Messages in this thread | | | Date | Fri, 27 Jan 2023 13:34:50 +0100 | Subject | Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator | From | David Hildenbrand <> |
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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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