netdev - Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <ZFER5ROgCUyywvfe@nvidia.com>
Date:   Tue, 2 May 2023 10:36:37 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
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        Richard Cochran <richardcochran@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Pavel Begunkov <asml.silence@...il.com>,
        Mika Penttila <mpenttil@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 03:28:40PM +0200, David Hildenbrand wrote:
> On 02.05.23 15:10, Jason Gunthorpe wrote:
> > On Tue, May 02, 2023 at 03:04:27PM +0200, Christian Borntraeger wrote:
> > \> > We can reintroduce a flag to permit exceptions if this is really broken, are you
> > > > able to test? I don't have an s390 sat around :)
> > > 
> > > Matt (Rosato on cc) probably can. In the end, it would mean having
> > >    <memoryBacking>
> > >      <source type="file"/>
> > >    </memoryBacking>
> > 
> > This s390 code is the least of the problems, after this series VFIO
> > won't startup at all with this configuration.
> 
> Good question if the domain would fail to start. I recall that IOMMUs for
> zPCI are special on s390x. [1]

Not upstream they aren't.

> Well, zPCI is special. I cannot immediately tell when we would trigger
> long-term pinning.

zPCI uses the standard IOMMU stuff, so it uses a normal VFIO container
and the normal pin_user_pages() path.

> [1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg875728.html

AFIACT this is talking about the nested IOMMU translation stuff.
RPCIT is the hypercall to invalidate the nested IOMMU table. I expect s390 is
going to have another go at implementing this using iommufd.

The stuff in that series like KVM_S390_ZPCIOP_REG_IOAT didn't make it
upstream.

Jason