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Date:   Fri, 28 Apr 2023 11:26:02 -0700
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
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To:     Jason Gunthorpe <jgg@nvidia.com>, Peter Xu <peterx@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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        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>,
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        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
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References: <6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@gmail.com>
 <afcc124e-7a9b-879c-dfdf-200426b84e24@redhat.com>
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 <094d2074-5b69-5d61-07f7-9f962014fa68@redhat.com>
 <400da248-a14e-46a4-420a-a3e075291085@redhat.com>
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 <62ec50da-5f73-559c-c4b3-bde4eb215e08@redhat.com> <ZEvsx998gDFig/zq@x1n>
 <ZEwDelszXBRwUGS0@nvidia.com>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <ZEwDelszXBRwUGS0@nvidia.com>
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On 4/28/23 10:33, Jason Gunthorpe wrote:
> On Fri, Apr 28, 2023 at 11:56:55AM -0400, Peter Xu wrote:
> 
>>> PageAnon(page) can be called from GUP-fast after grabbing a reference. See
>>> gup_must_unshare().
>>
>> Hmm.. Is it a good idea at all to sacrifise all "!anon" fast-gups for this?
>> People will silently got degrade even on legal pins on shmem/hugetlb, I
>> think, which seems to be still a very major use case.
> 
> Remember gup fast was like this until quite recently - DAX wrecked it.
> 
> I fixed it when I changed DAX to not post-scan the VMA list..
> 
> I'm not sure longterm and really fast need to go together.
> 

Exactly: gup_fast + FOLL_LONGTERM is asking too much of this system,
it's really excessive.

I like the idea of simply: "if (FOLL_LONGTERM) jump straight to slow gup". 

thanks,
-- 
John Hubbard
NVIDIA