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From:   Pavel Tatashin <pasha.tatashin@soleen.com>
Date:   Fri, 4 Dec 2020 13:10:05 -0500
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Subject: Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE
To:     David Hildenbrand <david@redhat.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, Joonsoo Kim <js1304@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>,
        Oscar Salvador <osalvador@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Sasha Levin <sashal@kernel.org>,
        Tyler Hicks <tyhicks@linux.microsoft.com>,
        mike.kravetz@oracle.com, Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        David Rientjes <rientjes@google.com>,
        John Hubbard <jhubbard@nvidia.com>
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> > ZONE_MOVABLE can be configured via kernel parameter, or when memory
> > nodes are onlined after hot-add; so this is something that admins
> > configure. ZONE_MOVABLE is designed to gurantee memory hot-plug
> > functionality, and not availability of THP, however, I did not know
> > about the use case where some admins might configure ZONE_MOVABLE to
> > increase availability of THP because pages are always migratable in
> > them. The thing is, if we fragment ZONE_MOVABLE by pinning pages in
> > it, the availability of THP also suffers.  We can migrate pages in
> > ZONE_NORMAL, just not guaranteed, so we can create THP in ZONE_NORMAL
> > as well, which is the usual case.
>
> Right, we should document this at some place to make admins aware of
> this. Something like
>
> "Techniques that rely on long-term pinnings of memory (especially, RDMA
> and vfio) are fundamentally problematic with ZONE_MOVABLE and,
> therefore, memory hotunplug. Pinned pages cannot reside on ZONE_MOVABLE,
> to guarantee that memory can still get hotunplugged - 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."

Thanks, I will add this.

>
> BTW, you might also want to update the comment for ZONE_MOVABLE in
> include/linux/mmzone.h at the end of this series, removing the special
> case of pinned pages (1.) and maybe adding what happens when trying to
> pin pages on ZONE_MOVABLE.

Will do it.

Thank you,
Pasha