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 // SPDX-License-Identifier: GPL-2.0-only
 /*
  *  linux/mm/mmu_notifier.c
  *
  *  Copyright (C) 2008  Qumranet, Inc.
  *  Copyright (C) 2008  SGI
  *             Christoph Lameter <cl@linux.com>
  */
 
 #include <linux/rculist.h>
 #include <linux/mmu_notifier.h>
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/err.h>
 #include <linux/interval_tree.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);
 
 #ifdef CONFIG_LOCKDEP
 struct lockdep_map __mmu_notifier_invalidate_range_start_map = {
     .name = "mmu_notifier_invalidate_range_start"
 };
 #endif
 
 /*
  * The mmu_notifier_subscriptions structure is allocated and installed in
  * mm->notifier_subscriptions inside the mm_take_all_locks() protected
  * critical section and it's released only when mm_count reaches zero
  * in mmdrop().
  */
 struct mmu_notifier_subscriptions {
     /* all mmu notifiers registered in this mm are queued in this list */
     struct hlist_head list;
     bool has_itree;
     /* to serialize the list modifications and hlist_unhashed */
     spinlock_t lock;
     unsigned long invalidate_seq;
     unsigned long active_invalidate_ranges;
     struct rb_root_cached itree;
     wait_queue_head_t wq;
     struct hlist_head deferred_list;
 };
 
 /*
  * This is a collision-retry read-side/write-side 'lock', a lot like a
  * seqcount, however this allows multiple write-sides to hold it at
  * once. Conceptually the write side is protecting the values of the PTEs in
  * this mm, such that PTES cannot be read into SPTEs (shadow PTEs) while any
  * writer exists.
  *
  * Note that the core mm creates nested invalidate_range_start()/end() regions
  * within the same thread, and runs invalidate_range_start()/end() in parallel
  * on multiple CPUs. This is designed to not reduce concurrency or block
  * progress on the mm side.
  *
  * As a secondary function, holding the full write side also serves to prevent
  * writers for the itree, this is an optimization to avoid extra locking
  * during invalidate_range_start/end notifiers.
  *
  * The write side has two states, fully excluded:
  *  - mm->active_invalidate_ranges != 0
  *  - subscriptions->invalidate_seq & 1 == True (odd)
  *  - some range on the mm_struct is being invalidated
  *  - the itree is not allowed to change
  *
  * And partially excluded:
  *  - mm->active_invalidate_ranges != 0
  *  - subscriptions->invalidate_seq & 1 == False (even)
  *  - some range on the mm_struct is being invalidated
  *  - the itree is allowed to change
  *
  * Operations on notifier_subscriptions->invalidate_seq (under spinlock):
  *    seq |= 1  # Begin writing
  *    seq++     # Release the writing state
  *    seq & 1   # True if a writer exists
  *
  * The later state avoids some expensive work on inv_end in the common case of
  * no mmu_interval_notifier monitoring the VA.
  */
 static bool
 mn_itree_is_invalidating(struct mmu_notifier_subscriptions *subscriptions)
 {
     lockdep_assert_held(&subscriptions->lock);
     return subscriptions->invalidate_seq & 1;
 }
 
 static struct mmu_interval_notifier *
 mn_itree_inv_start_range(struct mmu_notifier_subscriptions *subscriptions,
              const struct mmu_notifier_range *range,
              unsigned long *seq)
 {
     struct interval_tree_node *node;
     struct mmu_interval_notifier *res = NULL;
 
     spin_lock(&subscriptions->lock);
     subscriptions->active_invalidate_ranges++;
     node = interval_tree_iter_first(&subscriptions->itree, range->start,
                     range->end - 1);
     if (node) {
         subscriptions->invalidate_seq |= 1;
         res = container_of(node, struct mmu_interval_notifier,
                    interval_tree);
     }
 
