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Date:   Tue, 2 May 2023 15:39:50 +0200
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Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings
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To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
        Christian Benvenuti <benve@cisco.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Bernard Metzler <bmt@zurich.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Richard Cochran <richardcochran@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Mika Penttila <mpenttil@redhat.com>,
        Dave Chinner <david@fromorbit.com>,
        Theodore Ts'o <tytso@mit.edu>, Peter Xu <peterx@redhat.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>
References: <cover.1682981880.git.lstoakes@gmail.com>
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 <fbad9e18-f727-9703-33cf-545a2d33af76@linux.ibm.com>
 <7d56b424-ba79-4b21-b02c-c89705533852@lucifer.local>
 <a6bb0334-9aba-9fd8-6a9a-9d4a931b6da2@linux.ibm.com>
 <ZFEL20GQdomXGxko@nvidia.com>
 <c4f790fb-b18a-341a-6965-455163ec06d1@redhat.com>
 <ZFER5ROgCUyywvfe@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <ZFER5ROgCUyywvfe@nvidia.com>
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On 02.05.23 15:36, Jason Gunthorpe wrote:
> 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.


@Christian, Matthew: would we pin all guest memory when starting the 
domain (IIRC, like on x86-64) and fail early, or only when the guest 
issues rpcit instructions to map individual pages?

-- 
Thanks,

David / dhildenb