mm/hmm: Hold on to the mmget for the lifetime of the range

Range functions like hmm_range_snapshot() and hmm_range_fault() call
find_vma, which requires hodling the mmget() and the mmap_sem for the mm.

Make this simpler for the callers by holding the mmget() inside the range
for the lifetime of the range. Other functions that accept a range should
only be called if the range is registered.

This has the side effect of directly preventing hmm_release() from
happening while a range is registered. That means range->dead cannot be
false during the lifetime of the range, so remove dead and
hmm_mirror_mm_is_alive() entirely.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Philip Yang <Philip.Yang@amd.com>

include/linux/hmm.h

@@ -82,7 +82,6 @@
  * @mirrors_sem: read/write semaphore protecting the mirrors list
  * @wq: wait queue for user waiting on a range invalidation
  * @notifiers: count of active mmu notifiers
- * @dead: is the mm dead ?
  */
 struct hmm {
 	struct mm_struct	*mm;
@@ -95,7 +94,6 @@ struct hmm {
 	wait_queue_head_t	wq;
 	struct rcu_head		rcu;
 	long			notifiers;
-	bool			dead;
 };
 
 /*
@@ -459,30 +457,6 @@ struct hmm_mirror {
 int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm);
 void hmm_mirror_unregister(struct hmm_mirror *mirror);
 
-/*
- * hmm_mirror_mm_is_alive() - test if mm is still alive
- * @mirror: the HMM mm mirror for which we want to lock the mmap_sem
- * Return: false if the mm is dead, true otherwise
- *
- * This is an optimization, it will not always accurately return false if the
- * mm is dead; i.e., there can be false negatives (process is being killed but
- * HMM is not yet informed of that). It is only intended to be used to optimize
- * out cases where the driver is about to do something time consuming and it
- * would be better to skip it if the mm is dead.
- */
-static inline bool hmm_mirror_mm_is_alive(struct hmm_mirror *mirror)
-{
-	struct mm_struct *mm;
-
-	if (!mirror || !mirror->hmm)
-		return false;
-	mm = READ_ONCE(mirror->hmm->mm);
-	if (mirror->hmm->dead || !mm)
-		return false;
-
-	return true;
-}
-
 /*
  * Please see Documentation/vm/hmm.rst for how to use the range API.
  */

mm/hmm.c

@@ -67,7 +67,6 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
 	mutex_init(&hmm->lock);
 	kref_init(&hmm->kref);
 	hmm->notifiers = 0;
-	hmm->dead = false;
 	hmm->mm = mm;
 
 	hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
@@ -120,21 +119,16 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
 {
 	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
 	struct hmm_mirror *mirror;
-	struct hmm_range *range;
 
 	/* Bail out if hmm is in the process of being freed */
 	if (!kref_get_unless_zero(&hmm->kref))
 		return;
 
-	/* Report this HMM as dying. */
-	hmm->dead = true;
-
-	/* Wake-up everyone waiting on any range. */
-	mutex_lock(&hmm->lock);
-	list_for_each_entry(range, &hmm->ranges, list)
-		range->valid = false;
-	wake_up_all(&hmm->wq);
-	mutex_unlock(&hmm->lock);
+	/*
+	 * Since hmm_range_register() holds the mmget() lock hmm_release() is
+	 * prevented as long as a range exists.
+	 */
+	WARN_ON(!list_empty_careful(&hmm->ranges));
 
 	down_write(&hmm->mirrors_sem);
 	mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
@@ -903,8 +897,8 @@ int hmm_range_register(struct hmm_range *range,
 	range->start = start;
 	range->end = end;
 
-	/* Check if hmm_mm_destroy() was call. */
-	if (hmm->mm == NULL || hmm->dead)
+	/* Prevent hmm_release() from running while the range is valid */
+	if (!mmget_not_zero(hmm->mm))
 		return -EFAULT;
 
 	/* Initialize range to track CPU page table updates. */
@@ -942,11 +936,12 @@ void hmm_range_unregister(struct hmm_range *range)
 		return;
 
 	mutex_lock(&hmm->lock);
-	list_del(&range->list);
+	list_del_init(&range->list);
 	mutex_unlock(&hmm->lock);
 
 	/* Drop reference taken by hmm_range_register() */
 	range->valid = false;
+	mmput(hmm->mm);
 	hmm_put(hmm);
 	range->hmm = NULL;
 }
@@ -974,10 +969,7 @@ long hmm_range_snapshot(struct hmm_range *range)
 	struct vm_area_struct *vma;
 	struct mm_walk mm_walk;
 
-	/* Check if hmm_mm_destroy() was call. */
-	if (hmm->mm == NULL || hmm->dead)
-		return -EFAULT;
-
+	lockdep_assert_held(&hmm->mm->mmap_sem);
 	do {
 		/* If range is no longer valid force retry. */
 		if (!range->valid)
@@ -1072,9 +1064,7 @@ long hmm_range_fault(struct hmm_range *range, bool block)
 	struct mm_walk mm_walk;
 	int ret;
 
-	/* Check if hmm_mm_destroy() was call. */
-	if (hmm->mm == NULL || hmm->dead)
-		return -EFAULT;
+	lockdep_assert_held(&hmm->mm->mmap_sem);
 
 	do {
 		/* If range is no longer valid force retry. */