     *seq = subscriptions->invalidate_seq;
     spin_unlock(&subscriptions->lock);
     return res;
 }
 
 static struct mmu_interval_notifier *
 mn_itree_inv_next(struct mmu_interval_notifier *interval_sub,
           const struct mmu_notifier_range *range)
 {
     struct interval_tree_node *node;
 
     node = interval_tree_iter_next(&interval_sub->interval_tree,
                        range->start, range->end - 1);
     if (!node)
         return NULL;
     return container_of(node, struct mmu_interval_notifier, interval_tree);
 }
 
 static void mn_itree_inv_end(struct mmu_notifier_subscriptions *subscriptions)
 {
     struct mmu_interval_notifier *interval_sub;
     struct hlist_node *next;
 
     spin_lock(&subscriptions->lock);
     if (--subscriptions->active_invalidate_ranges ||
         !mn_itree_is_invalidating(subscriptions)) {
         spin_unlock(&subscriptions->lock);
         return;
     }
 
     /* Make invalidate_seq even */
     subscriptions->invalidate_seq++;
 
     /*
      * The inv_end incorporates a deferred mechanism like rtnl_unlock().
      * Adds and removes are queued until the final inv_end happens then
      * they are progressed. This arrangement for tree updates is used to
      * avoid using a blocking lock during invalidate_range_start.
      */
     hlist_for_each_entry_safe(interval_sub, next,
                   &subscriptions->deferred_list,
                   deferred_item) {
         if (RB_EMPTY_NODE(&interval_sub->interval_tree.rb))
             interval_tree_insert(&interval_sub->interval_tree,
                          &subscriptions->itree);
         else
             interval_tree_remove(&interval_sub->interval_tree,
                          &subscriptions->itree);
         hlist_del(&interval_sub->deferred_item);
     }
     spin_unlock(&subscriptions->lock);
 
     wake_up_all(&subscriptions->wq);
 }
 
 /**
  * mmu_interval_read_begin - Begin a read side critical section against a VA
  *                           range
  * @interval_sub: The interval subscription
  *
  * mmu_iterval_read_begin()/mmu_iterval_read_retry() implement a
  * collision-retry scheme similar to seqcount for the VA range under
  * subscription. If the mm invokes invalidation during the critical section
  * then mmu_interval_read_retry() will return true.
  *
  * This is useful to obtain shadow PTEs where teardown or setup of the SPTEs
  * require a blocking context.  The critical region formed by this can sleep,
  * and the required 'user_lock' can also be a sleeping lock.
  *
  * The caller is required to provide a 'user_lock' to serialize both teardown
  * and setup.
  *
  * The return value should be passed to mmu_interval_read_retry().
  */
 unsigned long
 mmu_interval_read_begin(struct mmu_interval_notifier *interval_sub)
 {
     struct mmu_notifier_subscriptions *subscriptions =
         interval_sub->mm->notifier_subscriptions;
     unsigned long seq;
     bool is_invalidating;
 
     /*
      * If the subscription has a different seq value under the user_lock
      * than we started with then it has collided.
      *
      * If the subscription currently has the same seq value as the
      * subscriptions seq, then it is currently between
      * invalidate_start/end and is colliding.
      *
      * The locking looks broadly like this:
      *   mn_tree_invalidate_start():          mmu_interval_read_begin():
      *                                         spin_lock
      *                                          seq = READ_ONCE(interval_sub->invalidate_seq);
      *                                          seq == subs->invalidate_seq
      *                                         spin_unlock
      *    spin_lock
      *     seq = ++subscriptions->invalidate_seq
      *    spin_unlock
      *     op->invalidate_range():
      *       user_lock
      *        mmu_interval_set_seq()
      *         interval_sub->invalidate_seq = seq
      *       user_unlock
      *
      *                          [Required: mmu_interval_read_retry() == true]
      *
      *   mn_itree_inv_end():
      *    spin_lock
      *     seq = ++subscriptions->invalidate_seq
      *    spin_unlock
      *
      *                                        user_lock
      *                                         mmu_interval_read_retry():
      *                                          interval_sub->invalidate_seq != seq
      *                                        user_unlock
      *
      * Barriers are not needed here as any races here are closed by an
      * eventual mmu_interval_read_retry(), which provides a barrier via the
      * user_lock.
      */
     spin_lock(&subscriptions->lock);
     /* Pairs with the WRITE_ONCE in mmu_interval_set_seq() */
     seq = READ_ONCE(interval_sub->invalidate_seq);
     is_invalidating = seq == subscriptions->invalidate_seq;
     spin_unlock(&subscriptions->lock);
 
