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

Shakeel Butt
2025-11-11T03:17:38Z

On Tue, Nov 11, 2025 at 11:04:09AM +0800, Qi Zheng wrote:
>
> On 11/11/25 12:47 AM, Shakeel Butt wrote:
> > On Mon, Nov 10, 2025 at 02:43:21PM +0900, Harry Yoo wrote:
> > > On Mon, Nov 10, 2025 at 12:30:06PM +0800, Qi Zheng wrote:
> > > > > 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?
> > >
> > > I meant using atomic operations instead of disabling IRQs, like, by
> > > using this_cpu_add() or cmpxchg() instead.
> >
> > We already have mod_node_page_state() which is safe from IRQs and is
> > optimized to not disable IRQs for archs with HAVE_CMPXCHG_LOCAL which
> > includes x86 and arm64.
>
> 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?
>

Yes we can disable irqs on PREEMPT_RT but it is usually frown upon and
it is usually requested to do so only for short window. However if
someone running PREEMPT_RT on an arch without HAVE_CMPXCHG_LOCAL and has
issues with mod_node_page_state() then they can solve it then. I don't
think we need to fix that now.