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

On 05.05.21 15:24, Michal Hocko wrote:

quoted

On Thu 29-04-21 14:25:17, David Hildenbrand wrote:

quoted

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.

quoted

quoted

Signed-off-by: David Hildenbrand <redacted>

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.