Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

[John Hubbard <jhubbard@nvidia.com>]

On 11/12/19 2:45 PM, Dan Williams wrote:
> On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> On 11/12/19 12:43 PM, Jason Gunthorpe wrote:
>> ...
>>>> -            }
>>>> +    ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
>>>> +                                page, vmas, NULL);
>>>> +    /*
>>>> +     * The lifetime of a vaddr_get_pfn() page pin is
>>>> +     * userspace-controlled. In the fs-dax case this could
>>>> +     * lead to indefinite stalls in filesystem operations.
>>>> +     * Disallow attempts to pin fs-dax pages via this
>>>> +     * interface.
>>>> +     */
>>>> +    if (ret > 0 && vma_is_fsdax(vmas[0])) {
>>>> +            ret = -EOPNOTSUPP;
>>>> +            put_page(page[0]);
>>>>      }
>>>
>>> AFAIK this chunk is redundant now as it is some hack to emulate
>>> FOLL_LONGTERM? So vmas can be deleted too.
>>
>> Let me first make sure I understand what Dan has in mind for the vma
>> checking, in the other thread...
> 
> It's not redundant relative to upstream which does not do anything the
> FOLL_LONGTERM in the gup-slow path... but I have not looked at patches
> 1-7 to see if something there made it redundant.
> 

There is nothing in patches 1-7 that would make it redundant. 

About the only thing that you might find interesting in that subset is
patch 4 ("mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages"),
for devmap and ZONE_DEVICE interest. But it doesn't affect this
discussion directly.


thanks,
-- 
John Hubbard
NVIDIA