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SubjectRe: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator
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On 27.01.23 13:30, Jan Kara wrote:
> On Fri 27-01-23 02:02:31, Al Viro wrote:
>> On Fri, Jan 27, 2023 at 12:44:08AM +0100, David Hildenbrand wrote:
>>> On 26.01.23 23:36, Al Viro wrote:
>>>> On Thu, Jan 26, 2023 at 09:59:36PM +0000, Al Viro wrote:
>>>>> On Thu, Jan 26, 2023 at 02:16:20PM +0000, David Howells wrote:
>>>>>
>>>>>> +/**
>>>>>> + * iov_iter_extract_will_pin - Indicate how pages from the iterator will be retained
>>>>>> + * @iter: The iterator
>>>>>> + *
>>>>>> + * Examine the iterator and indicate by returning true or false as to how, if
>>>>>> + * at all, pages extracted from the iterator will be retained by the extraction
>>>>>> + * function.
>>>>>> + *
>>>>>> + * %true indicates that the pages will have a pin placed in them that the
>>>>>> + * caller must unpin. This is must be done for DMA/async DIO to force fork()
>>>>>> + * to forcibly copy a page for the child (the parent must retain the original
>>>>>> + * page).
>>>>>> + *
>>>>>> + * %false indicates that no measures are taken and that it's up to the caller
>>>>>> + * to retain the pages.
>>>>>> + */
>>>>>> +static inline bool iov_iter_extract_will_pin(const struct iov_iter *iter)
>>>>>> +{
>>>>>> + return user_backed_iter(iter);
>>>>>> +}
>>>>>> +
>>>>>
>>>>> Wait a sec; why would we want a pin for pages we won't be modifying?
>>>>> A reference - sure, but...
>>>>
>>>> After having looked through the earlier iterations of the patchset -
>>>> sorry, but that won't fly for (at least) vmsplice(). There we can't
>>>> pin those suckers;
>>>
>>> We'll need a way to pass FOLL_LONGTERM to pin_user_pages_fast() to handle
>>> such long-term pinning as vmsplice() needs. But the release path (unpin)
>>> will be the same.
>>
>> Umm... Are you saying that if the source area contains DAX mmaps, vmsplice()
>> from it will fail?
>
> Yes, that's the plan. Because as you wrote elsewhere, it is otherwise too easy
> to lock up operations such as truncate(2) on DAX filesystems.

Right, it's then the same behavior as we already have for other
FOLL_LONGTERM users, such as RDMA or io_uring.

... if we're afraid of breaking existing setups we could add some kind
of fallback to copy to a buffer like ordinary pipe writes.

--
Thanks,

David / dhildenb

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