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 18:32:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Peter Xu <peterx@...hat.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        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>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

On 02.05.23 18:19, Jason Gunthorpe wrote:
> On Tue, May 02, 2023 at 06:12:39PM +0200, David Hildenbrand wrote:
> 
>>> It missses the general architectural point why we have all these
>>> shootdown mechanims in other places - plares are not supposed to make
>>> these kinds of assumptions. When the userspace unplugs the memory from
>>> KVM or unmaps it from VFIO it is not still being accessed by the
>>> kernel.
>>
>> Yes. Like having memory in a vfio iommu v1 and doing the same (mremap,
>> munmap, MADV_DONTNEED, ...). Which is why we disable MADV_DONTNEED (e.g.,
>> virtio-balloon) in QEMU with vfio.
> 
> That is different, VFIO has it's own contract how it consumes the
> memory from the MM and VFIO breaks all this stuff.
> 
> But when you tell VFIO to unmap the memory it doesn't keep accessing
> it in the background like this does.

To me, this is similar to when QEMU (user space) triggers 
KVM_S390_ZPCIOP_DEREG_AEN, to tell KVM to disable AIF and stop using the 
page (1) When triggered by the guest explicitly (2) when resetting the 
VM (3) when resetting the virtual PCI device / configuration.

Interrupt gets unregistered from HW (which stops using the page), the 
pages get unpinned. Pages get no longer used.

I guess I am still missing (a) how this is fundamentally different (b) 
how it could be done differently.

I'd really be happy to learn how a better approach would look like that 
does not use longterm pinnings.

I don't see an easy way to not use longterm pinnings. When using mmu 
notifiers and getting notified about unmapping of a page (for whatever 
reason ... migration, swapout, unmap), you'd have to disable aif. But 
when to reenable it (maybe there would be a way)? Also, I'm not sure if 
this could even be visible by the guest, if it's suddenly no longer enabled.

Something for the s390x people to explore ... if HW would be providing a 
way to deal with that somehow.

-- 
Thanks,

David / dhildenb