Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   David Hildenbrand <david@redhat.com>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone
Date:   Sat, 12 Dec 2020 08:29:11 +0100
Message-Id: <2D10D596-3159-483C-81B4-CD187806ED46@redhat.com>
References: <20201211235005.GE5487@ziepe.ca>
Cc:     David Hildenbrand <david@redhat.com>,
        Pavel Tatashin <pasha.tatashin@soleen.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>,
        Joonsoo Kim <iamjoonsoo.kim@lge.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>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>
In-Reply-To: <20201211235005.GE5487@ziepe.ca>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Precedence: bulk


> Am 12.12.2020 um 00:50 schrieb Jason Gunthorpe <jgg@ziepe.ca>:
>=20
> =EF=BB=BFOn Fri, Dec 11, 2020 at 10:53:00PM +0100, David Hildenbrand wrote=
:
>=20
>>> When check_and_migrate_movable_pages() is called, the pages are
>>> already pinned. If some of those pages are in movable zone, and we
>>> fail to migrate or isolate them what should we do: proceed, and
>>> keep it as exception of when movable zone can actually have pinned
>>> pages or unpin all pages in the array, and return an error, or
>>> unpin only pages in movable zone, and return an error?
>>=20
>> I guess revert what we did (unpin) and return an error. The
>> interesting question is what can make migration/isolation fail
>>=20
>> a) out of memory: smells like a zone setup issue. Failures are acceptable=
 I guess.
>=20
> Out of memory is reasonable..
>=20
>> b) short term pinnings: process dying - not relevant I guess. Other cases=
? (Fork?)
>=20
> Concurrent with non-longterm GUP users are less reasonable, fork is
> not reasonable, etc..

Concurrent alloc_contig_range(), memory offlining, compaction .. where we mi=
grate pages? Any experts on racing page migration in these scenarios?

(Also wondering what would happen if we are just about to swap)

>=20
> Racing with another GUP in another thread is also not reasonable, so
> failing to isolate can't be a failure

Having VMs with multiple vfio containers is certainly realistic, and optimiz=
ing in user space to do vfio mappings concurrently doesn=E2=80=98t sound too=
 crazy to me. But I haven=E2=80=98t checked if vfio common code already hand=
les such concurrency.

>=20
> Jasnon
>=20