#include <linux/sched/mm.h>
	#include <linux/slab.h>

	/* global SRCU for all MMs */
	DEFINE_STATIC_SRCU(srcu);

	/*
	* This function allows mmu_notifier::release callback to delay a call to
	* a function that will free appropriate resources. The function must be
	* quick and must not block.
	*/
	void mmu_notifier_call_srcu(struct rcu_head *rcu,
	void (func)(struct rcu_head rcu))
	{
	call_srcu(&srcu, rcu, func);
	}
	EXPORT_SYMBOL_GPL(mmu_notifier_call_srcu);

	void mmu_notifier_synchronize(void)
	{
	/* Wait for any running method to finish. */
	srcu_barrier(&srcu);
	}
	EXPORT_SYMBOL_GPL(mmu_notifier_synchronize);

	/*
	* This function can't run concurrently against mmu_notifier_register
	* because mm->mm_users > 0 during mmu_notifier_register and exit_mmap
	* runs with mm_users == 0. Other tasks may still invoke mmu notifiers
	* in parallel despite there being no task using this mm any more,
	* through the vmas outside of the exit_mmap context, such as with
	* vmtruncate. This serializes against mmu_notifier_unregister with
	* the mmu_notifier_mm->lock in addition to SRCU and it serializes
	* against the other mmu notifiers with SRCU. struct mmu_notifier_mm
	* can't go away from under us as exit_mmap holds an mm_count pin
	* itself.
	*/
	void __mmu_notifier_release(struct mm_struct *mm)
	{
	struct mmu_notifier *mn;
	int id;

	/*
	* SRCU here will block mmu_notifier_unregister until
	* ->release returns.
	*/
	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist)
	/*
	* If ->release runs before mmu_notifier_unregister it must be
	* handled, as it's the only way for the driver to flush all
	* existing sptes and stop the driver from establishing any more
	* sptes before all the pages in the mm are freed.
	*/
	if (mn->ops->release)
	mn->ops->release(mn, mm);

	spin_lock(&mm->mmu_notifier_mm->lock);
	while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
	mn = hlist_entry(mm->mmu_notifier_mm->list.first,
	struct mmu_notifier,
	hlist);
	/*
	* We arrived before mmu_notifier_unregister so
	* mmu_notifier_unregister will do nothing other than to wait
	* for ->release to finish and for mmu_notifier_unregister to
	* return.
	*/
	hlist_del_init_rcu(&mn->hlist);
	}
	spin_unlock(&mm->mmu_notifier_mm->lock);
	srcu_read_unlock(&srcu, id);

	/*
	* synchronize_srcu here prevents mmu_notifier_release from returning to
	* exit_mmap (which would proceed with freeing all pages in the mm)
	* until the ->release method returns, if it was invoked by
	* mmu_notifier_unregister.
	*
	* The mmu_notifier_mm can't go away from under us because one mm_count
	* is held by exit_mmap.
	*/
	synchronize_srcu(&srcu);
	}

	/*
	* If no young bitflag is supported by the hardware, ->clear_flush_young can
	* unmap the address and return 1 or 0 depending if the mapping previously
	* existed or not.
	*/
	int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
	unsigned long start,
	unsigned long end)
	{
	struct mmu_notifier *mn;
	int young = 0, id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	if (mn->ops->clear_flush_young)
	young \|= mn->ops->clear_flush_young(mn, mm, start, end);
	}
	srcu_read_unlock(&srcu, id);

	return young;
	}

	int __mmu_notifier_clear_young(struct mm_struct *mm,
	unsigned long start,
	unsigned long end)
	{
	struct mmu_notifier *mn;
	int young = 0, id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	if (mn->ops->clear_young)
	young \|= mn->ops->clear_young(mn, mm, start, end);
	}
	srcu_read_unlock(&srcu, id);

	return young;
	}

	int __mmu_notifier_test_young(struct mm_struct *mm,
	unsigned long address)
	{
	struct mmu_notifier *mn;
	int young = 0, id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	if (mn->ops->test_young) {
	young = mn->ops->test_young(mn, mm, address);
	if (young)
	break;
	}
	}
	srcu_read_unlock(&srcu, id);

