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

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Date:   Tue, 2 May 2023 15:39:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.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>,
        Christian Brauner <brauner@...nel.org>,
        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 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