virtio-mem: factor out fake-offlining into virtio_mem_fake_offline()

... which now matches virtio_mem_fake_online(). We'll reuse this
functionality soon.

Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-9-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -793,6 +793,27 @@ static void virtio_mem_fake_online(unsigned long pfn, unsigned long nr_pages)
 	}
 }
 
+/*
+ * Try to allocate a range, marking pages fake-offline, effectively
+ * fake-offlining them.
+ */
+static int virtio_mem_fake_offline(unsigned long pfn, unsigned long nr_pages)
+{
+	int rc;
+
+	rc = alloc_contig_range(pfn, pfn + nr_pages, MIGRATE_MOVABLE,
+				GFP_KERNEL);
+	if (rc == -ENOMEM)
+		/* whoops, out of memory */
+		return rc;
+	if (rc)
+		return -EBUSY;
+
+	virtio_mem_set_fake_offline(pfn, nr_pages, true);
+	adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
+	return 0;
+}
+
 static void virtio_mem_online_page_cb(struct page *page, unsigned int order)
 {
 	const unsigned long addr = page_to_phys(page);
@@ -1328,17 +1349,10 @@ static int virtio_mem_mb_unplug_sb_online(struct virtio_mem *vm,
 
 	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);
+	rc = virtio_mem_fake_offline(start_pfn, nr_pages);
+	if (rc)
+		return rc;
 
 	/* Try to unplug the allocated memory */
 	rc = virtio_mem_mb_unplug_sb(vm, mb_id, sb_id, count);