     /*
      * interval_sub->invalidate_seq must always be set to an odd value via
      * mmu_interval_set_seq() using the provided cur_seq from
      * mn_itree_inv_start_range(). This ensures that if seq does wrap we
      * will always clear the below sleep in some reasonable time as
      * subscriptions->invalidate_seq is even in the idle state.
      */
     lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
     lock_map_release(&__mmu_notifier_invalidate_range_start_map);
     if (is_invalidating)
         wait_event(subscriptions->wq,
                READ_ONCE(subscriptions->invalidate_seq) != seq);
 
     /*
      * Notice that mmu_interval_read_retry() can already be true at this
      * point, avoiding loops here allows the caller to provide a global
      * time bound.
      */
 
     return seq;
 }
 EXPORT_SYMBOL_GPL(mmu_interval_read_begin);
 
 static void mn_itree_release(struct mmu_notifier_subscriptions *subscriptions,
                  struct mm_struct *mm)
 {
     struct mmu_notifier_range range = {
         .flags = MMU_NOTIFIER_RANGE_BLOCKABLE,
         .event = MMU_NOTIFY_RELEASE,
         .mm = mm,
         .start = 0,
         .end = ULONG_MAX,
     };
     struct mmu_interval_notifier *interval_sub;
     unsigned long cur_seq;
     bool ret;
 
     for (interval_sub =
              mn_itree_inv_start_range(subscriptions, &range, &cur_seq);
          interval_sub;
          interval_sub = mn_itree_inv_next(interval_sub, &range)) {
         ret = interval_sub->ops->invalidate(interval_sub, &range,
                             cur_seq);
         WARN_ON(!ret);
     }
 
     mn_itree_inv_end(subscriptions);
 }
 
 /*
  * 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 notifier_subscriptions->lock in addition to SRCU and it serializes
  * against the other mmu notifiers with SRCU. struct mmu_notifier_subscriptions
  * can't go away from under us as exit_mmap holds an mm_count pin
  * itself.
  */
 static void mn_hlist_release(struct mmu_notifier_subscriptions *subscriptions,
                  struct mm_struct *mm)
 {
     struct mmu_notifier *subscription;
     int id;
 
     /*
      * SRCU here will block mmu_notifier_unregister until
      * ->release returns.
      */
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu))
         /*
          * 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 (subscription->ops->release)
             subscription->ops->release(subscription, mm);
 
     spin_lock(&subscriptions->lock);
     while (unlikely(!hlist_empty(&subscriptions->list))) {
         subscription = hlist_entry(subscriptions->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(&subscription->hlist);
     }
     spin_unlock(&subscriptions->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 notifier_subscriptions can't go away from under us because
      * one mm_count is held by exit_mmap.
      */
     synchronize_srcu(&srcu);
 }
 
 void __mmu_notifier_release(struct mm_struct *mm)
 {
     struct mmu_notifier_subscriptions *subscriptions =
         mm->notifier_subscriptions;
 
     if (subscriptions->has_itree)
         mn_itree_release(subscriptions, mm);
 
     if (!hlist_empty(&subscriptions->list))
         mn_hlist_release(subscriptions, mm);
 }
 
 /*
  * 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 *subscription;
     int young = 0, id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription,
                  &mm->notifier_subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         if (subscription->ops->clear_flush_young)
             young |= subscription->ops->clear_flush_young(
                 subscription, 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 *subscription;
     int young = 0, id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription,
                  &mm->notifier_subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         if (subscription->ops->clear_young)
             young |= subscription->ops->clear_young(subscription,
                                 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 *subscription;
     int young = 0, id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription,
                  &mm->notifier_subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         if (subscription->ops->test_young) {
             young = subscription->ops->test_young(subscription, 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 *subscription;
     int id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription,
                  &mm->notifier_subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         if (subscription->ops->change_pte)
             subscription->ops->change_pte(subscription, mm, address,
                               pte);
     }
     srcu_read_unlock(&srcu, id);
 }
 
 static int mn_itree_invalidate(struct mmu_notifier_subscriptions *subscriptions,
                    const struct mmu_notifier_range *range)
 {
     struct mmu_interval_notifier *interval_sub;
     unsigned long cur_seq;
 
     for (interval_sub =
              mn_itree_inv_start_range(subscriptions, range, &cur_seq);
          interval_sub;
          interval_sub = mn_itree_inv_next(interval_sub, range)) {
         bool ret;
 
