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Date:   Thu, 10 Feb 2022 12:20:47 -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>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 2/10/22 11:29, Minchan Kim wrote:
> On Thu, Feb 10, 2022 at 11:20:31AM -0800, John Hubbard wrote:
>> 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.
> 
> Yub, but pin_user_pages adds FOLL_PIN unconditinally and the comments
> says it supports NUU pages argument. Isn't it conflict?
>

Oh right, that is a conflict. The documentation should *not* say that a
NULL **pages arg is supported. Because the whole point of the FOLL_PIN
APIs is to actually pin struct pages. The NULL cases are only useful for
get_user_pages*().

So removing that last sentence is appropriate, plus also looking around
for similar documentation claims, including in pin_user_pages.rst. I
don't see anything from a very quick scan, though.

Sending out a fix will also trigger the observation that both the
kerneldoc headers mm/gup.c, and the writings in pin_user_pages.rst,
could use some updating. However, it is also true that this can be
reasonably treated as a documentation bug fix, and therefore allowed to
be limited to just this change.

Were you going to send out a formal patch? If not, I can include it in
an upcoming gup series, with your Reported-by tag. Up to you.


>>
>> Perhaps that should be part of the documentation. It sort of is already,
>> for get_user_pages*().
> 
> I expected it was just copied from get_user_pages.

Yes it was.

thanks,
-- 
John Hubbard
NVIDIA