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

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

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?

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.
  */

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.

> 
> 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?

>