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

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

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@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?
> 
> 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@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);
>>