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

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Message-ID: <20251112074930.mKu1J__U@linutronix.de>
Date: Wed, 12 Nov 2025 08:49:30 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Harry Yoo <harry.yoo@...cle.com>, 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>,
	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 2025-11-11 08:44:14 [-0800], Shakeel Butt wrote:
> On Tue, Nov 11, 2025 at 09:49:00AM +0100, Sebastian Andrzej Siewior wrote:
> > On 2025-11-11 12:16:43 [+0900], Harry Yoo wrote:
> > > > However, in the !CONFIG_HAVE_CMPXCHG_LOCAL case, mod_node_page_state()
> > > > still calls local_irq_save(). Is this feasible in the PREEMPT_RT kernel?
> > > 
> > > Hmm I was going to say it's necessary, but AFAICT we don't allocate
> > > or free memory in hardirq context on PREEMPT_RT (that's the policy)
> > > and so I'd say it's not necessary to disable IRQs.
> > > 
> > > Sounds like we still want to disable IRQs only on !PREEMPT_RT on
> > > such architectures?
> > > 
> > > Not sure how seriously do PREEMPT_RT folks care about architectures
> > > without HAVE_CMPXCHG_LOCAL. (riscv and loongarch have ARCH_SUPPORTS_RT
> > > but doesn't have HAVE_CMPXCHG_LOCAL).
> > 
> > We take things seriously and you shouldn't make assumption based on
> > implementation. Either the API can be used as such or not.
> > In case of mod_node_page_state(), the non-IRQ off version
> > (__mod_node_page_state()) has a preempt_disable_nested() to ensure
> > atomic update on PREEMPT_RT without disabling interrupts.
> > 
> 
> Harry is talking about mod_node_page_state() on
> !CONFIG_HAVE_CMPXCHG_LOCAL which is disabling irqs.
> 
> void mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
> 					long delta)
> {
> 	unsigned long flags;
> 
> 	local_irq_save(flags);
> 	__mod_node_page_state(pgdat, item, delta);
> 	local_irq_restore(flags);
> }
> 
> Is PREEMPT_RT fine with this?

Yes.
The local_irq_save() is not strictly needed but I am fine with it to
keep it simple. The inner part is just counting.

Sebastian