virtio-mem: check if the config changed before fake offlining memory

If we repeatedly fail to fake offline memory to unplug it, we won't be
sending any unplug requests to the device. However, we only check if the
config changed when sending such (un)plug requests.

We could end up trying for a long time to unplug memory, even though
the config changed already and we're not supposed to unplug memory
anymore. For example, the hypervisor might detect a low-memory situation
while unplugging memory and decide to replug some memory. Continuing
trying to unplug memory in that case can be problematic.

So let's check on a more regular basis.

Signed-off-by: David Hildenbrand <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/virtio/virtio_mem.c

···
  * 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)
+static int virtio_mem_fake_offline(struct virtio_mem *vm, unsigned long pfn,
+				   unsigned long nr_pages)
 {
 	const bool is_movable = is_zone_movable_page(pfn_to_page(pfn));
 	int rc, retry_count;
···
 	 * some guarantees.
 	 */
 	for (retry_count = 0; retry_count < 5; retry_count++) {
+		/*
+		 * If the config changed, stop immediately and go back to the
+		 * main loop: avoid trying to keep unplugging if the device
+		 * might have decided to not remove any more memory.
+		 */
+		if (atomic_read(&vm->config_changed))
+			return -EAGAIN;
+
 		rc = alloc_contig_range(pfn, pfn + nr_pages, MIGRATE_MOVABLE,
 					GFP_KERNEL);
 		if (rc == -ENOMEM)
···
 	start_pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
 			     sb_id * vm->sbm.sb_size);
 
-	rc = virtio_mem_fake_offline(start_pfn, nr_pages);
+	rc = virtio_mem_fake_offline(vm, start_pfn, nr_pages);
 	if (rc)
 		return rc;
 
···
 		if (!page)
 			continue;
 
-		rc = virtio_mem_fake_offline(pfn, PAGES_PER_SECTION);
+		rc = virtio_mem_fake_offline(vm, pfn, PAGES_PER_SECTION);
 		if (rc) {
 			end_pfn = pfn;
 			goto rollback;