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Date:   Tue, 2 May 2023 14:47:22 +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:     Peter Zijlstra <peterz@infradead.org>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, 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>,
        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>,
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        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>,
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        Oleg Nesterov <oleg@redhat.com>,
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 <20230502120810.GD1597538@hirez.programming.kicks-ass.net>
 <20230502124058.GB1597602@hirez.programming.kicks-ass.net>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230502124058.GB1597602@hirez.programming.kicks-ass.net>
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On 02.05.23 14:40, Peter Zijlstra wrote:
> On Tue, May 02, 2023 at 02:08:10PM +0200, Peter Zijlstra wrote:
> 
>>>>
>>>>
>>>> 	if (folio_test_anon(folio))
>>>> 		return true;
>>>
>>> This relies on the mapping so belongs below the lockdep assert imo.
>>
>> Oh, right you are.
>>
>>>>
>>>> 	/*
>>>> 	 * Having IRQs disabled (as per GUP-fast) also inhibits RCU
>>>> 	 * grace periods from making progress, IOW. they imply
>>>> 	 * rcu_read_lock().
>>>> 	 */
>>>> 	lockdep_assert_irqs_disabled();
>>>>
>>>> 	/*
>>>> 	 * Inodes and thus address_space are RCU freed and thus safe to
>>>> 	 * access at this point.
>>>> 	 */
>>>> 	mapping = folio_mapping(folio);
>>>> 	if (mapping && shmem_mapping(mapping))
>>>> 		return true;
>>>>
>>>> 	return false;
>>>>
>>>>> +}
> 
> So arguably you should do *one* READ_ONCE() load of mapping and
> consistently use that, this means open-coding both folio_test_anon() and
> folio_mapping().

Open-coding folio_test_anon() should not be required. We only care about 
PAGE_MAPPING_FLAGS stored alongside folio->mapping, that will stick 
around until the anon page was freed.

@Lorenzo, you might also want to special-case hugetlb directly using 
folio_test_hugetlb().

-- 
Thanks,

David / dhildenb