Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page()

[Yang Shi <shy828301@gmail.com>]

On Wed, Aug 31, 2022 at 11:58 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 01.09.22 00:18, Yang Shi wrote:
> > On Wed, Aug 31, 2022 at 12:43 PM Yang Shi <shy828301@gmail.com> wrote:
> >>
> >> On Wed, Aug 31, 2022 at 12:36 PM David Hildenbrand <david@redhat.com> wrote:
> >>>
> >>> On 31.08.22 21:34, Yang Shi wrote:
> >>>> On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand <david@redhat.com> wrote:
> >>>>>
> >>>>> On 31.08.22 21:08, Yang Shi wrote:
> >>>>>> On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand <david@redhat.com> wrote:
> >>>>>>>
> >>>>>>> On 31.08.22 19:55, Yang Shi wrote:
> >>>>>>>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand <david@redhat.com> wrote:
> >>>>>>>>>
> >>>>>>>>> The comment is stale, because a TLB flush is no longer sufficient and
> >>>>>>>>> required to synchronize against concurrent GUP-fast. This used to be true
> >>>>>>>>> in the past, whereby a TLB flush would have implied an IPI on architectures
> >>>>>>>>> that support GUP-fast, resulting in GUP-fast that disables local interrupts
> >>>>>>>>> from completing before completing the flush.
> >>>>>>>>
> >>>>>>>> Hmm... it seems there might be problem for THP collapse IIUC. THP
> >>>>>>>> collapse clears and flushes pmd before doing anything on pte and
> >>>>>>>> relies on interrupt disable of fast GUP to serialize against fast GUP.
> >>>>>>>> But if TLB flush is no longer sufficient, then we may run into the
> >>>>>>>> below race IIUC:
> >>>>>>>>
> >>>>>>>>          CPU A                                                CPU B
> >>>>>>>> THP collapse                                             fast GUP
> >>>>>>>>
> >>>>>>>> gup_pmd_range() <-- see valid pmd
> >>>>>>>>
> >>>>>>>> gup_pte_range() <-- work on pte
> >>>>>>>> clear pmd and flush TLB
> >>>>>>>> __collapse_huge_page_isolate()
> >>>>>>>>     isolate page <-- before GUP bump refcount
> >>>>>>>>
> >>>>>>>>    pin the page
> >>>>>>>> __collapse_huge_page_copy()
> >>>>>>>>     copy data to huge page
> >>>>>>>>     clear pte (don't flush TLB)
> >>>>>>>> Install huge pmd for huge page
> >>>>>>>>
> >>>>>>>> return the obsolete page
> >>>>>>>
> >>>>>>> Hm, the is_refcount_suitable() check runs while the PTE hasn't been
> >>>>>>> cleared yet. And we don't check if the PMD changed once we're in
> >>>>>>> gup_pte_range().
> >>>>>>
> >>>>>> Yes
> >>>>>>
> >>>>>>>
> >>>>>>> The comment most certainly should be stale as well -- unless there is
> >>>>>>> some kind of an implicit IPI broadcast being done.
> >>>>>>>
> >>>>>>> 2667f50e8b81 mentions: "The RCU page table free logic coupled with an
> >>>>>>> IPI broadcast on THP split (which is a rare event), allows one to
> >>>>>>> protect a page table walker by merely disabling the interrupts during
> >>>>>>> the walk."
> >>>>>>>
> >>>>>>> I'm not able to quickly locate that IPI broadcast -- maybe there is one
> >>>>>>> being done here (in collapse) as well?
> >>>>>>
> >>>>>> The TLB flush may call IPI. I'm supposed it is arch dependent, right?
> >>>>>> Some do use IPI, some may not.
> >>>>>
> >>>>> Right, and the whole idea of the RCU GUP-fast was to support
> >>>>> architectures that don't do it. x86-64 does it. IIRC, powerpc doesn't do
> >>>>> it -- but maybe it does so for PMDs?
> >>>>
> >>>> It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC.
> >>>>
> >>>> So maybe we should implement pmdp_collapse_flush() for those arches to
> >>>> issue IPI.
> >>>
> >>> ... or find another way to detect and handle this in GUP-fast?
> >>>
> >>> Not sure if, for handling PMDs, it could be sufficient to propagate the
> >>> pmdp pointer + value and double check that the values didn't change.
> >>
> >> Should work too, right before pinning the page.
> >
> > I actually mean the same place for checking pte. So, something like:
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 5abdaf487460..2b0703403902 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2392,7 +2392,8 @@ static int gup_pte_range(pmd_t pmd, unsigned
> > long addr, unsigned long end,
> >                         goto pte_unmap;
> >                 }
> >
> > -               if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> > +               if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
> > +                   unlikely(pte_val(pte) != pte_val(*ptep))) {
> >                         gup_put_folio(folio, 1, flags);
> >                         goto pte_unmap;
> >                 }
> >
> > It doesn't build, just shows the idea.
>
> Exactly what I had in mind. We should add a comment spelling out that
> this is for handling huge PMD collapse.

Yeah, I will prepare a patch soon.

>
>
> --
> Thanks,
>
> David / dhildenb
>