/*
   *  linux/mm/mmu_notifier.c
   *
   *  Copyright (C) 2008  Qumranet, Inc.
   *  Copyright (C) 2008  SGI
   *             Christoph Lameter <clameter@sgi.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/slab.h>
  
  /* global SRCU for all MMs */
  static struct srcu_struct srcu;
  
  /*
   * 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);
  	srcu_read_unlock(&srcu, id);
  
  	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);
  
  	/*
  	 * 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 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->clear_flush_young)
  			young |= mn->ops->clear_flush_young(mn, mm, address);
  	}
  	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);
  }
  
  void __mmu_notifier_invalidate_page(struct mm_struct *mm,
  					  unsigned long address)
  {
  	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_page)
  			mn->ops->invalidate_page(mn, mm, address);
  	}
  	srcu_read_unlock(&srcu, id);
  }
  
  void __mmu_notifier_invalidate_range_start(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_start)
  			mn->ops->invalidate_range_start(mn, mm, start, end);
  	}
  	srcu_read_unlock(&srcu, id);
  }
  EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_start);
  
  void __mmu_notifier_invalidate_range_end(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_end)
  			mn->ops->invalidate_range_end(mn, mm, start, end);
  	}
  	srcu_read_unlock(&srcu, id);
  }
  EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_end);
  
  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);
  
  	/*
  	 * Verify that mmu_notifier_init() already run and the global srcu is
  	 * initialized.
  	 */
  	BUG_ON(!srcu.per_cpu_ref);
  
  	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;
  	}
  	atomic_inc(&mm->mm_count);
  
  	/*
  	 * 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);
  
  static int __init mmu_notifier_init(void)
  {
  	return init_srcu_struct(&srcu);
  }
  subsys_initcall(mmu_notifier_init);