Re: [PATCH v1 5/7] mm: introduce page_offline_(begin|end|freeze|unfreeze) to synchronize setting PageOffline()

David Hildenbrand
2021-05-05T15:10:44Z

On 05.05.21 15:24, Michal Hocko wrote:
> On Thu 29-04-21 14:25:17, David Hildenbrand wrote:
>> A driver might set a page logically offline -- PageOffline() -- and
>> turn the page inaccessible in the hypervisor; after that, access to page
>> content can be fatal. One example is virtio-mem; while unplugged memory
>> -- marked as PageOffline() can currently be read in the hypervisor, this
>> will no longer be the case in the future; for example, when having
>> a virtio-mem device backed by huge pages in the hypervisor.
>>
>> Some special PFN walkers -- i.e., /proc/kcore -- read content of random
>> pages after checking PageOffline(); however, these PFN walkers can race
>> with drivers that set PageOffline().
>>
>> Let's introduce page_offline_(begin|end|freeze|unfreeze) for
>> synchronizing.
>>
>> page_offline_freeze()/page_offline_unfreeze() allows for a subsystem to
>> synchronize with such drivers, achieving that a page cannot be set
>> PageOffline() while frozen.
>>
>> page_offline_begin()/page_offline_end() is used by drivers that care about
>> such races when setting a page PageOffline().
>>
>> For simplicity, use a rwsem for now; neither drivers nor users are
>> performance sensitive.
>
> Please add a note to the PageOffline documentation as well. While are
> adding the api close enough an explicit note there wouldn't hurt.

Will do.

>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>
> As to the patch itself, I am slightly worried that other pfn walkers
> might be less tolerant to the locking than the proc ones. On the other
> hand most users shouldn't really care as they do not tend to touch the
> memory content and PageOffline check without any synchronization should
> be sufficient for those. Let's try this out and see where we get...

My thinking. Users that actually read random page content (as discussed
in the cover letter) are

1. Hibernation
2. Dumping (/proc/kcore, /proc/vmcore)
3. Physical memory access bypassing the kernel via /dev/mem
4. Live debug tools (kgdb)

Other PFN walkers really shouldn't (and don't) access random page content.

Thanks!

--
Thanks,

David / dhildenb