Let's properly use page_offline_(start|end) to synchronize setting
PageOffline(), so we won't have valid page access to unplugged memory
regions from /proc/kcore.

Existing balloon implementations usually allow reading inflated memory;
doing so might result in unnecessary overhead in the hypervisor, which
is currently the case with virtio-mem.

For future virtio-mem use cases, it will be different when using shmem,
huge pages, !anonymous private mappings, ... as backing storage for a VM.
virtio-mem unplugged memory must no longer be accessed and access might
result in undefined behavior. There will be a virtio spec extension to
document this change, including a new feature flag indicating the
changed behavior. We really don't want to race against PFN walkers
reading random page content.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <redacted>
---
 drivers/virtio/virtio_mem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index 10ec60d81e84..dc2a2e2b2ff8 100644
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c

@@ -1065,6 +1065,7 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 static void virtio_mem_set_fake_offline(unsigned long pfn,
 					unsigned long nr_pages, bool onlined)
 {
+	page_offline_begin();
 	for (; nr_pages--; pfn++) {
 		struct page *page = pfn_to_page(pfn);
 

@@ -1075,6 +1076,7 @@ static void virtio_mem_set_fake_offline(unsigned long pfn,
 			ClearPageReserved(page);
 		}
 	}
+	page_offline_end();
 }
 
 /*

-- 
2.31.1