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

Dan Williams
2019-11-12T22:46:04Z

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.