Re: [PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()

[David Hildenbrand]

On 17.05.21 08:43, Mike Rapoport wrote:
> 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@xxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
> One nit below, otherwise
>
> Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
>
> > ---
> >   include/linux/page-flags.h | 10 ++++++++++
> >   mm/util.c                  | 40 ++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 50 insertions(+)
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index daed82744f4b..ea2df9a247b3 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -769,9 +769,19 @@ PAGE_TYPE_OPS(Buddy, buddy)
> >    * relies on this feature is aware that re-onlining the memory block will
> >    * require to re-set the pages PageOffline() and not giving them to the
> >    * buddy via online_page_callback_t.
> > + *
> > + * There are drivers that mark a page PageOffline() and do not expect any
>
> Maybe "and expect there won't be any further access"...

Thanks, makes sense.

I'll wait a bit before I resend.

--
Thanks,

David / dhildenb