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Date:   Tue, 2 May 2023 17:09:06 +0200
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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:     Matthew Rosato <mjrosato@linux.ibm.com>,
        Jason Gunthorpe <jgg@nvidia.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Lorenzo Stoakes <lstoakes@gmail.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>,
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        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>,
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        Oleg Nesterov <oleg@redhat.com>,
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References: <dee4f4ad6532b0f94d073da263526de334d5d7e0.1682981880.git.lstoakes@gmail.com>
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 <a6bb0334-9aba-9fd8-6a9a-9d4a931b6da2@linux.ibm.com>
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 <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>
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 <ZFEVQmFGL3GxZMaf@nvidia.com>
 <1d4c9258-9423-7411-e722-8f6865b18886@linux.ibm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <1d4c9258-9423-7411-e722-8f6865b18886@linux.ibm.com>
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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.

-- 
Thanks,

David / dhildenb