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"...
> + * further access to page content. PFN walkers that read content of random
> + * pages should check PageOffline() and synchronize with such drivers using
> + * page_offline_freeze()/page_offline_thaw().
> */
> PAGE_TYPE_OPS(Offline, offline)
>
> +extern void page_offline_freeze(void);
> +extern void page_offline_thaw(void);
> +extern void page_offline_begin(void);
> +extern void page_offline_end(void);
> +
> /*
> * Marks pages in use as page tables.
> */
> diff --git a/mm/util.c b/mm/util.c
> index a8bf17f18a81..a034525e7ba2 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1010,3 +1010,43 @@ void mem_dump_obj(void *object)
> }
> EXPORT_SYMBOL_GPL(mem_dump_obj);
> #endif
> +
> +/*
> + * 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.
> + *
> + * 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().
> + *
> + * 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().
> + */
> +static DECLARE_RWSEM(page_offline_rwsem);
> +
> +void page_offline_freeze(void)
> +{
> + down_read(&page_offline_rwsem);
> +}
> +
> +void page_offline_thaw(void)
> +{
> + up_read(&page_offline_rwsem);
> +}
> +
> +void page_offline_begin(void)
> +{
> + down_write(&page_offline_rwsem);
> +}
> +EXPORT_SYMBOL(page_offline_begin);
> +
> +void page_offline_end(void)
> +{
> + up_write(&page_offline_rwsem);
> +}
> +EXPORT_SYMBOL(page_offline_end);
> --
> 2.31.1
>
--
Sincerely yours,
Mike.
