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

From: Jason Gunthorpe <[email-masked]>
Date: Tue, 12 Nov 2019 19:42:50 -0400

On Tue, Nov 12, 2019 at 02:45:51PM -0800, Dan Williams wrote:
> On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <[email-masked]> 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.

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.

Certainly no caller of FOLL_LONGTERM should have to do dax specific
VMA checking.

Jason