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

[Qi Zheng <qi.zheng@linux.dev>]

Hi Harry,

On 11/7/25 1:11 PM, Harry Yoo wrote:
> 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()?

We obtained lruvec through the following method:

memcg = mem_cgroup_iter(target_memcg, NULL, partial);
do {
     struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);

     shrink_lruvec(lruvec, sc);
     --> shrink_inactive_list
} while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));

The mem_cgroup_iter() will hold the refcount of this memcg, so IIUC,
the memcg will not disappear at this time.

> 
>>   	__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.

Thanks for this information.

> 
> 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.

But how do we determine if it's necessary?

Thanks,
Qi

>