Re: [PATCH v1 1/1] mm/gup: skip pinnable check for refs==1

[John Hubbard <jhubbard@nvidia.com>]

On 1/31/22 23:35, John Hubbard wrote:
> 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.
> ...oh, but I guess you're pointing out that it's always going to be
page-at-a-time this deep in the pin_user_pages() call path. Which is true.

I hadn't worked through how to fix this yet, my initial reaction was
that allowing single refs to go through, while prohibiting multiple refs,
was clearly *not* the way to go.

thanks,
-- 
John Hubbard
NVIDIA