diff mbox series

[v6,10/14] memory-hotplug.rst: add a note about ZONE_MOVABLE and page pinning

Message ID 20210120014333.222547-11-pasha.tatashin@soleen.com (mailing list archive)
State New
Headers show
Series prohibit pinning pages in ZONE_MOVABLE | expand

Commit Message

Pasha Tatashin Jan. 20, 2021, 1:43 a.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>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 Documentation/admin-guide/mm/memory-hotplug.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

Comments

Jason Gunthorpe Jan. 20, 2021, 1:22 p.m. UTC | #1 
On Tue, Jan 19, 2021 at 08:43:29PM -0500, Pavel Tatashin wrote:
> +.. 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.

Just to point out, if anyone is using RDMA/etc with hotplug memory,
this series is likekly going to be a major regression for those users.

Jason
Pasha Tatashin Jan. 20, 2021, 2:28 p.m. UTC | #2 
On Wed, Jan 20, 2021 at 8:22 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Jan 19, 2021 at 08:43:29PM -0500, Pavel Tatashin wrote:
> > +.. 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.
>
> Just to point out, if anyone is using RDMA/etc with hotplug memory,
> this series is likekly going to be a major regression for those users.

Right: one is because we now prohibit pinning any pages in a movable
zone, second reason is because we now check every page during
migration that fixes an existing bug.


>
> Jason
diff mbox series

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