mm/hmm: Use hmm_mirror not mm as an argument for hmm_range_register

Ralph observes that hmm_range_register() can only be called by a driver
while a mirror is registered. Make this clear in the API by passing in the
mirror structure as a parameter.

This also simplifies understanding the lifetime model for struct hmm, as
the hmm pointer must be valid as part of a registered mirror so all we
need in hmm_register_range() is a simple kref_get.

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

drivers/gpu/drm/nouveau/nouveau_svm.c

@@ -649,7 +649,7 @@ nouveau_svm_fault(struct nvif_notify *notify)
 		range.values = nouveau_svm_pfn_values;
 		range.pfn_shift = NVIF_VMM_PFNMAP_V0_ADDR_SHIFT;
 again:
-		ret = hmm_vma_fault(&range, true);
+		ret = hmm_vma_fault(&svmm->mirror, &range, true);
 		if (ret == 0) {
 			mutex_lock(&svmm->mutex);
 			if (!hmm_vma_range_done(&range)) {

include/linux/hmm.h

@@ -496,7 +496,7 @@ static inline bool hmm_mirror_mm_is_alive(struct hmm_mirror *mirror)
  * Please see Documentation/vm/hmm.rst for how to use the range API.
  */
 int hmm_range_register(struct hmm_range *range,
-		       struct mm_struct *mm,
+		       struct hmm_mirror *mirror,
 		       unsigned long start,
 		       unsigned long end,
 		       unsigned page_shift);
@@ -532,7 +532,8 @@ static inline bool hmm_vma_range_done(struct hmm_range *range)
 }
 
 /* This is a temporary helper to avoid merge conflict between trees. */
-static inline int hmm_vma_fault(struct hmm_range *range, bool block)
+static inline int hmm_vma_fault(struct hmm_mirror *mirror,
+				struct hmm_range *range, bool block)
 {
 	long ret;
 
@@ -545,7 +546,7 @@ static inline int hmm_vma_fault(struct hmm_range *range, bool block)
 	range->default_flags = 0;
 	range->pfn_flags_mask = -1UL;
 
-	ret = hmm_range_register(range, range->vma->vm_mm,
+	ret = hmm_range_register(range, mirror,
 				 range->start, range->end,
 				 PAGE_SHIFT);
 	if (ret)

mm/hmm.c

@@ -914,13 +914,13 @@ static void hmm_pfns_clear(struct hmm_range *range,
  * Track updates to the CPU page table see include/linux/hmm.h
  */
 int hmm_range_register(struct hmm_range *range,
-		       struct mm_struct *mm,
+		       struct hmm_mirror *mirror,
 		       unsigned long start,
 		       unsigned long end,
 		       unsigned page_shift)
 {
 	unsigned long mask = ((1UL << page_shift) - 1UL);
-	struct hmm *hmm;
+	struct hmm *hmm = mirror->hmm;
 
 	range->valid = false;
 	range->hmm = NULL;
@@ -934,20 +934,15 @@ int hmm_range_register(struct hmm_range *range,
 	range->start = start;
 	range->end = end;
 
-	hmm = hmm_get_or_create(mm);
-	if (!hmm)
-		return -EFAULT;
-
 	/* Check if hmm_mm_destroy() was call. */
-	if (hmm->mm == NULL || hmm->dead) {
-		hmm_put(hmm);
+	if (hmm->mm == NULL || hmm->dead)
 		return -EFAULT;
-	}
 
 	/* Initialize range to track CPU page table updates. */
 	mutex_lock(&hmm->lock);
 
 	range->hmm = hmm;
+	kref_get(&hmm->kref);
 	list_add_rcu(&range->list, &hmm->ranges);
 
 	/*