Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 23, 2020 at 02:07:59PM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 22, 2020 at 08:27:35PM -0400, Peter Xu wrote:
> > On Tue, Sep 22, 2020 at 04:11:16PM -0300, Jason Gunthorpe wrote:
> > > On Tue, Sep 22, 2020 at 01:54:15PM -0400, Peter Xu wrote:
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 8f3521be80ca..6591f3f33299 100644
> > > > +++ b/mm/memory.c
> > > > @@ -888,8 +888,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > >                  * Because we'll need to release the locks before doing cow,
> > > >                  * pass this work to upper layer.
> > > >                  */
> > > > -               if (READ_ONCE(src_mm->has_pinned) && wp &&
> > > > -                   page_maybe_dma_pinned(page)) {
> > > > +               if (wp && page_maybe_dma_pinned(page) &&
> > > > +                   READ_ONCE(src_mm->has_pinned)) {
> > > >                         /* We've got the page already; we're safe */
> > > >                         data->cow_old_page = page;
> > > >                         data->cow_oldpte = *src_pte;
> > > > 
> > > > I can also add some more comment to emphasize this.
> > > 
> > > It is not just that, but the ptep_set_wrprotect() has to be done
> > > earlier.
> > 
> > Now I understand your point, I think..  So I guess it's not only about
> > has_pinned, but it should be a race between the fast-gup and the fork() code,
> > even if has_pinned is always set.
> 
> Yes
> 
> > > The best algorithm I've thought of is something like:
> > > 
> > >  pte_map_lock()
> > >   if (page) {
> > >       if (wp) {
> > > 	  ptep_set_wrprotect()
> > > 	  /* Order with try_grab_compound_head(), either we see
> > > 	   * page_maybe_dma_pinned(), or they see the wrprotect */
> > > 	  get_page();
> > 
> > Is this get_page() a must to be after ptep_set_wrprotect()
> > explicitly?  
> 
> No, just before page_maybe_dma_pinned()
> 
> > IIUC what we need is to order ptep_set_wrprotect() and
> > page_maybe_dma_pinned() here.  E.g., would a "mb()" work?
> 
> mb() is not needed because page_maybe_dma_pinned() has an atomic
> barrier too. I like to see get_page() followed immediately by
> page_maybe_dma_pinned() since they are accessing the same atomic and
> could be fused together someday

If so, I'd hope you won't disagree that I still move the get_page() out of the
"if (wp)".  Not only it's a shared operation no matter whether "if (wp)" or
not, but I'm afraid it would confuse future readers on a special ordering on
the get_page() and the wrprotect(), especially with the comment above.

> 
> > Another thing is, do we need similar thing for e.g. gup_pte_range(), so that
> > to guarantee ordering of try_grab_compound_head() and the pte change
> > check?
> 
> gup_pte_range() is as I quoted? The gup slow path ends up in
> follow_page_pte() which uses the pte lock so is OK.
> > 
> > Another question is, how about read fast-gup for pinning?  Because we can't use
> > the write-protect mechanism to block a read gup.  I remember we've discussed
> > similar things and iirc your point is "pinned pages should always be with
> > WRITE".  However now I still doubt it...  Because I feel like read gup is still
> > legal (as I mentioned previously - when device purely writes to the page and
> > the processor only reads from it).
> 
> We need a definition for what FOLL_PIN means. After this work on fork
> I propose that FOLL_PIN means:
> 
>   The page is in-use for DMA and the CPU PTE should not be changed
>   without explicit involvement of the application (eg via mmap/munmap)
> 
> If GUP encounters a read-only page during FOLL_PIN the behavior should
> depend on what the fault handler would do. If the fault handler would
> trigger COW and replace the PTE then it violates the above. GUP should
> do the COW before pinning.
> 
> If the fault handler would SIGSEGV then GUP can keep the read-only
> page and allow !FOLL_WRITE access. The PTE should not be replaced for
> other reasons (though I think there is work there too).
> 
> For COW related issues the idea is the mm_struct doing the pin will
> never trigger a COW. When other processes hit the COW they copy the
> page into their mm and don't touch the source MM's PTE.
> 
> Today we do this roughly with FOLL_FORCE and FOLL_WRITE in the users,
> but a more nuanced version and documentation would be much clearer.
> 
> Unfortunately just doing simple read GUP potentially exposes things to
> various COW related data corruption races.
> 
> This is a discussion beyond this series though..

Yes.  It's kind of related here on whether we can still use wrprotect() to
guard against fast-gup, though.  So my understanding is that we should still at
least need the other patch [1] that I proposed in the other thread to force
break-cow for read-only gups (that patch is not only for fast-gup, of course).

But I agree that should be another bigger topic.  I hope we don't need to pick
that patch up someday by another dma report on read-only pinned pages...

Regarding the solution here, I think we can also cover read-only fast-gup too
in the future - IIUC what we need to do is to make it pte_protnone() instead of
pte_wrprotect(), then in the fault handler we should identify this special
pte_protnone() against numa balancing (change_prot_numa()).  I think it should
work fine too, iiuc, because I don't think we should migrate a page at all if
it's pinned for any reason...

So I think I'll focus on the wrprotect() solution for now.  Thanks!

[1] https://lore.kernel.org/lkml/20200915151746.GB2949@xz-x1/

-- 
Peter Xu