Re: [PATCH v4 08/10] userfaultfd: add UFFDIO_CONTINUE ioctl

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 10, 2021 at 10:00:21AM -0800, Axel Rasmussen wrote:
> > >  static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> > > @@ -417,10 +416,14 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> > >                                               unsigned long dst_addr,
> > >                                               unsigned long src_addr,
> > >                                               struct page **page,
> > > -                                             bool zeropage,
> > > +                                             enum mcopy_atomic_mode mode,
> > >                                               bool wp_copy)
> > >  {
> > >       ssize_t err;
> > > +     bool zeropage = (mode == MCOPY_ATOMIC_ZEROPAGE);
> > > +
> > > +     if (mode == MCOPY_ATOMIC_CONTINUE)
> > > +             return -EINVAL;
> >
> > So you still passed in the mode into mfill_atomic_pte() just to make sure
> > CONTINUE is not called there.  It's okay, but again I think it's not extremely
> > necessary: we should make sure to fail early at the entry of uffdio_continue()
> > by checking against the vma type to be hugetlb, rather than reaching here.
> 
> Hmm, it's not quite as simple as that. We don't have the dst_vma yet
> in uffdio_continue(), __mcopy_atomic looks it up.
> 
> I'd prefer not to look it up in uffdio_continue(), because I think
> that means changing the API so all the ioctls look up the vma, and
> then plumb it into __mcopy_atomic. (We don't want to look it up twice,
> since each lookup has to traverse the rbtree.) This is complicated too
> by the fact that the ioctl handlers would need to perform various
> validation / checks - e.g., acquiring mmap_lock, dealing with
> *mmap_changing, validating the range, ....

Sure.

> 
> We can move the enforcement up one more layer, into __mcopy_atomic,
> easily enough, though.

Right, that sounds good to me.  It should be right after the "if
(!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))" check.

Thanks,

-- 
Peter Xu