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

[Yang Shi]

On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 31.08.22 21:08, Yang Shi wrote:
> > On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 31.08.22 19:55, Yang Shi wrote:
> >>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand <david@xxxxxxxxxx> 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.

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