On 11/4/19 12:37 PM, Jason Gunthorpe wrote:
> On Mon, Nov 04, 2019 at 03:31:53PM -0500, Jerome Glisse wrote:
>>> Note for Jason: the (a) or (b) items are talking about the vfio case, which is
>>> one of the two call sites that now use pin_longterm_pages_remote(), and the
>>> other one is infiniband:
>>>
>>> drivers/infiniband/core/umem_odp.c:646: npages = pin_longterm_pages_remote(owning_process, owning_mm,
>>> drivers/vfio/vfio_iommu_type1.c:353: ret = pin_longterm_pages_remote(NULL, mm, vaddr, 1,
>>
>> vfio should be reverted until it can be properly implemented.
>> The issue is that when you fix the implementation you might
>> break vfio existing user and thus regress the kernel from user
>> point of view. So i rather have the change to vfio reverted,
>> i believe it was not well understood when it got upstream,
>> between in my 5.4 tree it is still gup_remote not longterm.
>
> It is clearly a bug, vfio must use LONGTERM, and does right above this
> remote call:
>
> if (mm == current->mm) {
> ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
> vmas);
> } else {
> ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
> vmas, NULL);
>
>
> I'm not even sure that it really makes any sense to build a 'if' like
> that, surely just always call remote??
>
Right, and I thought about this when converting, and realized that the above
code is working around the current gup.c limitations, which are "cannot support
gup remote with FOLL_LONGTERM".
Given that observation, the code is getting itself some FOLL_LONGTERM support
for the non-remote case, and only hitting the limitation if the mm really is
non-current.
And if you look at my patch, it keeps the same behavior, while adding in the
new wrapper calls.
So...thoughts, preferences?
thanks,
John Hubbard
NVIDIA
