linux-kernel - Re: [PATCH v1 1/1] mm/khugepaged: move tlb_remove_table_sync

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Message-ID: <62e637cf-91e6-454d-a943-e5946bdf7784@linux.dev>
Date: Fri, 16 Jan 2026 09:25:54 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: ioworker0@...il.com, lorenzo.stoakes@...cle.com, david@...nel.org,
 akpm@...ux-foundation.org, Liam.Howlett@...cle.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, ziy@...dia.com,
 npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
 baohua@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 "hughd@...gle.com" <hughd@...gle.com>
Subject: Re: [PATCH v1 1/1] mm/khugepaged: move tlb_remove_table_sync_one out
 from under PTL



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@...ux.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