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

[Sebastian Andrzej Siewior]

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.

Sebastian