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

Mike Rapoport
2021-05-02T06:33:26Z

On Thu, Apr 29, 2021 at 02:25:17PM +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|unfreeze) for

Bikeshed: freeze|thaw?

> 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.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> include/linux/page-flags.h | 5 +++++
> mm/util.c | 38 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 43 insertions(+)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index b8c56672a588..e3d00c72f459 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -767,6 +767,11 @@ PAGE_TYPE_OPS(Buddy, buddy)
> */
> PAGE_TYPE_OPS(Offline, offline)
>
> +extern void page_offline_freeze(void);
> +extern void page_offline_unfreeze(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 54870226cea6..95395d4e4209 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1013,3 +1013,41 @@ void mem_dump_obj(void *object)
> }
> pr_cont(" non-slab/vmalloc memory.\n");
> }
> +
> +/*
> + * 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_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().
> + */
> +static DECLARE_RWSEM(page_offline_rwsem);
> +
> +void page_offline_freeze(void)
> +{
> + down_read(&page_offline_rwsem);
> +}
> +
> +void page_offline_unfreeze(void)
> +{
> + up_read(&page_offline_rwsem);
> +}
> +
> +void page_offline_begin(void)
> +{
> + down_write(&page_offline_rwsem);
> +}
> +
> +void page_offline_end(void)
> +{
> + up_write(&page_offline_rwsem);
> +}
> --
> 2.30.2
>

--
Sincerely yours,
Mike.