virtio-mem: Try to unplug the complete online memory block first

Right now, we always try to unplug single subblocks when processing an
online memory block. Let's try to unplug the complete online memory block
first, in case it is fully plugged and the unplug request is large
enough. Fallback to single subblocks in case the memory block cannot get
unplugged as a whole.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-16-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -1307,6 +1307,46 @@ static int virtio_mem_mb_unplug_any_sb_offline(struct virtio_mem *vm,
 	return 0;
 }
 
+/*
+ * Unplug the given plugged subblocks of an online memory block.
+ *
+ * Will modify the state of the memory block.
+ */
+static int virtio_mem_mb_unplug_sb_online(struct virtio_mem *vm,
+					  unsigned long mb_id, int sb_id,
+					  int count)
+{
+	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size) * count;
+	unsigned long start_pfn;
+	int rc;
+
+	start_pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
+			     sb_id * vm->subblock_size);
+	rc = alloc_contig_range(start_pfn, start_pfn + nr_pages,
+				MIGRATE_MOVABLE, GFP_KERNEL);
+	if (rc == -ENOMEM)
+		/* whoops, out of memory */
+		return rc;
+	if (rc)
+		return -EBUSY;
+
+	/* Mark it as fake-offline before unplugging it */
+	virtio_mem_set_fake_offline(start_pfn, nr_pages, true);
+	adjust_managed_page_count(pfn_to_page(start_pfn), -nr_pages);
+
+	/* Try to unplug the allocated memory */
+	rc = virtio_mem_mb_unplug_sb(vm, mb_id, sb_id, count);
+	if (rc) {
+		/* Return the memory to the buddy. */
+		virtio_mem_fake_online(start_pfn, nr_pages);
+		return rc;
+	}
+
+	virtio_mem_mb_set_state(vm, mb_id,
+				VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL);
+	return 0;
+}
+
 /*
  * Unplug the desired number of plugged subblocks of an online memory block.
  * Will skip subblock that are busy.
@@ -1321,16 +1361,21 @@ static int virtio_mem_mb_unplug_any_sb_online(struct virtio_mem *vm,
 					      unsigned long mb_id,
 					      uint64_t *nb_sb)
 {
-	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size);
-	unsigned long start_pfn;
 	int rc, sb_id;
 
-	/*
-	 * TODO: To increase the performance we want to try bigger, consecutive
-	 * subblocks first before falling back to single subblocks. Also,
-	 * we should sense via something like is_mem_section_removable()
-	 * first if it makes sense to go ahead any try to allocate.
-	 */
+	/* If possible, try to unplug the complete block in one shot. */
+	if (*nb_sb >= vm->nb_sb_per_mb &&
+	    virtio_mem_mb_test_sb_plugged(vm, mb_id, 0, vm->nb_sb_per_mb)) {
+		rc = virtio_mem_mb_unplug_sb_online(vm, mb_id, 0,
+						    vm->nb_sb_per_mb);
+		if (!rc) {
+			*nb_sb -= vm->nb_sb_per_mb;
+			goto unplugged;
+		} else if (rc != -EBUSY)
+			return rc;
+	}
+
+	/* Fallback to single subblocks. */
 	for (sb_id = vm->nb_sb_per_mb - 1; sb_id >= 0 && *nb_sb; sb_id--) {
 		/* Find the next candidate subblock */
 		while (sb_id >= 0 &&
@@ -1339,34 +1384,15 @@ static int virtio_mem_mb_unplug_any_sb_online(struct virtio_mem *vm,
 		if (sb_id < 0)
 			break;
 
-		start_pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
-				     sb_id * vm->subblock_size);
-		rc = alloc_contig_range(start_pfn, start_pfn + nr_pages,
-					MIGRATE_MOVABLE, GFP_KERNEL);
-		if (rc == -ENOMEM)
-			/* whoops, out of memory */
-			return rc;
-		if (rc)
-			/* memory busy, we can't unplug this chunk */
+		rc = virtio_mem_mb_unplug_sb_online(vm, mb_id, sb_id, 1);
+		if (rc == -EBUSY)
 			continue;
-
-		/* Mark it as fake-offline before unplugging it */
-		virtio_mem_set_fake_offline(start_pfn, nr_pages, true);
-		adjust_managed_page_count(pfn_to_page(start_pfn), -nr_pages);
-
-		/* Try to unplug the allocated memory */
-		rc = virtio_mem_mb_unplug_sb(vm, mb_id, sb_id, 1);
-		if (rc) {
-			/* Return the memory to the buddy. */
-			virtio_mem_fake_online(start_pfn, nr_pages);
+		else if (rc)
 			return rc;
-		}
-
-		virtio_mem_mb_set_state(vm, mb_id,
-					VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL);
 		*nb_sb -= 1;
 	}
 
+unplugged:
 	/*
 	 * Once all subblocks of a memory block were unplugged, offline and
 	 * remove it. This will usually not fail, as no memory is in use