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

[Harry Yoo <harry.yoo@oracle.com>]

On Tue, Oct 28, 2025 at 09:58:17PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@bytedance.com>
> 
> In a subsequent patch, we'll reparent the LRU folios. The folios that are
> moved to the appropriate LRU list can undergo reparenting during the
> move_folios_to_lru() process. Hence, it's incorrect for the caller to hold
> a lruvec lock. Instead, we should utilize the more general interface of
> folio_lruvec_relock_irq() to obtain the correct lruvec lock.
> 
> This patch involves only code refactoring and doesn't introduce any
> functional changes.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  mm/vmscan.c | 46 +++++++++++++++++++++++-----------------------
>  1 file changed, 23 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 3a1044ce30f1e..660cd40cfddd4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2016,9 +2016,9 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  	nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false,
>  					 lruvec_memcg(lruvec));
>  
> -	spin_lock_irq(&lruvec->lru_lock);
> -	move_folios_to_lru(lruvec, &folio_list);
> +	move_folios_to_lru(&folio_list);
>  
> +	spin_lock_irq(&lruvec->lru_lock);
>  	__mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(sc),
>  					stat.nr_demoted);

Maybe I'm missing something or just confused for now, but let me ask...

How do we make sure the lruvec (and the mem_cgroup containing the
lruvec) did not disappear (due to offlining) after move_folios_to_lru()?

>  	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
> @@ -2166,11 +2166,10 @@ static void shrink_active_list(unsigned long nr_to_scan,
>  	/*
>  	 * Move folios back to the lru list.
>  	 */
> -	spin_lock_irq(&lruvec->lru_lock);
> -
> -	nr_activate = move_folios_to_lru(lruvec, &l_active);
> -	nr_deactivate = move_folios_to_lru(lruvec, &l_inactive);
> +	nr_activate = move_folios_to_lru(&l_active);
> +	nr_deactivate = move_folios_to_lru(&l_inactive);
>  
> +	spin_lock_irq(&lruvec->lru_lock);
>  	__count_vm_events(PGDEACTIVATE, nr_deactivate);
>  	count_memcg_events(lruvec_memcg(lruvec), PGDEACTIVATE, nr_deactivate);
>  
> @@ -4735,14 +4734,15 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  			set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_active));
>  	}
>  
> -	spin_lock_irq(&lruvec->lru_lock);
> -
> -	move_folios_to_lru(lruvec, &list);
> +	move_folios_to_lru(&list);
>  
> +	local_irq_disable();
>  	walk = current->reclaim_state->mm_walk;
>  	if (walk && walk->batched) {
>  		walk->lruvec = lruvec;
> +		spin_lock(&lruvec->lru_lock);
>  		reset_batch_size(walk);
> +		spin_unlock(&lruvec->lru_lock);
>  	}

Cc'ing RT folks as they may not want to disable IRQ on PREEMPT_RT.

IIRC there has been some effort in MM to reduce the scope of
IRQ-disabled section in MM when PREEMPT_RT config was added to the
mainline. spin_lock_irq() doesn't disable IRQ on PREEMPT_RT.

Also, this will break RT according to Documentation/locking/locktypes.rst:
> The changes in spinlock_t and rwlock_t semantics on PREEMPT_RT kernels
> have a few implications. For example, on a non-PREEMPT_RT kernel
> the following code sequence works as expected:
>
> local_irq_disable();
> spin_lock(&lock);
>
> and is fully equivalent to:
>
> spin_lock_irq(&lock);
> Same applies to rwlock_t and the _irqsave() suffix variants.
>
> On PREEMPT_RT kernel this code sequence breaks because RT-mutex requires
> a fully preemptible context. Instead, use spin_lock_irq() or
> spin_lock_irqsave() and their unlock counterparts.
>
> In cases where the interrupt disabling and locking must remain separate,
> PREEMPT_RT offers a local_lock mechanism. Acquiring the local_lock pins
> the task to a CPU, allowing things like per-CPU interrupt disabled locks
> to be acquired. However, this approach should be used only where absolutely
> necessary.

-- 
Cheers,
Harry / Hyeonggon