Re: [PATCH] mm: switch deferred split shrinker to list_lru

[Johannes Weiner]

On Wed, Mar 11, 2026 at 08:00:29PM +0300, Usama Arif wrote:
> On 11/03/2026 18:43, Johannes Weiner wrote:
> > @@ -3802,33 +3706,25 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
> >  	struct folio *new_folio, *next;
> >  	int old_order = folio_order(folio);
> >  	int ret = 0;
> > -	struct deferred_split *ds_queue;
> > +	struct list_lru_one *l;
> >  
> >  	VM_WARN_ON_ONCE(!mapping && end);
> >  	/* Prevent deferred_split_scan() touching ->_refcount */
> > -	ds_queue = folio_split_queue_lock(folio);
> > +	l = list_lru_lock(&deferred_split_lru, folio_nid(folio), folio_memcg(folio));
> 
> Hello Johannes!
> 
> I think we need folio_memcg() to be under rcu_read_lock()? 
> folio_memcg() calls obj_cgroup_memcg() which has lockdep_assert_once(rcu_read_lock_held()).
> 
> folio_split_queue_lock() wraps split_queue_lock() under rcu_read_lock() so wasnt an issue.

In this case, the caller has interrupts disabled, which implies an RCU
read-side critical section.

That's also why it's using the plain list_lru_lock(), not the irq
variant. Same for the lruvec lock a few lines down. I followed the
example of none of that being documented, either ;)

How do folks feel about a VM_WARN_ON(!irqs_disabled()) at the top?