         ret = interval_sub->ops->invalidate(interval_sub, range,
                             cur_seq);
         if (!ret) {
             if (WARN_ON(mmu_notifier_range_blockable(range)))
                 continue;
             goto out_would_block;
         }
     }
     return 0;
 
 out_would_block:
     /*
      * On -EAGAIN the non-blocking caller is not allowed to call
      * invalidate_range_end()
      */
     mn_itree_inv_end(subscriptions);
     return -EAGAIN;
 }
 
 static int mn_hlist_invalidate_range_start(
     struct mmu_notifier_subscriptions *subscriptions,
     struct mmu_notifier_range *range)
 {
     struct mmu_notifier *subscription;
     int ret = 0;
     int id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         const struct mmu_notifier_ops *ops = subscription->ops;
 
         if (ops->invalidate_range_start) {
             int _ret;
 
             if (!mmu_notifier_range_blockable(range))
                 non_block_start();
             _ret = ops->invalidate_range_start(subscription, range);
             if (!mmu_notifier_range_blockable(range))
                 non_block_end();
             if (_ret) {
                 pr_info("%pS callback failed with %d in %sblockable context.\n",
                     ops->invalidate_range_start, _ret,
                     !mmu_notifier_range_blockable(range) ?
                         "non-" :
                         "");
                 WARN_ON(mmu_notifier_range_blockable(range) ||
                     _ret != -EAGAIN);
                 /*
                  * We call all the notifiers on any EAGAIN,
                  * there is no way for a notifier to know if
                  * its start method failed, thus a start that
                  * does EAGAIN can't also do end.
                  */
                 WARN_ON(ops->invalidate_range_end);
                 ret = _ret;
             }
         }
     }
 
     if (ret) {
         /*
          * Must be non-blocking to get here.  If there are multiple
          * notifiers and one or more failed start, any that succeeded
          * start are expecting their end to be called.  Do so now.
          */
         hlist_for_each_entry_rcu(subscription, &subscriptions->list,
                      hlist, srcu_read_lock_held(&srcu)) {
             if (!subscription->ops->invalidate_range_end)
                 continue;
 
             subscription->ops->invalidate_range_end(subscription,
                                 range);
         }
     }
     srcu_read_unlock(&srcu, id);
 
     return ret;
 }
 
 int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
 {
     struct mmu_notifier_subscriptions *subscriptions =
         range->mm->notifier_subscriptions;
     int ret;
 
     if (subscriptions->has_itree) {
         ret = mn_itree_invalidate(subscriptions, range);
         if (ret)
             return ret;
     }
     if (!hlist_empty(&subscriptions->list))
         return mn_hlist_invalidate_range_start(subscriptions, range);
     return 0;
 }
 
 static void
 mn_hlist_invalidate_end(struct mmu_notifier_subscriptions *subscriptions,
             struct mmu_notifier_range *range, bool only_end)
 {
     struct mmu_notifier *subscription;
     int id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         /*
          * 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 && subscription->ops->invalidate_range)
             subscription->ops->invalidate_range(subscription,
                                 range->mm,
                                 range->start,
                                 range->end);
         if (subscription->ops->invalidate_range_end) {
             if (!mmu_notifier_range_blockable(range))
                 non_block_start();
             subscription->ops->invalidate_range_end(subscription,
                                 range);
             if (!mmu_notifier_range_blockable(range))
                 non_block_end();
         }
     }
     srcu_read_unlock(&srcu, id);
 }
 
 void __mmu_notifier_invalidate_range_end(struct mmu_notifier_range *range,
                      bool only_end)
 {
     struct mmu_notifier_subscriptions *subscriptions =
         range->mm->notifier_subscriptions;
 
     lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
     if (subscriptions->has_itree)
         mn_itree_inv_end(subscriptions);
 
     if (!hlist_empty(&subscriptions->list))
         mn_hlist_invalidate_end(subscriptions, range, only_end);
     lock_map_release(&__mmu_notifier_invalidate_range_start_map);
 }
 
 void __mmu_notifier_invalidate_range(struct mm_struct *mm,
                   unsigned long start, unsigned long end)
 {
     struct mmu_notifier *subscription;
     int id;
 
     id = srcu_read_lock(&srcu);
     hlist_for_each_entry_rcu(subscription,
                  &mm->notifier_subscriptions->list, hlist,
                  srcu_read_lock_held(&srcu)) {
         if (subscription->ops->invalidate_range)
             subscription->ops->invalidate_range(subscription, mm,
                                 start, end);
     }
     srcu_read_unlock(&srcu, id);
 }
 
