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Date:   Thu, 10 Feb 2022 11:20:31 -0800
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Subject: Re: pin_user_pages supports NULL pages arguments?
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To:     Minchan Kim <minchan@kernel.org>
Cc:     linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
References: <YgVkxwYtLov6Z5WZ@google.com>
From:   John Hubbard <jhubbard@nvidia.com>
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On 2/10/22 11:17, Minchan Kim wrote:
>   * 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)));

Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So
if a caller passes a null **pages arg, then that caller must not also
set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing.

Perhaps that should be part of the documentation. It sort of is already,
for get_user_pages*().

thanks,
-- 
John Hubbard
NVIDIA