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

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Message-ID: <ad60d5d2-cfdf-df9f-aef1-7a0d3facbece@redhat.com>
Date:   Tue, 2 May 2023 15:47:43 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        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>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

On 02.05.23 15:43, Matthew Rosato wrote:
> On 5/2/23 9:39 AM, David Hildenbrand wrote:
>> 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?
>>
> 
> Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
> 

Okay, so IIRC we'll fail starting the domain early, that's good. And if 
we pin all guest memory (instead of small pieces dynamically), there is 
little existing use for file-backed RAM in such zPCI configurations 
(because memory cannot be reclaimed either way if it's all pinned), so 
likely there are no real existing users.

> However, per Jason's prior suggestion, the initial implementation for s390 nesting via iommufd will pin all of guest memory when starting the domain.  I have something already working via iommufd built on top of the nesting infrastructure patches and QEMU iommufd series that are floating around; needs some cleanup, hoping to send an RFC in the coming weeks.  I can CC you if you'd like.

Yes, please.

-- 
Thanks,

David / dhildenb