Re: [PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge_page()

[David Hildenbrand (Red Hat)]

On 12/18/25 15:35, Lance Yang wrote:
> On 2025/12/18 21:13, David Hildenbrand (Red Hat) wrote:
> > On 12/13/25 09:00, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > >
> > > Similar to the hugetlb PMD unsharing optimization, skip the second IPI
> > > in collapse_huge_page() when the TLB flush already provides necessary
> > > synchronization.
> > >
> > > Before commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE
> > > unconditional"), bare metal x86 didn't enable MMU_GATHER_RCU_TABLE_FREE.
> > > In that configuration, tlb_remove_table_sync_one() was a NOP. GUP-fast
> > > synchronization relied on IRQ disabling, which blocks TLB flush IPIs.
> > >
> > > When Rik made MMU_GATHER_RCU_TABLE_FREE unconditional to support AMD's
> > > INVLPGB, all x86 systems started sending the second IPI. However, on
> > > native x86 this is redundant:
> > >
> > >     - pmdp_collapse_flush() calls flush_tlb_range(), sending IPIs to all
> > >       CPUs to invalidate TLB entries
> > >
> > >     - GUP-fast runs with IRQs disabled, so when the flush IPI completes,
> > >       any concurrent GUP-fast must have finished
> > >
> > >     - tlb_remove_table_sync_one() provides no additional synchronization
> > >
> > > On x86, skip the second IPI when running native (without paravirt) and
> > > without INVLPGB. For paravirt with non-native flush_tlb_multi and for
> > > INVLPGB, conservatively keep both IPIs.
> > >
> > > Use tlb_table_flush_implies_ipi_broadcast(), consistent with the hugetlb
> > > optimization.
> > >
> > > Suggested-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>
> > > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > > ---
> > >    mm/khugepaged.c | 7 ++++++-
> > >    1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 97d1b2824386..06ea793a8190 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1178,7 +1178,12 @@ static int collapse_huge_page(struct mm_struct
> > > *mm, unsigned long address,
> > >        _pmd = pmdp_collapse_flush(vma, address, pmd);
> > >        spin_unlock(pmd_ptl);
> > >        mmu_notifier_invalidate_range_end(&range);
> > > -    tlb_remove_table_sync_one();
> > > +    /*
> > > +     * Skip the second IPI if the TLB flush above already synchronized
> > > +     * with concurrent GUP-fast via broadcast IPIs.
> > > +     */
> > > +    if (!tlb_table_flush_implies_ipi_broadcast())
> > > +        tlb_remove_table_sync_one();
> >
> > We end up calling
> >
> >       flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> >
> >       -> flush_tlb_mm_range(freed_tables = true)
> >
> >       -> flush_tlb_multi(mm_cpumask(mm), info);
> >
> > So freed_tables=true and we should be doing the right thing.
>
> Yep ;)
>
> > BTW, I was wondering whether we should embed that
> > tlb_table_flush_implies_ipi_broadcast() check in
> > tlb_remove_table_sync_one() instead.
> > It then relies on the caller to do the right thing (flush with
> > freed_tables=true or unshared_tables = true).
> >
> > Thoughts?
>
> Good point! Let me check the other callers to ensure they
> are all preceded by a flush with freed_tables=true (or unshared_tables).
>
> Will get back to you with what I find :)

The use case in tlb_table_flush() is a bit confusing. But I would assume 
that we have a TLB flush with remove_tables=true beforehand. Otherwise 
we cannot possibly free the page table.

--
Cheers

David