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

John Hubbard
2019-11-12T23:17:37Z

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