Re: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user_pages*()

From: Dan Carpenter <dan.carpenter@oracle.com>
To: Souptick Joarder <jrdr.linux@gmail.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
	"open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, Rob Springer <rspringer@google.com>,
	Todd Poynor <toddpoynor@google.com>
Subject: Re: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user_pages*()
Date: Fri, 29 May 2020 10:46:58 +0300	[thread overview]
Message-ID: <20200529074658.GM30374@kadam> (raw)
In-Reply-To: <CAFqt6zbtg0NWbAnDGPC0ZddEiTeohz=8JN+S_KxqM0bnnvar3g@mail.gmail.com>

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.

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).
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  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