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

[Yang Shi <shy828301@gmail.com>]

On Wed, Aug 31, 2022 at 11:52 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Aug 31, 2022 at 10:55:43AM -0700, 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
>
> Maybe the pmd level tlb flush is still needed, but on pte level it's
> optional (where we can rely on fast-gup rechecking on the pte change)?

Do you mean in khugepaged? It does TLB flush, but some arches may not use IPI.

>
> --
> Peter Xu
>