drm/amdkfd: Remove prefault before migrating to VRAM

Prefaulting potentially allocates system memory pages before a
migration. This adds unnecessary overhead. Instead we can skip
unallocated pages in the migration and just point migrate->dst to a
0-initialized VRAM page directly. Then the VRAM page will be inserted
to the PTE. A subsequent CPU page fault will migrate the page back to
system memory.
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_migrate.c

@@ -322,12 +322,13 @@ svm_migrate_copy_to_vram(struct amdgpu_device *adev, struct svm_range *prange,
 	for (i = j = 0; i < npages; i++) {
 		struct page *spage;
 
+		dst[i] = cursor.start + (j << PAGE_SHIFT);
+		migrate->dst[i] = svm_migrate_addr_to_pfn(adev, dst[i]);
+		svm_migrate_get_vram_page(prange, migrate->dst[i]);
+		migrate->dst[i] = migrate_pfn(migrate->dst[i]);
+
 		spage = migrate_pfn_to_page(migrate->src[i]);
 		if (spage && !is_zone_device_page(spage)) {
-			dst[i] = cursor.start + (j << PAGE_SHIFT);
-			migrate->dst[i] = svm_migrate_addr_to_pfn(adev, dst[i]);
-			svm_migrate_get_vram_page(prange, migrate->dst[i]);
-			migrate->dst[i] = migrate_pfn(migrate->dst[i]);
 			src[i] = dma_map_page(dev, spage, 0, PAGE_SIZE,
 					      DMA_TO_DEVICE);
 			r = dma_mapping_error(dev, src[i]);
@@ -522,9 +523,6 @@ svm_migrate_ram_to_vram(struct svm_range *prange, uint32_t best_loc,
 	pr_debug("svms 0x%p [0x%lx 0x%lx] to gpu 0x%x\n", prange->svms,
 		 prange->start, prange->last, best_loc);
 
-	/* FIXME: workaround for page locking bug with invalid pages */
-	svm_range_prefault(prange, mm, SVM_ADEV_PGMAP_OWNER(adev));
-
 	start = prange->start << PAGE_SHIFT;
 	end = (prange->last + 1) << PAGE_SHIFT;
 

drivers/gpu/drm/amd/amdkfd/kfd_svm.c

@@ -3181,28 +3181,6 @@ svm_range_best_prefetch_location(struct svm_range *prange)
 	return best_loc;
 }
 
-/* FIXME: This is a workaround for page locking bug when some pages are
- * invalid during migration to VRAM
- */
-void svm_range_prefault(struct svm_range *prange, struct mm_struct *mm,
-			void *owner)
-{
-	struct hmm_range *hmm_range;
-	int r;
-
-	if (prange->validated_once)
-		return;
-
-	r = amdgpu_hmm_range_get_pages(&prange->notifier, mm, NULL,
-				       prange->start << PAGE_SHIFT,
-				       prange->npages, &hmm_range,
-				       false, true, owner);
-	if (!r) {
-		amdgpu_hmm_range_get_pages_done(hmm_range);
-		prange->validated_once = true;
-	}
-}
-
 /* svm_range_trigger_migration - start page migration if prefetch loc changed
  * @mm: current process mm_struct
  * @prange: svm range structure

drivers/gpu/drm/amd/amdkfd/kfd_svm.h

@@ -181,8 +181,6 @@ void schedule_deferred_list_work(struct svm_range_list *svms);
 void svm_range_dma_unmap(struct device *dev, dma_addr_t *dma_addr,
 			 unsigned long offset, unsigned long npages);
 void svm_range_free_dma_mappings(struct svm_range *prange);
-void svm_range_prefault(struct svm_range *prange, struct mm_struct *mm,
-			void *owner);
 int svm_range_get_info(struct kfd_process *p, uint32_t *num_svm_ranges,
 		       uint64_t *svm_priv_data_size);
 int kfd_criu_checkpoint_svm(struct kfd_process *p,