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Subject: Re: [PATCH] mm/gup: don't permit users to call get_user_pages with
 FOLL_LONGTERM
To:     Souptick Joarder <jrdr.linux@gmail.com>
CC:     Barry Song <song.bao.hua@hisilicon.com>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 9/3/20 12:12 AM, Souptick Joarder wrote:
> On Wed, Aug 19, 2020 at 11:45 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> On 8/19/20 4:01 AM, Barry Song wrote:
>>> gug prohibits users from calling get_user_pages() with FOLL_PIN. But it
>>
>> Maybe Andrew can fix the typo above: gug --> gup.
>>
>>
>>> allows users to call get_user_pages() with FOLL_LONGTERM only. It seems
>>> insensible.
>>>
>>> since FOLL_LONGTERM is a stricter case of FOLL_PIN, we should prohibit
>>> users from calling get_user_pages() with FOLL_LONGTERM while not with
>>> FOLL_PIN.
>>>
>>> mm/gup_benchmark.c used to be the only user who did this improperly.
>>> But it has been fixed by moving to use pin_user_pages().
>>
>> For future patches, you don't have to write everything in the
>> commit log. Some things are better placed in a cover letter or after
>> the "---" line, because they don't need to be recorded forever.
>>
>> Anyway, the diffs seem fine, assuming that you've audited the call sites.
> 
> We can use is_valid_gup_flags() inside ->
> get_user_pages_locked(),
> get_user_pages_unlocked(),
> pin_user_pages_locked() as well.

Probably it's best to discern between valid pup flags, and valid gup flags.
As in: separate functions for those. Maybe one is a subset of the other, but
still.

> 
> Are you planning to add it in future patches ?
> 

It's not on my list. I don't see anything wrong with doing so, other
than avoiding the minor pitfall I called out above. So if you want to
do that, then feel free...


thanks,
-- 
John Hubbard
NVIDIA