virtio-mem: simplify high-level unplug handling in Big Block Mode

Let's simplify high-level big block selection when unplugging in
Big Block Mode.

Combine handling of offline and online blocks. We can get rid of
virtio_mem_bbm_bb_is_offline() and simply use
virtio_mem_bbm_offline_remove_and_unplug_bb(), as that already tolerates
offline parts.

We can race with concurrent onlining/offlining either way, so we don;t
have to be super correct by failing if an offline big block we'd like to
unplug just got (partially) onlined.
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20210602185720.31821-7-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -702,18 +702,6 @@ static int virtio_mem_sbm_remove_mb(struct virtio_mem *vm, unsigned long mb_id)
 	return virtio_mem_remove_memory(vm, addr, size);
 }
 
-/*
- * See virtio_mem_remove_memory(): Try to remove all Linux memory blocks covered
- * by the big block.
- */
-static int virtio_mem_bbm_remove_bb(struct virtio_mem *vm, unsigned long bb_id)
-{
-	const uint64_t addr = virtio_mem_bb_id_to_phys(vm, bb_id);
-	const uint64_t size = vm->bbm.bb_size;
-
-	return virtio_mem_remove_memory(vm, addr, size);
-}
-
 /*
  * Try offlining and removing memory from Linux.
  *
@@ -2114,35 +2102,6 @@ static int virtio_mem_bbm_offline_remove_and_unplug_bb(struct virtio_mem *vm,
 	return rc;
 }
 
-/*
- * Try to remove a big block from Linux and unplug it. Will fail with
- * -EBUSY if some memory is online.
- *
- * Will modify the state of the memory block.
- */
-static int virtio_mem_bbm_remove_and_unplug_bb(struct virtio_mem *vm,
-					       unsigned long bb_id)
-{
-	int rc;
-
-	if (WARN_ON_ONCE(virtio_mem_bbm_get_bb_state(vm, bb_id) !=
-			 VIRTIO_MEM_BBM_BB_ADDED))
-		return -EINVAL;
-
-	rc = virtio_mem_bbm_remove_bb(vm, bb_id);
-	if (rc)
-		return -EBUSY;
-
-	rc = virtio_mem_bbm_unplug_bb(vm, bb_id);
-	if (rc)
-		virtio_mem_bbm_set_bb_state(vm, bb_id,
-					    VIRTIO_MEM_BBM_BB_PLUGGED);
-	else
-		virtio_mem_bbm_set_bb_state(vm, bb_id,
-					    VIRTIO_MEM_BBM_BB_UNUSED);
-	return rc;
-}
-
 /*
  * Test if a big block is completely offline.
  */
@@ -2166,42 +2125,35 @@ static int virtio_mem_bbm_unplug_request(struct virtio_mem *vm, uint64_t diff)
 {
 	uint64_t nb_bb = diff / vm->bbm.bb_size;
 	uint64_t bb_id;
-	int rc;
+	int rc, i;
 
 	if (!nb_bb)
 		return 0;
 
-	/* Try to unplug completely offline big blocks first. */
-	virtio_mem_bbm_for_each_bb_rev(vm, bb_id, VIRTIO_MEM_BBM_BB_ADDED) {
-		cond_resched();
-		/*
-		 * As we're holding no locks, this check is racy as memory
-		 * can get onlined in the meantime - but we'll fail gracefully.
-		 */
-		if (!virtio_mem_bbm_bb_is_offline(vm, bb_id))
-			continue;
-		rc = virtio_mem_bbm_remove_and_unplug_bb(vm, bb_id);
-		if (rc == -EBUSY)
-			continue;
-		if (!rc)
-			nb_bb--;
-		if (rc || !nb_bb)
-			return rc;
-	}
-
-	if (!unplug_online)
-		return 0;
+	/*
+	 * Try to unplug big blocks. Similar to SBM, start with offline
+	 * big blocks.
+	 */
+	for (i = 0; i < 2; i++) {
+		virtio_mem_bbm_for_each_bb_rev(vm, bb_id, VIRTIO_MEM_BBM_BB_ADDED) {
+			cond_resched();
 
-	/* Try to unplug any big blocks. */
-	virtio_mem_bbm_for_each_bb_rev(vm, bb_id, VIRTIO_MEM_BBM_BB_ADDED) {
-		cond_resched();
-		rc = virtio_mem_bbm_offline_remove_and_unplug_bb(vm, bb_id);
-		if (rc == -EBUSY)
-			continue;
-		if (!rc)
-			nb_bb--;
-		if (rc || !nb_bb)
-			return rc;
+			/*
+			 * As we're holding no locks, these checks are racy,
+			 * but we don't care.
+			 */
+			if (i == 0 && !virtio_mem_bbm_bb_is_offline(vm, bb_id))
+				continue;
+			rc = virtio_mem_bbm_offline_remove_and_unplug_bb(vm, bb_id);
+			if (rc == -EBUSY)
+				continue;
+			if (!rc)
+				nb_bb--;
+			if (rc || !nb_bb)
+				return rc;
+		}
+		if (i == 0 && !unplug_online)
+			return 0;
 	}
 
 	return nb_bb ? -EBUSY : 0;