linux-kernel - Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <e59f14d0-4fbf-b8e4-bdf0-19eac3db540f@linux.ibm.com>
Date:   Tue, 2 May 2023 11:20:22 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>,
        David Hildenbrand <david@...hat.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.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>,
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        Dave Chinner <david@...morbit.com>,
        "Theodore Ts'o" <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

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!