virtio-mem: Big Block Mode (BBM) - safe memory hotunplug

Let's add a safe mechanism to unplug memory, avoiding long/endless loops
when trying to offline memory - similar to in SBM.

Fake-offline all memory (via alloc_contig_range()) before trying to
offline+remove it. Use this mode as default, but allow to enable the other
mode explicitly (which could give better memory hotunplug guarantees in
some environments).

The "unsafe" mode can be enabled e.g., via virtio_mem.bbm_safe_unplug=0
on the cmdline.
Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-30-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -37,6 +37,11 @@ module_param(bbm_block_size, ulong, 0444);
 MODULE_PARM_DESC(bbm_block_size,
 		 "Big Block size in bytes. Default is 0 (auto-detection).");
 
+static bool bbm_safe_unplug = true;
+module_param(bbm_safe_unplug, bool, 0444);
+MODULE_PARM_DESC(bbm_safe_unplug,
+	     "Use a safe unplug mechanism in BBM, avoiding long/endless loops");
+
 /*
  * virtio-mem currently supports the following modes of operation:
  *
@@ -87,6 +92,8 @@ enum virtio_mem_bbm_bb_state {
 	VIRTIO_MEM_BBM_BB_PLUGGED,
 	/* Plugged and added to Linux. */
 	VIRTIO_MEM_BBM_BB_ADDED,
+	/* All online parts are fake-offline, ready to remove. */
+	VIRTIO_MEM_BBM_BB_FAKE_OFFLINE,
 	VIRTIO_MEM_BBM_BB_COUNT
 };
 
@@ -889,6 +896,32 @@ static void virtio_mem_sbm_notify_cancel_offline(struct virtio_mem *vm,
 	}
 }
 
+static void virtio_mem_bbm_notify_going_offline(struct virtio_mem *vm,
+						unsigned long bb_id,
+						unsigned long pfn,
+						unsigned long nr_pages)
+{
+	/*
+	 * When marked as "fake-offline", all online memory of this device block
+	 * is allocated by us. Otherwise, we don't have any memory allocated.
+	 */
+	if (virtio_mem_bbm_get_bb_state(vm, bb_id) !=
+	    VIRTIO_MEM_BBM_BB_FAKE_OFFLINE)
+		return;
+	virtio_mem_fake_offline_going_offline(pfn, nr_pages);
+}
+
+static void virtio_mem_bbm_notify_cancel_offline(struct virtio_mem *vm,
+						 unsigned long bb_id,
+						 unsigned long pfn,
+						 unsigned long nr_pages)
+{
+	if (virtio_mem_bbm_get_bb_state(vm, bb_id) !=
+	    VIRTIO_MEM_BBM_BB_FAKE_OFFLINE)
+		return;
+	virtio_mem_fake_offline_cancel_offline(pfn, nr_pages);
+}
+
 /*
  * This callback will either be called synchronously from add_memory() or
  * asynchronously (e.g., triggered via user space). We have to be careful
@@ -949,6 +982,10 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 		vm->hotplug_active = true;
 		if (vm->in_sbm)
 			virtio_mem_sbm_notify_going_offline(vm, id);
+		else
+			virtio_mem_bbm_notify_going_offline(vm, id,
+							    mhp->start_pfn,
+							    mhp->nr_pages);
 		break;
 	case MEM_GOING_ONLINE:
 		mutex_lock(&vm->hotplug_mutex);
@@ -999,6 +1036,10 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 			break;
 		if (vm->in_sbm)
 			virtio_mem_sbm_notify_cancel_offline(vm, id);
+		else
+			virtio_mem_bbm_notify_cancel_offline(vm, id,
+							     mhp->start_pfn,
+							     mhp->nr_pages);
 		vm->hotplug_active = false;
 		mutex_unlock(&vm->hotplug_mutex);
 		break;
@@ -1189,7 +1230,13 @@ static void virtio_mem_online_page_cb(struct page *page, unsigned int order)
 			do_online = virtio_mem_sbm_test_sb_plugged(vm, id,
 								   sb_id, 1);
 		} else {
-			do_online = true;
+			/*
+			 * If the whole block is marked fake offline, keep
+			 * everything that way.
+			 */
+			id = virtio_mem_phys_to_bb_id(vm, addr);
+			do_online = virtio_mem_bbm_get_bb_state(vm, id) !=
+				    VIRTIO_MEM_BBM_BB_FAKE_OFFLINE;
 		}
 		if (do_online)
 			generic_online_page(page, order);
@@ -1969,15 +2016,50 @@ static int virtio_mem_sbm_unplug_request(struct virtio_mem *vm, uint64_t diff)
 static int virtio_mem_bbm_offline_remove_and_unplug_bb(struct virtio_mem *vm,
 						       unsigned long bb_id)
 {
+	const unsigned long start_pfn = PFN_DOWN(virtio_mem_bb_id_to_phys(vm, bb_id));
+	const unsigned long nr_pages = PFN_DOWN(vm->bbm.bb_size);
+	unsigned long end_pfn = start_pfn + nr_pages;
+	unsigned long pfn;
+	struct page *page;
 	int rc;
 
 	if (WARN_ON_ONCE(virtio_mem_bbm_get_bb_state(vm, bb_id) !=
 			 VIRTIO_MEM_BBM_BB_ADDED))
 		return -EINVAL;
 
+	if (bbm_safe_unplug) {
+		/*
+		 * Start by fake-offlining all memory. Once we marked the device
+		 * block as fake-offline, all newly onlined memory will
+		 * automatically be kept fake-offline. Protect from concurrent
+		 * onlining/offlining until we have a consistent state.
+		 */
+		mutex_lock(&vm->hotplug_mutex);
+		virtio_mem_bbm_set_bb_state(vm, bb_id,
+					    VIRTIO_MEM_BBM_BB_FAKE_OFFLINE);
+
+		for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+			page = pfn_to_online_page(pfn);
+			if (!page)
+				continue;
+
+			rc = virtio_mem_fake_offline(pfn, PAGES_PER_SECTION);
+			if (rc) {
+				end_pfn = pfn;
+				goto rollback_safe_unplug;
+			}
+		}
+		mutex_unlock(&vm->hotplug_mutex);
+	}
+
 	rc = virtio_mem_bbm_offline_and_remove_bb(vm, bb_id);
-	if (rc)
+	if (rc) {
+		if (bbm_safe_unplug) {
+			mutex_lock(&vm->hotplug_mutex);
+			goto rollback_safe_unplug;
+		}
 		return rc;
+	}
 
 	rc = virtio_mem_bbm_unplug_bb(vm, bb_id);
 	if (rc)
@@ -1987,6 +2069,17 @@ static int virtio_mem_bbm_offline_remove_and_unplug_bb(struct virtio_mem *vm,
 		virtio_mem_bbm_set_bb_state(vm, bb_id,
 					    VIRTIO_MEM_BBM_BB_UNUSED);
 	return rc;
+
+rollback_safe_unplug:
+	for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+		page = pfn_to_online_page(pfn);
+		if (!page)
+			continue;
+		virtio_mem_fake_online(pfn, PAGES_PER_SECTION);
+	}
+	virtio_mem_bbm_set_bb_state(vm, bb_id, VIRTIO_MEM_BBM_BB_ADDED);
+	mutex_unlock(&vm->hotplug_mutex);
+	return rc;
 }
 
 /*