Re: [PATCH v4 06/10] memory-hotplug.rst: add a note about ZONE_MOVABLE and page pinning

From: Michal Hocko
Date: Fri Dec 18 2020 - 04:45:33 EST


On Thu 17-12-20 13:52:39, Pavel Tatashin wrote:
> Document the special handling of page pinning when ZONE_MOVABLE present.
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> Documentation/admin-guide/mm/memory-hotplug.rst | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index 5c4432c96c4b..c6618f99f765 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -357,6 +357,15 @@ creates ZONE_MOVABLE as following.
> Unfortunately, there is no information to show which memory block belongs
> to ZONE_MOVABLE. This is TBD.
>
> +.. note::
> + Techniques that rely on long-term pinnings of memory (especially, RDMA and
> + vfio) are fundamentally problematic with ZONE_MOVABLE and, therefore, memory
> + hot remove. Pinned pages cannot reside on ZONE_MOVABLE, to guarantee that
> + memory can still get hot removed - be aware that pinning can fail even if
> + there is plenty of free memory in ZONE_MOVABLE. In addition, using
> + ZONE_MOVABLE might make page pinning more expensive, because pages have to be
> + migrated off that zone first.
> +
> .. _memory_hotplug_how_to_offline_memory:
>
> How to offline memory
> --
> 2.25.1
>

--
Michal Hocko
SUSE Labs