netdev - Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <afc2f63e-67a5-803b-883b-bc1f13ba452f@nvidia.com>
Date:   Fri, 28 Apr 2023 11:26:02 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.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>,
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        Jonathan Lemon <jonathan.lemon@...il.com>,
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        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
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        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

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