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

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Message-ID: <aRF7eYlBKmG3hEFF@hyeyoo>
Date: Mon, 10 Nov 2025 14:43:21 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: Shakeel Butt <shakeel.butt@...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 Mon, Nov 10, 2025 at 12:30:06PM +0800, Qi Zheng wrote:
> 
> 
> On 11/10/25 10:13 AM, Harry Yoo wrote:
> > On Fri, Nov 07, 2025 at 10:32:52PM -0800, Shakeel Butt wrote:
> > > 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.
> > 
> > Right.
> > 
> > > For __mod_lruvec_state, the
> > > __mod_node_page_state() inside needs preemption disabled.
> > 
> > The function __mod_node_page_state() disables preemption.
> > And there's a comment in __mod_zone_page_state():
> > 
> > > /*
> > >   * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels,
> > >   * atomicity is provided by IRQs being disabled -- either explicitly
> > >   * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables
> > >   * CPU migrations and preemption potentially corrupts a counter so
> > >   * disable preemption.
> > >   */
> > > preempt_disable_nested();
> > 
> > We're relying on IRQs being disabled on !PREEMPT_RT.
> 
> So it's possible for us to update vmstat within an interrupt context,
> right?

Yes, for instance when freeing memory in an interrupt context we can
update vmstat and that's why we disable interrupts now.

> There is also a comment above __mod_zone_page_state():
> 
> /*
>  * For use when we know that interrupts are disabled,
>  * or when we know that preemption is disabled and that
>  * particular counter cannot be updated from interrupt context.
>  */

Yeah we don't have to disable IRQs when we already know it's disabled.

> BTW, the comment inside __mod_node_page_state() should be:
> 
> /* See __mod_zone_page_state */
> 
> instead of
> 
> /* See __mod_node_page_state */
> 
> Will fix it.

Right :) Thanks!

> > 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.

-- 
Cheers,
Harry / Hyeonggon