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Date:   Tue, 2 May 2023 09:35:33 -0400
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Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings
To:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, 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>,
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        Oleg Nesterov <oleg@redhat.com>,
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        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
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        Dave Chinner <david@fromorbit.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Peter Xu <peterx@redhat.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>
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From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <a6bb0334-9aba-9fd8-6a9a-9d4a931b6da2@linux.ibm.com>
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On 5/2/23 9:04 AM, Christian Borntraeger wrote:
> 
> 
> Am 02.05.23 um 14:54 schrieb Lorenzo Stoakes:
>> On Tue, May 02, 2023 at 02:46:28PM +0200, Christian Borntraeger wrote:
>>> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes:
>>>> Writing to file-backed dirty-tracked mappings via GUP is inherently broken
>>>> as we cannot rule out folios being cleaned and then a GUP user writing to
>>>> them again and possibly marking them dirty unexpectedly.
>>>>
>>>> This is especially egregious for long-term mappings (as indicated by the
>>>> use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as
>>>> we have already done in the slow path.
>>>
>>> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery?
>>> It would no longer work with file backed memory, correct?
>>>
>>> See
>>> arch/s390/kvm/pci.c
>>>
>>> kvm_s390_pci_aif_enable
>>> which does have
>>> FOLL_WRITE | FOLL_LONGTERM
>>> to
>>>
>>
>> Does this memory map a dirty-tracked file? It's kind of hard to dig into where
>> the address originates from without going through a ton of code. In worst case
>> if the fast code doesn't find a whitelist it'll fall back to slow path which
>> explicitly checks for dirty-tracked filesystem.
> 
> It does pin from whatever QEMU uses as backing for the guest.
>>
>> 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>
> 
> In libvirt I guess.

I am running with this series applied using a QEMU guest with memory-backend-file (using the above libvirt snippet) for a few different PCI device types and AEN forwarding (e.g. what is setup in kvm_s390_pci_aif_enable) is still working.