	return young;
	}

	void __mmu_notifier_change_pte(struct mm_struct *mm, unsigned long address,
	pte_t pte)
	{
	struct mmu_notifier *mn;
	int id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	if (mn->ops->change_pte)
	mn->ops->change_pte(mn, mm, address, pte);
	}
	srcu_read_unlock(&srcu, id);
	}

	int __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
	unsigned long start, unsigned long end,
	bool blockable)
	{
	struct mmu_notifier *mn;
	int ret = 0;
	int id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	if (mn->ops->invalidate_range_start) {
	int _ret = mn->ops->invalidate_range_start(mn, mm, start, end, blockable);
	if (_ret) {
	pr_info("%pS callback failed with %d in %sblockable context.\n",
	mn->ops->invalidate_range_start, _ret,
	!blockable ? "non-" : "");
	ret = _ret;
	}
	}
	}
	srcu_read_unlock(&srcu, id);

	return ret;
	}
	EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_start);

	void __mmu_notifier_invalidate_range_end(struct mm_struct *mm,
	unsigned long start,
	unsigned long end,
	bool only_end)
	{
	struct mmu_notifier *mn;
	int id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	/*
	* Call invalidate_range here too to avoid the need for the
	* subsystem of having to register an invalidate_range_end
	* call-back when there is invalidate_range already. Usually a
	* subsystem registers either invalidate_range_start()/end() or
	* invalidate_range(), so this will be no additional overhead
	* (besides the pointer check).
	*
	* We skip call to invalidate_range() if we know it is safe ie
	* call site use mmu_notifier_invalidate_range_only_end() which
	* is safe to do when we know that a call to invalidate_range()
	* already happen under page table lock.
	*/
	if (!only_end && mn->ops->invalidate_range)
	mn->ops->invalidate_range(mn, mm, start, end);
	if (mn->ops->invalidate_range_end)
	mn->ops->invalidate_range_end(mn, mm, start, end);
	}
	srcu_read_unlock(&srcu, id);
	}
	EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_end);

	void __mmu_notifier_invalidate_range(struct mm_struct *mm,
	unsigned long start, unsigned long end)
	{
	struct mmu_notifier *mn;
	int id;

	id = srcu_read_lock(&srcu);
	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
	if (mn->ops->invalidate_range)
	mn->ops->invalidate_range(mn, mm, start, end);
	}
	srcu_read_unlock(&srcu, id);
	}
	EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range);

	static int do_mmu_notifier_register(struct mmu_notifier *mn,
	struct mm_struct *mm,
	int take_mmap_sem)
	{
	struct mmu_notifier_mm *mmu_notifier_mm;
	int ret;

	BUG_ON(atomic_read(&mm->mm_users) <= 0);

	ret = -ENOMEM;
	mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
	if (unlikely(!mmu_notifier_mm))
	goto out;

	if (take_mmap_sem)
	down_write(&mm->mmap_sem);
	ret = mm_take_all_locks(mm);
	if (unlikely(ret))
	goto out_clean;

	if (!mm_has_notifiers(mm)) {
	INIT_HLIST_HEAD(&mmu_notifier_mm->list);
	spin_lock_init(&mmu_notifier_mm->lock);

	mm->mmu_notifier_mm = mmu_notifier_mm;
	mmu_notifier_mm = NULL;
	}
	mmgrab(mm);

	/*
	* Serialize the update against mmu_notifier_unregister. A
	* side note: mmu_notifier_release can't run concurrently with
	* us because we hold the mm_users pin (either implicitly as
	* current->mm or explicitly with get_task_mm() or similar).
	* We can't race against any other mmu notifier method either
	* thanks to mm_take_all_locks().
	*/
	spin_lock(&mm->mmu_notifier_mm->lock);
	hlist_add_head(&mn->hlist, &mm->mmu_notifier_mm->list);
	spin_unlock(&mm->mmu_notifier_mm->lock);

	mm_drop_all_locks(mm);
	out_clean:
	if (take_mmap_sem)
	up_write(&mm->mmap_sem);
	kfree(mmu_notifier_mm);
	out:
	BUG_ON(atomic_read(&mm->mm_users) <= 0);
	return ret;
	}

