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

[Dan Williams]

On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <jhubbard at 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.