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

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Message-ID: <hfutmuh4g5jtmrgeemq2aqr2tvxz6mnqaxo5l5vddqnjasyagi@gcscu5khrjxm>
Date: Fri, 7 Nov 2025 22:32:52 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Qi Zheng <qi.zheng@...ux.dev>, 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 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. For __mod_lruvec_state, the
__mod_node_page_state() inside needs preemption disabled.

Easy way would be to just disable/enable preemption instead of IRQs.
Otherwise go a bit more fine-grained approach i.e. replace
__count_vm_events with count_vm_events and just disable preemption
across __mod_node_page_state().