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

[David Hildenbrand <david@redhat.com>]

On 02.05.23 14:52, Lorenzo Stoakes wrote:
> On Tue, May 02, 2023 at 02:47:22PM +0200, David Hildenbrand wrote:
>> 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.
>>
> 
> Ack, good point!
> 
>> @Lorenzo, you might also want to special-case hugetlb directly using
>> folio_test_hugetlb().
>>
> 
> I already am :) I guess you mean when I respin along these lines? Will port
> that across to.

Ha, I only stared at that reply, good! :)

-- 
Thanks,

David / dhildenb