linux-kernel - pin_user_pages supports NULL pages arguments?

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Date:   Thu, 10 Feb 2022 11:17:27 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: pin_user_pages supports NULL pages arguments?

 * pin_user_pages() - pin user pages in memory for use by other devices
 < snip >
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.

pin_user_pages(,, pages = NULL, );
  gup_flags |= FOLL_PIN
  __get_user_pages_locked
    __get_user_pages
      ..
      VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));