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Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN
To:     Jason Gunthorpe <jgg@ziepe.ca>, Jerome Glisse <jglisse@redhat.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
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From:   John Hubbard <jhubbard@nvidia.com>
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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