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 17:45:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>
Cc:     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 17:36, Jason Gunthorpe wrote:
> On Tue, May 02, 2023 at 11:32:57AM -0400, Peter Xu wrote:
>>> How does s390 avoid mmu notifiers without having lots of problems?? It
>>> is not really optional to hook the invalidations if you need to build
>>> a shadow page table..
>>
>> Totally no idea on s390 details, but.. per my read above, if the firmware
>> needs to make sure the page is always available (so no way to fault it in
>> on demand), which means a longterm pinning seems appropriate here.
>>
>> Then if pinned a must, there's no need for mmu notifiers (as the page will
>> simply not be invalidated anyway)?
> 
> And what if someone deliberately changes the mapping?  memory hotplug
> in the VM, or whatever?

Besides s390 not supporting memory hotplug in VMs (yet): if the guest 
wants a different guest physical address, I guess that's the problem of 
the guest, and it can update it:

KVM_S390_ZPCIOP_REG_AEN is triggered from QEMU via 
s390_pci_kvm_aif_enable(), triggered by the guest via a special instruction.

If the hypervisor changes the mapping, it's just the same thing as 
mixing e.g. MADV_DONTNEED with longterm pinning in vfio: don't do it. 
And if you do it, you get to keep the mess you created for your VM.

Linux will make sure to not change the mapping: for example, page 
migration of a pinned page will fail.

But maybe I am missing something important here.

-- 
Thanks,

David / dhildenb