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

[Dan Williams]

On Tue, Nov 12, 2019 at 3:43 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Tue, Nov 12, 2019 at 02:45:51PM -0800, Dan Williams wrote:
> > On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <jhubbard@xxxxxxxxxx> 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.

Oh true, good eye. It is redundant if it does additionally call
__gup_longterm_locked(), and it needs to do that otherwises it undoes
the CMA migration magic that Aneesh added.

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

Agree, that was my comment about cleaning up the vma_is_fsdax() check
to be internal to the gup core.