linux-kernel - Re: [PATCH v1 04/26] mm: vmscan: refactor move_folios_to

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Message-ID: <2a68bddf-e6e6-4960-b5bc-1a39d747ea9b@linux.dev>
Date: Mon, 10 Nov 2025 12:30:06 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>, Shakeel Butt <shakeel.butt@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
 roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...hat.com,
 lorenzo.stoakes@...cle.com, ziy@...dia.com, imran.f.khan@...cle.com,
 kamalesh.babulal@...cle.com, axelrasmussen@...gle.com, yuanchu@...gle.com,
 weixugc@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 Muchun Song <songmuchun@...edance.com>, Qi Zheng
 <zhengqi.arch@...edance.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH v1 04/26] mm: vmscan: refactor move_folios_to_lru()



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?

>