Re: [PATCH v1 04/26] mm: vmscan: refactor move_folios_to_lru()

[Qi Zheng <qi.zheng@linux.dev>]

On 11/10/25 1:43 PM, Harry Yoo wrote:
> On Mon, Nov 10, 2025 at 12:30:06PM +0800, Qi Zheng wrote:
>>
>>
>> On 11/10/25 10:13 AM, Harry Yoo wrote:
>>> On Fri, Nov 07, 2025 at 10:32:52PM -0800, Shakeel Butt wrote:
>>>> On Fri, Nov 07, 2025 at 10:20:57PM +0900, Harry Yoo wrote:
>>>>>
>>>>> Although it's mentioned in the locking documentation, I'm afraid that
>>>>> local_lock is not the right interface to use here. Preemption will be
>>>>> disabled anyway (on both PREEMPT_RT and !PREEMPT_RT) when the stats are
>>>>> updated (in __mod_node_page_state()).
>>>>>
>>>>> Here we just want to disable IRQ only on !PREEMPT_RT (to update
>>>>> the stats safely).
>>>>
>>>> I don't think there is a need to disable IRQs. There are three stats
>>>> update functions called in that hunk.
>>>>
>>>> 1) __mod_lruvec_state
>>>> 2) __count_vm_events
>>>> 3) count_memcg_events
>>>>
>>>> count_memcg_events() can be called with IRQs. __count_vm_events can be
>>>> replaced with count_vm_events.
>>>
>>> Right.
>>>
>>>> For __mod_lruvec_state, the
>>>> __mod_node_page_state() inside needs preemption disabled.
>>>
>>> The function __mod_node_page_state() disables preemption.
>>> And there's a comment in __mod_zone_page_state():
>>>
>>>> /*
>>>>    * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels,
>>>>    * atomicity is provided by IRQs being disabled -- either explicitly
>>>>    * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables
>>>>    * CPU migrations and preemption potentially corrupts a counter so
>>>>    * disable preemption.
>>>>    */
>>>> preempt_disable_nested();
>>>
>>> We're relying on IRQs being disabled on !PREEMPT_RT.
>>
>> So it's possible for us to update vmstat within an interrupt context,
>> right?
> 
> Yes, for instance when freeing memory in an interrupt context we can
> update vmstat and that's why we disable interrupts now.

Got it.

> 
>> There is also a comment above __mod_zone_page_state():
>>
>> /*
>>   * For use when we know that interrupts are disabled,
>>   * or when we know that preemption is disabled and that
>>   * particular counter cannot be updated from interrupt context.
>>   */
> 
> Yeah we don't have to disable IRQs when we already know it's disabled.
> 
>> BTW, the comment inside __mod_node_page_state() should be:
>>
>> /* See __mod_zone_page_state */
>>
>> instead of
>>
>> /* See __mod_node_page_state */
>>
>> Will fix it.
> 
> Right :) Thanks!
> 
>>> Maybe we could make it safe against re-entrant IRQ handlers by using
>>> read-modify-write operations?
>>
>> Isn't it because of the RMW operation that we need to use IRQ to
>> guarantee atomicity? Or have I misunderstood something?
> 
> I meant using atomic operations instead of disabling IRQs, like, by
> using this_cpu_add() or cmpxchg() instead.

Got it. I will give it a try.

Thanks,
Qi

>