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

[Mike Rapoport <rppt@kernel.org>]

On Fri, May 14, 2021 at 07:22:46PM +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.
> 
> 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>

Acked-by: Mike Rapoport <rppt@linux.ibm.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
> 
> 

-- 
Sincerely yours,
Mike.