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SubjectRe: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user_pages*()
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On 2020-05-29 00:46, Dan Carpenter wrote:
> On Fri, May 29, 2020 at 11:57:09AM +0530, Souptick Joarder wrote:
>> On Fri, May 29, 2020 at 11:46 AM Souptick Joarder <jrdr.linux@gmail.com> wrote:
>>>
>>> On Thu, May 28, 2020 at 4:34 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
>>>>
>>>> On Thu, May 28, 2020 at 02:32:42AM +0530, Souptick Joarder wrote:
>>>>> This code was using get_user_pages_fast(), in a "Case 2" scenario
>>>>> (DMA/RDMA), using the categorization from [1]. That means that it's
>>>>> time to convert the get_user_pages_fast() + put_page() calls to
>>>>> pin_user_pages_fast() + unpin_user_page() calls.
>>>>
>>>> You are saying that the page is used for DIO and not DMA, but it sure
>>>> looks to me like it is used for DMA.
>>>
>>> No, I was referring to "Case 2" scenario in change log which means it is
>>> used for DMA, not DIO.
>
> You can't use pin_user_pages() for DMA. This was second reason that I
> was confused.


OK, now it is getting interesting!


>
> mm/gup.c
> 2863 /**
> 2864 * pin_user_pages_fast() - pin user pages in memory without taking locks
> 2865 *
> 2866 * @start: starting user address
> 2867 * @nr_pages: number of pages from start to pin
> 2868 * @gup_flags: flags modifying pin behaviour
> 2869 * @pages: array that receives pointers to the pages pinned.
> 2870 * Should be at least nr_pages long.
> 2871 *
> 2872 * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
> 2873 * get_user_pages_fast() for documentation on the function arguments, because
> 2874 * the arguments here are identical.
> 2875 *
> 2876 * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
> 2877 * see Documentation/core-api/pin_user_pages.rst for further details.
> 2878 *
> 2879 * This is intended for Case 1 (DIO) in Documentation/core-api/pin_user_pages.rst. It
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 2880 * is NOT intended for Case 2 (RDMA: long-term pins).
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


I'm trying to figure out why I wrote that. It seems just wrong, because once the
page is dma-pinned, it will work just fine for either Case 1 or Case 2. hmmm, I
think this was from a few design ideas ago, when we were still working through the
FOLL_LONGTERM and FOLL_PIN thoughts and how the pin_user_pages*() API set should
look.

At this point, it's looking very much like a (my) documentation bug: all 4 of the
"intended for Case 1 (DIO)" comments in mm/gup.c probably need to be simply deleted.
Good catch.



> 2881 */
> 2882 int pin_user_pages_fast(unsigned long start, int nr_pages,
> 2883 unsigned int gup_flags, struct page **pages)
> 2884 {
> 2885 /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> 2886 if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> 2887 return -EINVAL;
> 2888
> 2889 gup_flags |= FOLL_PIN;
> 2890 return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> 2891 }
> 2892 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
>
> regards,
> dan carpenter
>

thanks,
--
John Hubbard
NVIDIA

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