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

John Hubbard
2019-11-13T02:09:20Z

On 11/12/19 5:35 PM, Dan Williams wrote:
> On Tue, Nov 12, 2019 at 5:08 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> On 11/12/19 4:58 PM, Dan Williams wrote:
>> ...
>>>>> 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.
>>>>
>>>> Oh, the hunk John had below for get_user_pages_remote() also needs to
>>>> call __gup_longterm_locked() when FOLL_LONGTERM is specified, then
>>>> that calls check_dax_vmas() which duplicates the vma_is_fsdax() check
>>>> above.
>>>
>>> Oh true, good eye. It is redundant if it does additionally call
>>> __gup_longterm_locked(), and it needs to do that otherwises it undoes
>>> the CMA migration magic that Aneesh added.
>>>
>>
>> OK. So just to be clear, I'll be removing this from the patch:
>>
>> /*
>> * 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]);
>> }
>>
>> (and the declaration of "vmas", as well).
>
> ...and add a call to __gup_longterm_locked internal to
> get_user_pages_remote(), right?
>

Yes, and thanks for double-checking. I think I got a little dizzy following
the call stack there. :) And now I see that this also affects the
implementation of pin_longterm_pages_remote(), because that will need the
same logic that get_user_pages_remote() has.

thanks,
--
John Hubbard
NVIDIA