Re: [PATCH v3 7/9] userfaultfd: add UFFDIO_CONTINUE ioctl

[Peter Xu]

On Mon, Feb 01, 2021 at 02:11:55PM -0800, Axel Rasmussen wrote:
> On Mon, Feb 1, 2021 at 11:21 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > On Thu, Jan 28, 2021 at 02:48:17PM -0800, Axel Rasmussen wrote:
> > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > > index f94a35296618..79e1f0155afa 100644
> > > --- a/include/linux/hugetlb.h
> > > +++ b/include/linux/hugetlb.h
> > > @@ -135,11 +135,14 @@ void hugetlb_show_meminfo(void);
> > >  unsigned long hugetlb_total_pages(void);
> > >  vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > >                       unsigned long address, unsigned int flags);
> > > +#ifdef CONFIG_USERFAULTFD
> >
> > I'm confused why this is needed.. hugetlb_mcopy_atomic_pte() should only be
> > called in userfaultfd.c, but if without uffd config set it won't compile
> > either:
> >
> >         obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
> 
> With this series as-is, but *without* the #ifdef CONFIG_USERFAULTFD
> here, we introduce a bunch of build warnings like this:
> 
> 
> 
> In file included from ./include/linux/migrate.h:8:0,
>                  from kernel/sched/sched.h:53,
>                  from kernel/sched/isolation.c:10:
> ./include/linux/hugetlb.h:143:12: warning: 'enum mcopy_atomic_mode'
> declared inside parameter list
>      struct page **pagep);
>             ^
> ./include/linux/hugetlb.h:143:12: warning: its scope is only this
> definition or declaration, which is probably not what you want
> 
> And similarly we get an error about the "mode" parameter having an
> incomplete type in hugetlb.c.
> 
> 
> 
> This is because enum mcopy_atomic_mode is defined in userfaultfd_k.h,
> and that entire header is wrapped in a #ifdef CONFIG_USERFAULTFD. So
> we either need to define enum mcopy_atomic_mode unconditionally, or we
> need to #ifdef CONFIG_USERFAULTFD the references to it also.
> 
> - I opted not to move it outside the #ifdef CONFIG_USERFAULTFD in
> userfaultfd_k.h (defining it unconditionally), because that seemed
> messy to me.
> - I opted not to define it unconditionally in hugetlb.h, because we'd
> have to move it to userfaultfd_k.h anyway when shmem or other users
> are introduced. I'm planning to send a series to add this a few days
> or so after this series is merged, so it seems churn-y to move it
> then.
> - It seemed optimal to not compile hugetlb_mcopy_atomic_pte anyway
> (even ignoring adding the continue ioctl), since as you point out
> userfaultfd is the only caller.
> 
> Hopefully this clarifies this and the next two comments. Let me know
> if you still feel strongly, I don't hate any of the alternatives, just
> wanted to clarify that I had considered them and thought this approach
> was best.

Then I'd suggest you use a standalone patch to put hugetlb_mcopy_atomic_pte()
into CONFIG_USERFAULTFD blocks, then propose your change with the minor mode.
Note that there're two hugetlb_mcopy_atomic_pte() defined in hugetlb.h.
Although I don't think it a problem since the other one is inlined - I think
you should still put that one into the same ifdef:

#ifdef CONFIG_USERFAULTFD
static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
						pte_t *dst_pte,
						struct vm_area_struct *dst_vma,
						unsigned long dst_addr,
						unsigned long src_addr,
						struct page **pagep)
{
	BUG();
	return 0;
}
#endif /* CONFIG_USERFAULTFD */

Let's also see whether Mike would have a preference on this.

Thanks,

-- 
Peter Xu