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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Hi Lance,

On 1/15/26 3:16 PM, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
> 
> 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?

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.

> 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@linux.dev>
> ---
>   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);
>