[PATCH v2 5/6] virtio-mem: use page_offline_(start|end) when setting PageOffline()

From: David Hildenbrand <david@redhat.com>
To: linux-kernel@vger.kernel.org
Cc: David Hildenbrand <david@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Mike Rapoport <rppt@kernel.org>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Oscar Salvador <osalvador@suse.de>,
	Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
	Alex Shi <alex.shi@linux.alibaba.com>,
	Steven Price <steven.price@arm.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Aili Yao <yaoaili@kingsoft.com>, Jiri Bohac <jbohac@suse.cz>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	linux-hyperv@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH v2 5/6] virtio-mem: use page_offline_(start|end) when setting PageOffline()
Date: Fri, 14 May 2021 19:22:46 +0200	[thread overview]
Message-ID: <20210514172247.176750-6-david@redhat.com> (raw)
In-Reply-To: <20210514172247.176750-1-david@redhat.com>

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 <david@redhat.com>
---
 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

