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

Pavel Tatashin
2021-01-20T14:29:36Z

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