[PATCH v5 10/14] memory-hotplug.rst: add a note about ZONE_MOVABLE and page pinning

From: Pavel Tatashin
Date: Mon Jan 18 2021 - 23:48:59 EST

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

+.. 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