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Subject: Re: [PATCH v1 6/7] virtio-mem: use page_offline_(start|end) when
 setting PageOffline()
To:     Mike Rapoport <rppt@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "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
References: <20210429122519.15183-1-david@redhat.com>
 <20210429122519.15183-7-david@redhat.com> <YI5HzXN7+ZTNXtcI@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <98f22a46-da8b-2891-fade-09937c0ccf69@redhat.com>
Date:   Mon, 3 May 2021 10:16:03 +0200
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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@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