HMM: fork copy migrated memory into system memory for child process.

When forking if process being fork had any memory migrated to some
device memory, we need to make a system copy for the child process.
Latter patches can revisit this and use the same COW semantic for
device memory.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>

1 files changed, 37 insertions, 1 deletions

diff --git a/mm/hmm.c b/mm/hmm.c
index abe2fba8063e..b473fe90f8a6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -480,7 +480,37 @@ int hmm_mm_fork(struct mm_struct *src_mm,
 		unsigned long start,
 		unsigned long end)
 {
-	return -ENOMEM;
+	unsigned long npages = (end - start) >> PAGE_SHIFT;
+	struct hmm_event event;
+	dma_addr_t *dst;
+	struct hmm *hmm;
+	pte_t *new_pte;
+	int ret;
+
+	hmm = hmm_ref(src_mm->hmm);
+	if (!hmm)
+		return -EINVAL;
+
+
+	dst = kcalloc(npages, sizeof(*dst), GFP_KERNEL);
+	if (!dst) {
+		hmm_unref(hmm);
+		return -ENOMEM;
+	}
+	new_pte = kcalloc(npages, sizeof(*new_pte), GFP_KERNEL);
+	if (!new_pte) {
+		kfree(dst);
+		hmm_unref(hmm);
+		return -ENOMEM;
+	}
+
+	hmm_event_init(&event, hmm, start, end, HMM_FORK);
+	ret = hmm_migrate_back(hmm, &event, dst_mm, dst_vma, new_pte,
+			       dst, start, end);
+	hmm_unref(hmm);
+	kfree(new_pte);
+	kfree(dst);
+	return ret;
 }
 EXPORT_SYMBOL(hmm_mm_fork);
 
@@ -656,6 +686,12 @@ static void hmm_mirror_update_pte(struct hmm_mirror *mirror,
 	}
 
 	if (hmm_pte_test_valid_dev(hmm_pte)) {
+		/*
+		 * On fork device memory is duplicated so no need to write
+		 * protect it.
+		 */
+		if (event->etype == HMM_FORK)
+			return;
 		*hmm_pte &= event->pte_mask;
 		if (!hmm_pte_test_valid_dev(hmm_pte))
 			hmm_pt_iter_directory_unref(iter);