On Mon, Nov 04, 2019 at 12:57:59PM -0800, John Hubbard wrote:
> 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".
But AFAICT it doesn't have a problem, the protection test is just too
strict, and I guess the control flow needs a bit of fixing..
The issue is this:
static __always_inline long __get_user_pages_locked():
{
if (locked) {
/* if VM_FAULT_RETRY can be returned, vmas become invalid */
BUG_ON(vmas);
/* check caller initialized locked */
BUG_ON(*locked != 1);
}
so remote could be written as:
if (gup_flags & FOLL_LONGTERM) {
if (WARN_ON_ONCE(locked))
return -EINVAL;
return __gup_longterm_locked(...)
}
return __get_user_pages_locked(...)
??
Jason