 /*
  * Same as mmu_notifier_register but here the caller must hold the mmap_lock in
  * write mode. A NULL mn signals the notifier is being registered for itree
  * mode.
  */
 int __mmu_notifier_register(struct mmu_notifier *subscription,
                 struct mm_struct *mm)
 {
     struct mmu_notifier_subscriptions *subscriptions = NULL;
     int ret;
 
     mmap_assert_write_locked(mm);
     BUG_ON(atomic_read(&mm->mm_users) <= 0);
 
     if (!mm->notifier_subscriptions) {
         /*
          * kmalloc cannot be called under mm_take_all_locks(), but we
          * know that mm->notifier_subscriptions can't change while we
          * hold the write side of the mmap_lock.
          */
         subscriptions = kzalloc(
             sizeof(struct mmu_notifier_subscriptions), GFP_KERNEL);
         if (!subscriptions)
             return -ENOMEM;
 
         INIT_HLIST_HEAD(&subscriptions->list);
         spin_lock_init(&subscriptions->lock);
         subscriptions->invalidate_seq = 2;
         subscriptions->itree = RB_ROOT_CACHED;
         init_waitqueue_head(&subscriptions->wq);
         INIT_HLIST_HEAD(&subscriptions->deferred_list);
     }
 
     ret = mm_take_all_locks(mm);
     if (unlikely(ret))
         goto out_clean;
 
     /*
      * 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().
      *
      * release semantics on the initialization of the
      * mmu_notifier_subscriptions's contents are provided for unlocked
      * readers.  acquire can only be used while holding the mmgrab or
      * mmget, and is safe because once created the
      * mmu_notifier_subscriptions is not freed until the mm is destroyed.
      * As above, users holding the mmap_lock or one of the
      * mm_take_all_locks() do not need to use acquire semantics.
      */
     if (subscriptions)
         smp_store_release(&mm->notifier_subscriptions, subscriptions);
 
     if (subscription) {
         /* Pairs with the mmdrop in mmu_notifier_unregister_* */
         mmgrab(mm);
         subscription->mm = mm;
         subscription->users = 1;
 
         spin_lock(&mm->notifier_subscriptions->lock);
         hlist_add_head_rcu(&subscription->hlist,
                    &mm->notifier_subscriptions->list);
         spin_unlock(&mm->notifier_subscriptions->lock);
     } else
         mm->notifier_subscriptions->has_itree = true;
 
     mm_drop_all_locks(mm);
     BUG_ON(atomic_read(&mm->mm_users) <= 0);
     return 0;
 
 out_clean:
     kfree(subscriptions);
     return ret;
 }
 EXPORT_SYMBOL_GPL(__mmu_notifier_register);
 
 /**
  * mmu_notifier_register - Register a notifier on a mm
  * @subscription: The notifier to attach
  * @mm: The mm to attach the notifier to
  *
  * Must not hold mmap_lock 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() or mmu_notifier_put() must be always called to
  * unregister the notifier.
  *
  * While the caller has a mmu_notifier get the subscription->mm pointer will remain
  * valid, and can be converted to an active mm pointer via mmget_not_zero().
  */
 int mmu_notifier_register(struct mmu_notifier *subscription,
               struct mm_struct *mm)
 {
     int ret;
 
     mmap_write_lock(mm);
     ret = __mmu_notifier_register(subscription, mm);
     mmap_write_unlock(mm);
     return ret;
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_register);
 
 static struct mmu_notifier *
 find_get_mmu_notifier(struct mm_struct *mm, const struct mmu_notifier_ops *ops)
 {
     struct mmu_notifier *subscription;
 
     spin_lock(&mm->notifier_subscriptions->lock);
     hlist_for_each_entry_rcu(subscription,
                  &mm->notifier_subscriptions->list, hlist,
                  lockdep_is_held(&mm->notifier_subscriptions->lock)) {
         if (subscription->ops != ops)
             continue;
 
         if (likely(subscription->users != UINT_MAX))
             subscription->users++;
         else
             subscription = ERR_PTR(-EOVERFLOW);
         spin_unlock(&mm->notifier_subscriptions->lock);
         return subscription;
     }
     spin_unlock(&mm->notifier_subscriptions->lock);
     return NULL;
 }
 
