Re: [PATCH v5 4/4] mm: thp: reparent the split queue during memcg offline

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

On Tue, Oct 21, 2025 at 02:21:55PM +0800, Qi Zheng wrote:
> 
> 
> On 10/21/25 2:09 PM, Harry Yoo wrote:
> > On Wed, Oct 15, 2025 at 02:35:33PM +0800, Qi Zheng wrote:
> > > From: Qi Zheng <zhengqi.arch@bytedance.com>
> > > 
> > > Similar to list_lru, the split queue is relatively independent and does
> > > not need to be reparented along with objcg and LRU folios (holding
> > > objcg lock and lru lock). So let's apply the similar mechanism as list_lru
> > > to reparent the split queue separately when memcg is offine.
> > > 
> > > This is also a preparation for reparenting LRU folios.
> > > 
> > > Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> > > Acked-by: Zi Yan <ziy@nvidia.com>
> > > Reviewed-by: Muchun Song <muchun.song@linux.dev>
> > > Acked-by: David Hildenbrand <david@redhat.com>
> > > Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> > > ---
> > 
> > Looks good to me,
> > Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> 
> Thanks.
> 
> > 
> > with a question:
> > 
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index e850bc10da3e2..9323039418201 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -1117,8 +1117,19 @@ static struct deferred_split *split_queue_lock(int nid, struct mem_cgroup *memcg
> > >   {
> > >   	struct deferred_split *queue;
> > > +retry:
> > >   	queue = memcg_split_queue(nid, memcg);
> > >   	spin_lock(&queue->split_queue_lock);
> > > +	/*
> > > +	 * There is a period between setting memcg to dying and reparenting
> > > +	 * deferred split queue, and during this period the THPs in the deferred
> > > +	 * split queue will be hidden from the shrinker side.
> > > +	 */
> > 
> > You mean it will be hidden if the shrinker bit is not set for the node
> > in the parent memcg, right?
> 
> Look at the following situation:
> 
> CPU 0                   CPU 1
> -----                   -----
> 
> set CSS_DYING
>                         deferred_split_scan
>                             /*
>                              * See CSS_DYING, and return the parent
>                              * memcg's ds_queue. But the pages on the
>                              * child memcg's ds_queue has not yet been
>                              * reparented to the parent memcg, that is,
>                              * it is hidden.
>                              */
>                         --> ds_queue = split_queue_lock_irqsave()
> 
> reparent_deferred_split_queue

Ah, I see what you meant. Thanks.

So we may end up shrinking the parent memcg twice if it's
hidden, but I guess it's fine as it'll be rare?

> Thanks,
> Qi

-- 
Cheers,
Harry / Hyeonggon