Re: [PATCH v2 2/2] mm/huge_memory: dequeue the deferred split after the split freeze

[Kiryl Shutsemau <kirill@shutemov.name>]

On Mon, Sep 07, 2026 at 03:55:53PM +0200, David Hildenbrand (Arm) wrote:
> On 8/31/26 11:15, Kiryl Shutsemau wrote:
> > From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> > 
> > __folio_freeze_and_split_unmapped() takes the deferred split list_lru lock
> > across the freeze. It is only there to stop deferred_split_scan() from
> > touching the folio under split.
> > 
> > With deferred_split_isolate() fixed, the workaround can be dropped.
> > 
> > Unqueue the folio after folio_ref_freeze(), the way
> > __folio_migrate_mapping() does: folio_unqueue_deferred_split() needs a
> > zero refcount and a memcg still set, and both hold there.
> > 
> > If the split is called from deferred_split_scan(), the unqueue is a
> > no-op -- the folio is already removed from the list. But
> > PG_partially_mapped is still set, so it has to be cleared here or
> > MTHP_STAT_NR_ANON_PARTIALLY_MAPPED never comes back down.
> > 
> > Assisted-by: Claude-Code:claude-opus-5
> > Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> > Reviewed-by: Zi Yan <ziy@nvidia.com>
> > Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
> > Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> > ---
> >  mm/huge_memory.c | 44 +++++++++++++-------------------------------
> >  1 file changed, 13 insertions(+), 31 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 6281ed993243..c84e8cbc986d 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3931,41 +3931,27 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
> >  	struct folio *end_folio = folio_next(folio);
> >  	struct folio *new_folio, *next;
> >  	int old_order = folio_order(folio);
> > -	struct list_lru_one *lru;
> > -	bool dequeue_deferred;
> >  	int ret = 0;
> >  
> >  	VM_WARN_ON_ONCE(!mapping && end);
> > -	/*
> > -	 * If this folio can be on the deferred split queue, lock out
> > -	 * the shrinker before freezing the ref. If the shrinker sees
> > -	 * a 0-ref folio, it assumes it beat folio_put() to the list
> > -	 * lock and must clean up the LRU state - the same dequeue we
> > -	 * will do below as part of the split.
> > -	 */
> > -	dequeue_deferred = folio_test_anon(folio) && old_order > 1;
> > -	if (dequeue_deferred) {
> > -		struct mem_cgroup *memcg;
> >  
> > -		rcu_read_lock();
> > -		memcg = folio_memcg(folio);
> > -		lru = list_lru_lock(&deferred_split_lru,
> > -				    folio_nid(folio), &memcg);
> > -	}
> 
> Nice cleanup.
> 
> >  	if (folio_ref_freeze(folio, folio_cache_ref_count(folio) + 1)) {
> >  		struct swap_cluster_info *ci = NULL;
> >  		struct lruvec *lruvec;
> >  
> > -		if (dequeue_deferred) {
> > -			__list_lru_del(&deferred_split_lru, lru,
> > -				       &folio->_deferred_list, folio_nid(folio));
> > -			if (folio_test_partially_mapped(folio)) {
> > -				folio_clear_partially_mapped(folio);
> > -				mod_mthp_stat(old_order,
> > -					MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> > -			}
> > -			list_lru_unlock(lru);
> > -			rcu_read_unlock();
> > +		/* Take off the deferred split queue while frozen and memcg set */
> > +		folio_unqueue_deferred_split(folio);
> > +
> > +		/*
> > +		 * deferred_split_scan() takes the folio off the queue before it
> > +		 * splits it, so the unqueue above finds an empty list and
> > +		 * leaves PG_partially_mapped set.
> > +		 * Clear it here: the flag does not survive the split.
> > +		 */
> > +		if (folio_test_partially_mapped(folio)) {
> > +			folio_clear_partially_mapped(folio);
> > +			mod_mthp_stat(old_order,
> > +				      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> >  		}
> 
> In general,
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> 
> But I do wonder whether this sequence (that also
> __folio_unqueue_deferred_split()) performs would deserve a small local helper in
> mm/huge_memory.c
> 
> Could be done as a separate cleanup.

Just to be sure, do you want a helper like this:

static void folio_clear_partially_mapped_stat(struct folio *folio)
{
      if (!folio_test_partially_mapped(folio))
              return;

      folio_clear_partially_mapped(folio);
      mod_mthp_stat(folio_order(folio),
                    MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
}

?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov