Re: [PATCH v1 1/1] mm/khugepaged: move tlb_remove_table_sync_one out from under PTL

[Baolin Wang]

On 1/15/26 8:28 PM, Lance Yang wrote:
> On 2026/1/15 18:00, Baolin Wang wrote:
> > Hi Lance,
> >
> > On 1/15/26 3:16 PM, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > >
> > > tlb_remove_table_sync_one() sends IPIs to all CPUs and waits for them,
> > > which we really don't want to do while holding PTL.
> >
> > Could you add more comments to explain why this is safe for the PAE case?
>
> Yep, IIUC, it is safe because we've already done pmdp_collapse_flush()
> which ensures the PMD change is visible.
>
> pmdp_get_lockless_sync() (which calls tlb_remove_table_sync_one() on PAE)
> is just to ensure any ongoing lockless pmd readers (e.g., GUP-fast) 
> complete
> before we proceed. It sends IPIs to all CPUs and waits for responses - a 
> CPU
> can only respond when it's not between local_irq_save() and 
> local_irq_restore().
>
> Moving it out from under PTL doesn't change the synchronization semantics,
> since lockless readers don't depend on PTL anyway.

Cc Hugh who introduced the pmdp_get_lockless_sync(), to double check.

Sounds reasonable to me, please add these comments into the commit 
message. Thanks.

> > For the non-PAE case, you added a new tlb_remove_table_sync_one(), why 
> > we need this (to solve what problem)? Please also add more comments to 
> > explain.
>
> Oops, you're right, the original macro was a no-op for non-PAE.
>
> I should just move the macro call out from under PTL, rather than
> replacing it with direct tlb_remove_table_sync_one() calls.

OK.

> > > Just move the call to after we release PTL, and drop the macro wrapper
> > > while we're at it.
> > >
> > > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > > ---
> > >   include/linux/pgtable.h | 4 ----
> > >   mm/khugepaged.c         | 5 +++--
> > >   2 files changed, 3 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > > index eb8aacba3698..fb04ed22052c 100644
> > > --- a/include/linux/pgtable.h
> > > +++ b/include/linux/pgtable.h
> > > @@ -755,7 +755,6 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp)
> > >       return pmd;
> > >   }
> > >   #define pmdp_get_lockless pmdp_get_lockless
> > > -#define pmdp_get_lockless_sync() tlb_remove_table_sync_one()
> > >   #endif /* CONFIG_PGTABLE_LEVELS > 2 */
> > >   #endif /* CONFIG_GUP_GET_PXX_LOW_HIGH */
> > > @@ -774,9 +773,6 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp)
> > >   {
> > >       return pmdp_get(pmdp);
> > >   }
> > > -static inline void pmdp_get_lockless_sync(void)
> > > -{
> > > -}
> > >   #endif
> > >   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 9f790ec34400..0a6cebf880e0 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1664,10 +1664,10 @@ static enum scan_result 
> > > try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
> > >           }
> > >       }
> > >       pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd);
> > > -    pmdp_get_lockless_sync();
> > >       pte_unmap_unlock(start_pte, ptl);
> > >       if (ptl != pml)
> > >           spin_unlock(pml);
> > > +    tlb_remove_table_sync_one();
> > >       mmu_notifier_invalidate_range_end(&range);
> > > @@ -1818,7 +1818,6 @@ static void retract_page_tables(struct 
> > > address_space *mapping, pgoff_t pgoff)
> > >            */
> > >           if (likely(file_backed_vma_is_retractable(vma))) {
> > >               pgt_pmd = pmdp_collapse_flush(vma, addr, pmd);
> > > -            pmdp_get_lockless_sync();
> > >               success = true;
> > >           }
> > > @@ -1826,6 +1825,8 @@ static void retract_page_tables(struct 
> > > address_space *mapping, pgoff_t pgoff)
> > >               spin_unlock(ptl);
> > >   drop_pml:
> > >           spin_unlock(pml);
> > > +        if (success)
> > > +            tlb_remove_table_sync_one();
> > >           mmu_notifier_invalidate_range_end(&range);