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[v3,6/6] memory-hotplug.rst: add a note about ZONE_MOVABLE and page pinning

Message ID 20201211202140.396852-7-pasha.tatashin@soleen.com (mailing list archive)
State New, archived
Headers show
Series prohibit pinning pages in ZONE_MOVABLE | expand

Commit Message

Pasha Tatashin Dec. 11, 2020, 8:21 p.m. UTC 
Document the special handling of page pinning when ZONE_MOVABLE present.

Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Suggested-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/memory-hotplug.rst | 9 +++++++++
 1 file changed, 9 insertions(+)
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Patch

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