 /**
  * mmu_notifier_get_locked - Return the single struct mmu_notifier for
  *                           the mm & ops
  * @ops: The operations struct being subscribe with
  * @mm : The mm to attach notifiers too
  *
  * This function either allocates a new mmu_notifier via
  * ops->alloc_notifier(), or returns an already existing notifier on the
  * list. The value of the ops pointer is used to determine when two notifiers
  * are the same.
  *
  * Each call to mmu_notifier_get() must be paired with a call to
  * mmu_notifier_put(). The caller must hold the write side of mm->mmap_lock.
  *
  * While the caller has a mmu_notifier get the mm pointer will remain valid,
  * and can be converted to an active mm pointer via mmget_not_zero().
  */
 struct mmu_notifier *mmu_notifier_get_locked(const struct mmu_notifier_ops *ops,
                          struct mm_struct *mm)
 {
     struct mmu_notifier *subscription;
     int ret;
 
     mmap_assert_write_locked(mm);
 
     if (mm->notifier_subscriptions) {
         subscription = find_get_mmu_notifier(mm, ops);
         if (subscription)
             return subscription;
     }
 
     subscription = ops->alloc_notifier(mm);
     if (IS_ERR(subscription))
         return subscription;
     subscription->ops = ops;
     ret = __mmu_notifier_register(subscription, mm);
     if (ret)
         goto out_free;
     return subscription;
 out_free:
     subscription->ops->free_notifier(subscription);
     return ERR_PTR(ret);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_get_locked);
 
 /* this is called after the last mmu_notifier_unregister() returned */
 void __mmu_notifier_subscriptions_destroy(struct mm_struct *mm)
 {
     BUG_ON(!hlist_empty(&mm->notifier_subscriptions->list));
     kfree(mm->notifier_subscriptions);
     mm->notifier_subscriptions = 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 *subscription,
                  struct mm_struct *mm)
 {
     BUG_ON(atomic_read(&mm->mm_count) <= 0);
 
     if (!hlist_unhashed(&subscription->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 (subscription->ops->release)
             subscription->ops->release(subscription, mm);
         srcu_read_unlock(&srcu, id);
 
         spin_lock(&mm->notifier_subscriptions->lock);
         /*
          * Can not use list_del_rcu() since __mmu_notifier_release
          * can delete it before we hold the lock.
          */
         hlist_del_init_rcu(&subscription->hlist);
         spin_unlock(&mm->notifier_subscriptions->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 void mmu_notifier_free_rcu(struct rcu_head *rcu)
 {
     struct mmu_notifier *subscription =
         container_of(rcu, struct mmu_notifier, rcu);
     struct mm_struct *mm = subscription->mm;
 
     subscription->ops->free_notifier(subscription);
     /* Pairs with the get in __mmu_notifier_register() */
     mmdrop(mm);
 }
 
 /**
  * mmu_notifier_put - Release the reference on the notifier
  * @subscription: The notifier to act on
  *
  * This function must be paired with each mmu_notifier_get(), it releases the
  * reference obtained by the get. If this is the last reference then process
  * to free the notifier will be run asynchronously.
  *
  * Unlike mmu_notifier_unregister() the get/put flow only calls ops->release
  * when the mm_struct is destroyed. Instead free_notifier is always called to
  * release any resources held by the user.
  *
  * As ops->release is not guaranteed to be called, the user must ensure that
  * all sptes are dropped, and no new sptes can be established before
  * mmu_notifier_put() is called.
  *
  * This function can be called from the ops->release callback, however the
  * caller must still ensure it is called pairwise with mmu_notifier_get().
  *
  * Modules calling this function must call mmu_notifier_synchronize() in
  * their __exit functions to ensure the async work is completed.
  */
 void mmu_notifier_put(struct mmu_notifier *subscription)
 {
     struct mm_struct *mm = subscription->mm;
 