	/*
	* Must not hold mmap_sem nor any other VM related lock when calling
	* this registration function. Must also ensure mm_users can't go down
	* to zero while this runs to avoid races with mmu_notifier_release,
	* so mm has to be current->mm or the mm should be pinned safely such
	* as with get_task_mm(). If the mm is not current->mm, the mm_users
	* pin should be released by calling mmput after mmu_notifier_register
	* returns. mmu_notifier_unregister must be always called to
	* unregister the notifier. mm_count is automatically pinned to allow
	* mmu_notifier_unregister to safely run at any time later, before or
	* after exit_mmap. ->release will always be called before exit_mmap
	* frees the pages.
	*/
	int mmu_notifier_register(struct mmu_notifier mn, struct mm_struct mm)
	{
	return do_mmu_notifier_register(mn, mm, 1);
	}
	EXPORT_SYMBOL_GPL(mmu_notifier_register);

	/*
	* Same as mmu_notifier_register but here the caller must hold the
	* mmap_sem in write mode.
	*/
	int __mmu_notifier_register(struct mmu_notifier mn, struct mm_struct mm)
	{
	return do_mmu_notifier_register(mn, mm, 0);
	}
	EXPORT_SYMBOL_GPL(__mmu_notifier_register);

	/* this is called after the last mmu_notifier_unregister() returned */
	void __mmu_notifier_mm_destroy(struct mm_struct *mm)
	{
	BUG_ON(!hlist_empty(&mm->mmu_notifier_mm->list));
	kfree(mm->mmu_notifier_mm);
	mm->mmu_notifier_mm = LIST_POISON1; /* debug */
	}

	/*
	* This releases the mm_count pin automatically and frees the mm
	* structure if it was the last user of it. It serializes against
	* running mmu notifiers with SRCU and against mmu_notifier_unregister
	* with the unregister lock + SRCU. All sptes must be dropped before
	* calling mmu_notifier_unregister. ->release or any other notifier
	* method may be invoked concurrently with mmu_notifier_unregister,
	* and only after mmu_notifier_unregister returned we're guaranteed
	* that ->release or any other method can't run anymore.
	*/
	void mmu_notifier_unregister(struct mmu_notifier mn, struct mm_struct mm)
	{
	BUG_ON(atomic_read(&mm->mm_count) <= 0);

	if (!hlist_unhashed(&mn->hlist)) {
	/*
	* SRCU here will force exit_mmap to wait for ->release to
	* finish before freeing the pages.
	*/
	int id;

	id = srcu_read_lock(&srcu);
	/*
	* exit_mmap will block in mmu_notifier_release to guarantee
	* that ->release is called before freeing the pages.
	*/
	if (mn->ops->release)
	mn->ops->release(mn, mm);
	srcu_read_unlock(&srcu, id);

	spin_lock(&mm->mmu_notifier_mm->lock);
	/*
	* Can not use list_del_rcu() since __mmu_notifier_release
	* can delete it before we hold the lock.
	*/
	hlist_del_init_rcu(&mn->hlist);
	spin_unlock(&mm->mmu_notifier_mm->lock);
	}

	/*
	* Wait for any running method to finish, of course including
	* ->release if it was run by mmu_notifier_release instead of us.
	*/
	synchronize_srcu(&srcu);

	BUG_ON(atomic_read(&mm->mm_count) <= 0);

	mmdrop(mm);
	}
	EXPORT_SYMBOL_GPL(mmu_notifier_unregister);

	/*
	* Same as mmu_notifier_unregister but no callback and no srcu synchronization.
	*/
	void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
	struct mm_struct *mm)
	{
	spin_lock(&mm->mmu_notifier_mm->lock);
	/*
	* Can not use list_del_rcu() since __mmu_notifier_release
	* can delete it before we hold the lock.
	*/
	hlist_del_init_rcu(&mn->hlist);
	spin_unlock(&mm->mmu_notifier_mm->lock);

	BUG_ON(atomic_read(&mm->mm_count) <= 0);
	mmdrop(mm);
	}
	EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release);