linux-kernel - Re: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user

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Message-ID: <88daa048-7fc3-2d46-d582-02a7aaa7a970@nvidia.com>
Date:   Fri, 29 May 2020 01:00:22 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Souptick Joarder <jrdr.linux@...il.com>
CC:     "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>,
        Rob Springer <rspringer@...gle.com>,
        Todd Poynor <toddpoynor@...gle.com>
Subject: Re: [PATCH] staging: gasket: Convert get_user_pages*() -->
 pin_user_pages*()

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@...il.com> wrote:
>>>
>>> On Thu, May 28, 2020 at 4:34 PM Dan Carpenter <dan.carpenter@...cle.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