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: <a597947b-6aba-bd8d-7a97-582fa7f88ad2@redhat.com>
Date:   Tue, 2 May 2023 14:47:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
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        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>,
        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>,
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        Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

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