# Re: \[PATCH v2 4/6\] mm: introduce page\_offline\_(begin\|end\|freeze\|thaw) to synchronize setting PageOffline()

- From: Oscar Salvador \<osalvador@suse.de\>
- Date: 2021-05-25 08:16:35 
- List: linux-hyperv
- Page: https://ratatoskr.run/linux-hyperv/2021/05/206526
- Thread: https://ratatoskr.run/linux-hyperv/2021/05/206036/t.md

```text
On Fri, May 14, 2021 at 07:22:45PM +0200, 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|thaw) for
> synchronizing.
> 
> page_offline_freeze()/page_offline_thaw() 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.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: David Hildenbrand <redacted>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

-- 
Oscar Salvador
SUSE L3
```