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

[Mike Rapoport]

On Wed, May 05, 2021 at 05:10:33PM +0200, David Hildenbrand wrote:
> 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@xxxxxxxxxx>
> > 
> > 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)

I think you can add

5. Very old drivers
 
> Other PFN walkers really shouldn't (and don't) access random page content.
> 
> Thanks!
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 
> 

-- 
Sincerely yours,
Mike.