     spin_lock(&mm->notifier_subscriptions->lock);
     if (WARN_ON(!subscription->users) || --subscription->users)
         goto out_unlock;
     hlist_del_init_rcu(&subscription->hlist);
     spin_unlock(&mm->notifier_subscriptions->lock);
 
     call_srcu(&srcu, &subscription->rcu, mmu_notifier_free_rcu);
     return;
 
 out_unlock:
     spin_unlock(&mm->notifier_subscriptions->lock);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_put);
 
 static int __mmu_interval_notifier_insert(
     struct mmu_interval_notifier *interval_sub, struct mm_struct *mm,
     struct mmu_notifier_subscriptions *subscriptions, unsigned long start,
     unsigned long length, const struct mmu_interval_notifier_ops *ops)
 {
     interval_sub->mm = mm;
     interval_sub->ops = ops;
     RB_CLEAR_NODE(&interval_sub->interval_tree.rb);
     interval_sub->interval_tree.start = start;
     /*
      * Note that the representation of the intervals in the interval tree
      * considers the ending point as contained in the interval.
      */
     if (length == 0 ||
         check_add_overflow(start, length - 1,
                    &interval_sub->interval_tree.last))
         return -EOVERFLOW;
 
     /* Must call with a mmget() held */
     if (WARN_ON(atomic_read(&mm->mm_users) <= 0))
         return -EINVAL;
 
     /* pairs with mmdrop in mmu_interval_notifier_remove() */
     mmgrab(mm);
 
     /*
      * If some invalidate_range_start/end region is going on in parallel
      * we don't know what VA ranges are affected, so we must assume this
      * new range is included.
      *
      * If the itree is invalidating then we are not allowed to change
      * it. Retrying until invalidation is done is tricky due to the
      * possibility for live lock, instead defer the add to
      * mn_itree_inv_end() so this algorithm is deterministic.
      *
      * In all cases the value for the interval_sub->invalidate_seq should be
      * odd, see mmu_interval_read_begin()
      */
     spin_lock(&subscriptions->lock);
     if (subscriptions->active_invalidate_ranges) {
         if (mn_itree_is_invalidating(subscriptions))
             hlist_add_head(&interval_sub->deferred_item,
                        &subscriptions->deferred_list);
         else {
             subscriptions->invalidate_seq |= 1;
             interval_tree_insert(&interval_sub->interval_tree,
                          &subscriptions->itree);
         }
         interval_sub->invalidate_seq = subscriptions->invalidate_seq;
     } else {
         WARN_ON(mn_itree_is_invalidating(subscriptions));
         /*
          * The starting seq for a subscription not under invalidation
          * should be odd, not equal to the current invalidate_seq and
          * invalidate_seq should not 'wrap' to the new seq any time
          * soon.
          */
         interval_sub->invalidate_seq =
             subscriptions->invalidate_seq - 1;
         interval_tree_insert(&interval_sub->interval_tree,
                      &subscriptions->itree);
     }
     spin_unlock(&subscriptions->lock);
     return 0;
 }
 
 /**
  * mmu_interval_notifier_insert - Insert an interval notifier
  * @interval_sub: Interval subscription to register
  * @start: Starting virtual address to monitor
  * @length: Length of the range to monitor
  * @mm: mm_struct to attach to
  * @ops: Interval notifier operations to be called on matching events
  *
  * This function subscribes the interval notifier for notifications from the
  * mm.  Upon return the ops related to mmu_interval_notifier will be called
  * whenever an event that intersects with the given range occurs.
  *
  * Upon return the range_notifier may not be present in the interval tree yet.
  * The caller must use the normal interval notifier read flow via
  * mmu_interval_read_begin() to establish SPTEs for this range.
  */
 int mmu_interval_notifier_insert(struct mmu_interval_notifier *interval_sub,
                  struct mm_struct *mm, unsigned long start,
                  unsigned long length,
                  const struct mmu_interval_notifier_ops *ops)
 {
     struct mmu_notifier_subscriptions *subscriptions;
     int ret;
 
     might_lock(&mm->mmap_lock);
 
     subscriptions = smp_load_acquire(&mm->notifier_subscriptions);
     if (!subscriptions || !subscriptions->has_itree) {
         ret = mmu_notifier_register(NULL, mm);
         if (ret)
             return ret;
         subscriptions = mm->notifier_subscriptions;
     }
     return __mmu_interval_notifier_insert(interval_sub, mm, subscriptions,
                           start, length, ops);
 }
 EXPORT_SYMBOL_GPL(mmu_interval_notifier_insert);
 
