[PATCH v12 28/29] HMM: CPU page fault on migrated memory.

["Jérôme Glisse" <jglisse@redhat.com>]

When CPU try to access memory that have been migrated to device memory
we have to copy it back to system memory. This patch implement the CPU
page fault handler for special HMM pte swap entry.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
 mm/hmm.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 4dcd98f..38943a7 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -472,7 +472,59 @@ int hmm_handle_cpu_fault(struct mm_struct *mm,
 			pmd_t *pmdp, unsigned long addr,
 			unsigned flags, pte_t orig_pte)
 {
-	return VM_FAULT_SIGBUS;
+	unsigned long start, end;
+	struct hmm_event event;
+	swp_entry_t entry;
+	struct hmm *hmm;
+	dma_addr_t dst;
+	pte_t new_pte;
+	int ret;
+
+	/* First check for poisonous entry. */
+	entry = pte_to_swp_entry(orig_pte);
+	if (is_hmm_entry_poisonous(entry))
+		return VM_FAULT_SIGBUS;
+
+	hmm = hmm_ref(mm->hmm);
+	if (!hmm) {
+		pte_t poison = swp_entry_to_pte(make_hmm_entry_poisonous());
+		spinlock_t *ptl;
+		pte_t *ptep;
+
+		/* Check if cpu pte is already updated. */
+		ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
+		if (!pte_same(*ptep, orig_pte)) {
+			pte_unmap_unlock(ptep, ptl);
+			return 0;
+		}
+		set_pte_at(mm, addr, ptep, poison);
+		pte_unmap_unlock(ptep, ptl);
+		return VM_FAULT_SIGBUS;
+	}
+
+	/*
+	 * TODO we likely want to migrate more then one page at a time, we need
+	 * to call into the device driver to get good hint on the range to copy
+	 * back to system memory.
+	 *
+	 * For now just live with the one page at a time solution.
+	 */
+	start = addr & PAGE_MASK;
+	end = start + PAGE_SIZE;
+	hmm_event_init(&event, hmm, start, end, HMM_COPY_FROM_DEVICE);
+
+	ret = hmm_migrate_back(hmm, &event, mm, vma, &new_pte,
+			       &dst, start, end);
+	hmm_unref(hmm);
+	switch (ret) {
+	case 0:
+		return VM_FAULT_MAJOR;
+	case -ENOMEM:
+		return VM_FAULT_OOM;
+	case -EINVAL:
+	default:
+		return VM_FAULT_SIGBUS;
+	}
 }
 EXPORT_SYMBOL(hmm_handle_cpu_fault);
 
-- 
2.4.3