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

[Lance Yang <lance.yang@linux.dev>]

On 2026/1/16 09:03, Baolin Wang wrote:
> 
> 
> 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.

Yes, will do. 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.

Cheers,
Lance