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Date:   Tue, 2 May 2023 11:20:22 -0400
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Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings
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To:     Lorenzo Stoakes <lstoakes@gmail.com>,
        David Hildenbrand <david@redhat.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        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>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Mika Penttila <mpenttil@redhat.com>,
        Dave Chinner <david@fromorbit.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Peter Xu <peterx@redhat.com>
References: <a6bb0334-9aba-9fd8-6a9a-9d4a931b6da2@linux.ibm.com>
 <ZFEL20GQdomXGxko@nvidia.com>
 <c4f790fb-b18a-341a-6965-455163ec06d1@redhat.com>
 <ZFER5ROgCUyywvfe@nvidia.com>
 <ce3aa7b9-723c-6ad3-3f03-3f1736e1c253@redhat.com>
 <ff99f2d8-804d-924f-3c60-b342ffc2173c@linux.ibm.com>
 <ad60d5d2-cfdf-df9f-aef1-7a0d3facbece@redhat.com>
 <ZFEVQmFGL3GxZMaf@nvidia.com>
 <1d4c9258-9423-7411-e722-8f6865b18886@linux.ibm.com>
 <1f3231c0-34b2-1e78-0bf0-f32d5b67811d@redhat.com>
 <25b7aa40-fad0-4886-90b2-c5d68d75d28b@lucifer.local>
From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <25b7aa40-fad0-4886-90b2-c5d68d75d28b@lucifer.local>
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On 5/2/23 11:19 AM, Lorenzo Stoakes wrote:
> On Tue, May 02, 2023 at 05:09:06PM +0200, David Hildenbrand wrote:
>> On 02.05.23 15:56, Matthew Rosato wrote:
>>> On 5/2/23 9:50 AM, Jason Gunthorpe wrote:
>>>> On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
>>>>>> 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.
>>>>
>>>> Right, this is VFIO, the physical HW can't tolerate not having pinned
>>>> memory, so something somewhere is always pinning it.
>>>
>>> I might have mis-explained above.
>>>
>>> With iommufd nesting, we will pin everything upfront as a starting point.
>>>
>>> The current usage of vfio type1 iommu for s390 does not pin the entirety of guest memory upfront, it happens as guest RPCITs occur / type1 mappings are made.
>>
>> ... so, after the domain started successfully on the libvirt/QEMU side ? :/
>>
>> It would be great to confirm that. There might be a BUG in patch #2 (see my
>> reply to patch #2) that might not allow you to reproduce it right now.
>>
> 
> Yes apologies - thank you VERY much for doing this Matthew, but apologies, I
> made rather a clanger in patch 2 which would mean fast patch degrading to slow
> path would pass even for file-backed.
> 
> Will respin a v7 + cc you on that, if you could be so kind as to test that?
> 

Sure, will do!