[058/192] virtio-mem: use page_offline_(start|end) when setting PageOffline()

Commit Message

Andrew Morton July 1, 2021, 1:50 a.m. UTC

From: David Hildenbrand <[e-mail]>
Subject: virtio-mem: use page_offline_(start|end) when setting PageOffline()

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.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Acked-by: Mike Rapoport <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Cc: Aili Yao <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Alex Shi <[e-mail]>
Cc: Haiyang Zhang <[e-mail]>
Cc: Jason Wang <[e-mail]>
Cc: Jiri Bohac <[e-mail]>
Cc: "K. Y. Srinivasan" <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Naoya Horiguchi <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Cc: Steven Price <[e-mail]>
Cc: Wei Liu <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 drivers/virtio/virtio_mem.c |    2 ++
 1 file changed, 2 insertions(+)

Patch

--- a/drivers/virtio/virtio_mem.c~virtio-mem-use-page_offline_startend-when-setting-pageoffline
+++ a/drivers/virtio/virtio_mem.c
@@ -1065,6 +1065,7 @@  static int virtio_mem_memory_notifier_cb
 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(
 			ClearPageReserved(page);
 		}
 	}
+	page_offline_end();
 }
 
 /*