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From:   Pavel Tatashin <pasha.tatashin@soleen.com>
Date:   Fri, 4 Dec 2020 12:50:56 -0500
Message-ID: <CA+CK2bCGGoBXg7FbhGMDdWRnePKFgvtsM_PJmA2qtMNsvPMZbg@mail.gmail.com>
Subject: Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     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>,
        David Hildenbrand <david@redhat.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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> > Yes, this indeed could be a problem for some configurations. I will
> > add your comment to the commit log of one of the patches.
>
> It sounds like there is some inherent tension here, breaking THP's
> when doing pin_user_pages() is a really nasty thing to do. DMA
> benefits greatly from THP.
>
> I know nothing about ZONE_MOVABLE, is this auto-setup or an admin
> option? If the result of this patch is standard systems can no longer
> pin > 80% of their memory I have some regression concerns..

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.