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Date:   Fri, 18 Dec 2020 10:44:42 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, david@...hat.com,
        osalvador@...e.de, dan.j.williams@...el.com, sashal@...nel.org,
        tyhicks@...ux.microsoft.com, iamjoonsoo.kim@....com,
        mike.kravetz@...cle.com, rostedt@...dmis.org, mingo@...hat.com,
        jgg@...pe.ca, peterz@...radead.org, mgorman@...e.de,
        willy@...radead.org, rientjes@...gle.com, jhubbard@...dia.com,
        linux-doc@...r.kernel.org, ira.weiny@...el.com,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 06/10] memory-hotplug.rst: add a note about
 ZONE_MOVABLE and page pinning

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@...een.com>
> Suggested-by: David Hildenbrand <david@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  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

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