 int mmu_interval_notifier_insert_locked(
     struct mmu_interval_notifier *interval_sub, struct mm_struct *mm,
     unsigned long start, unsigned long length,
     const struct mmu_interval_notifier_ops *ops)
 {
     struct mmu_notifier_subscriptions *subscriptions =
         mm->notifier_subscriptions;
     int ret;
 
     mmap_assert_write_locked(mm);
 
     if (!subscriptions || !subscriptions->has_itree) {
         ret = __mmu_notifier_register(NULL, mm);
         if (ret)
             return ret;
         subscriptions = mm->notifier_subscriptions;
     }
     return __mmu_interval_notifier_insert(interval_sub, mm, subscriptions,
                           start, length, ops);
 }
 EXPORT_SYMBOL_GPL(mmu_interval_notifier_insert_locked);
 
 static bool
 mmu_interval_seq_released(struct mmu_notifier_subscriptions *subscriptions,
               unsigned long seq)
 {
     bool ret;
 
     spin_lock(&subscriptions->lock);
     ret = subscriptions->invalidate_seq != seq;
     spin_unlock(&subscriptions->lock);
     return ret;
 }
 
 /**
  * mmu_interval_notifier_remove - Remove a interval notifier
  * @interval_sub: Interval subscription to unregister
  *
  * This function must be paired with mmu_interval_notifier_insert(). It cannot
  * be called from any ops callback.
  *
  * Once this returns ops callbacks are no longer running on other CPUs and
  * will not be called in future.
  */
 void mmu_interval_notifier_remove(struct mmu_interval_notifier *interval_sub)
 {
     struct mm_struct *mm = interval_sub->mm;
     struct mmu_notifier_subscriptions *subscriptions =
         mm->notifier_subscriptions;
     unsigned long seq = 0;
 
     might_sleep();
 
     spin_lock(&subscriptions->lock);
     if (mn_itree_is_invalidating(subscriptions)) {
         /*
          * remove is being called after insert put this on the
          * deferred list, but before the deferred list was processed.
          */
         if (RB_EMPTY_NODE(&interval_sub->interval_tree.rb)) {
             hlist_del(&interval_sub->deferred_item);
         } else {
             hlist_add_head(&interval_sub->deferred_item,
                        &subscriptions->deferred_list);
             seq = subscriptions->invalidate_seq;
         }
     } else {
         WARN_ON(RB_EMPTY_NODE(&interval_sub->interval_tree.rb));
         interval_tree_remove(&interval_sub->interval_tree,
                      &subscriptions->itree);
     }
     spin_unlock(&subscriptions->lock);
 
     /*
      * The possible sleep on progress in the invalidation requires the
      * caller not hold any locks held by invalidation callbacks.
      */
     lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
     lock_map_release(&__mmu_notifier_invalidate_range_start_map);
     if (seq)
         wait_event(subscriptions->wq,
                mmu_interval_seq_released(subscriptions, seq));
 
     /* pairs with mmgrab in mmu_interval_notifier_insert() */
     mmdrop(mm);
 }
 EXPORT_SYMBOL_GPL(mmu_interval_notifier_remove);
 
 /**
  * mmu_notifier_synchronize - Ensure all mmu_notifiers are freed
  *
  * This function ensures that all outstanding async SRU work from
  * mmu_notifier_put() is completed. After it returns any mmu_notifier_ops
  * associated with an unused mmu_notifier will no longer be called.
  *
  * Before using the caller must ensure that all of its mmu_notifiers have been
  * fully released via mmu_notifier_put().
  *
  * Modules using the mmu_notifier_put() API should call this in their __exit
  * function to avoid module unloading races.
  */
 void mmu_notifier_synchronize(void)
 {
     synchronize_srcu(&srcu);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_synchronize);
 
 bool
 mmu_notifier_range_update_to_read_only(const struct mmu_notifier_range *range)
 {
     if (!range->vma || range->event != MMU_NOTIFY_PROTECTION_VMA)
         return false;
     /* Return true if the vma still have the read flag set. */
     return range->vma->vm_flags & VM_READ;
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_range_update_to_read_only);

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