linux-kernel - Re: [PATCH v6 10/14] memory-hotplug.rst: add a note about ZONE

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Message-ID: <CA+CK2bAcbLzWTC3b=FqtSODG2ZWEK56AoWAQKnPWbJTzn3w6+g@mail.gmail.com>
Date:   Wed, 20 Jan 2021 09:28:56 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 10/14] memory-hotplug.rst: add a note about
 ZONE_MOVABLE and page pinning

On Wed, Jan 20, 2021 at 8:22 AM Jason Gunthorpe <jgg@...pe.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