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Date:   Mon, 31 Jan 2022 23:35:14 -0800
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Subject: Re: [PATCH v1 1/1] mm/gup: skip pinnable check for refs==1
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To:     Minchan Kim <minchan@kernel.org>
Cc:     Will McVicker <willmcvicker@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        kernel-team@android.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20220131203504.3458775-1-willmcvicker@google.com>
 <27e5f98a-0709-1a80-18ed-e4ccaaf39fe6@nvidia.com>
 <YfjhGKCkQ6h5H5uI@google.com>
From:   John Hubbard <jhubbard@nvidia.com>
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On 1/31/22 23:28, Minchan Kim wrote:
...
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -135,7 +135,7 @@ struct page *try_grab_compound_head(struct page *page,
>>>    		 * right zone, so fail and let the caller fall back to the slow
>>>    		 * path.
>>>    		 */
>>> -		if (unlikely((flags & FOLL_LONGTERM) &&
>>> +		if (refs > 1 && unlikely((flags & FOLL_LONGTERM) &&
>>>    			     !is_pinnable_page(page)))
>>>    			return NULL;
>>
>> ...but are you really sure that this is the best way to "fix" the
>> problem? This trades correctness for "bug-for-bug compatibility" with
>> the previous code. It says, "it's OK to violate the pinnable and
>> longterm checks, as long as you do it one page at a time, rather than in
>> larger chunks.
>>
>> Wouldn't it be better to try to fix up the calling code so that it's
>> not in violation of these zone rules?
> 
> I think the problem is before pin_user_pages can work with CMA pages
> in the fallback path but now it doesn't work with CMA page. Driver

Actually, it "worked" only if the caller did it one page at a time.
(See how the above attempted fix restores the "make it work for
refs == 1.)

> couldn't know whether it will work with CMA page or not in advance.
> 
> pin_user_pages
>    __get_user_pages_locked
>      follow_page_mask
>        follow_page_pte
>          try_grab_page
>            !is_pinnable_page(page)
>              return NULL;
>          return ERR_PTR(-ENOMEM);
>       return -ENOMEM without faultin_page

Yes, that's all clear.


thanks,
-- 
John Hubbard
NVIDIA