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

Shakeel Butt
2025-11-11T16:44:31Z

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?