David Hildenbrand
2021-May-03 08:16 UTC
[PATCH v1 6/7] virtio-mem: use page_offline_(start|end) when setting PageOffline()
On 02.05.21 08:33, Mike Rapoport wrote:> On Thu, Apr 29, 2021 at 02:25:18PM +0200, David Hildenbrand wrote: >> 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. >> >> Signed-off-by: David Hildenbrand <david at redhat.com> >> --- >> drivers/virtio/virtio_mem.c | 2 ++ >> mm/util.c | 2 ++ >> 2 files changed, 4 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(); > > I'm not really familiar with ballooning and memory hotplug, but is it the > only place that needs page_offline_{begin,end} ?Existing balloon implementations that I am aware of (Hyper-V, XEN, virtio-balloon, vmware-balloon) usually allow reading inflated memory; doing so might result in unnecessary overhead in the hypervisor, so we really want to avoid it -- but it's strictly not forbidden and has been working forever. So we barely care about races: if there would be a rare race, we'd still be able to read that memory. For virtio-mem, it'll be different in the future when using shmem, huge pages, !anonymous private mappings, ... as backing storage for a VM; there will be a virtio spec extension to document that virtio-mem changes that indicate the new behavior won't allow reading unplugged memory and doing so will result in undefined behavior. -- Thanks, David / dhildenb