Re: [PATCH 1/5] mm/huge_memory: do not touch frozen folios in deferred_split_isolate()

[Zi Yan]

On Thu Aug 27, 2026 at 12:38 PM EDT, Usama Arif wrote:
> On Wed, 26 Aug 2026 17:20:57 +0100 Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:
>
>> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
>> 
>> deferred_split_isolate() probes each queued folio with folio_try_get().
>> folio_try_get() failure is treated as a lost race with folio_put(): clear
>> PG_partially_mapped, correct MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, take
>> the folio off the queue.
>> 
>> The folio_put() race is the most common case for !folio_try_get(), but
>> it is not the only option. Another scenario is folio_ref_freeze().
>> 
>> A zero refcount in such cases does not mean the folio is going away.  It
>> means "don't touch me" and current deferred_split_isolate() doesn't
>> respect it. It can lead to unqueueing folios from the deferred list for
>> no reason:
>> 
>>     CPU 0                            CPU 1
>>     ---------------------------      ------------------------------
>>     freeze a mapped folio            deferred_split_scan()
>>       folio_ref_freeze()               folio_try_get() fails
>>                                        folio_clear_partially_mapped()
>>                                        NR_ANON_PARTIALLY_MAPPED--
>>                                        folio off the queue
>>     give up, put it back
>>       folio_ref_unfreeze()
>> 
>> The folio is still partially mapped, but it is no longer a split candidate.
>> Nothing queues it again until part of it is unmapped once more.
>> 
>> Skip the folio instead: whoever freezes the folio, owns it and owner is
>> responsible for its fate. It also covers the folio_put() case:
>> __folio_put() unqueues the folio via folio_unqueue_deferred_split().
>> 
>> Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
>> Link: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@xxxxxxxxx/
>> Assisted-by: Claude-Code:claude-opus-5
>> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
>> ---
>>  mm/huge_memory.c | 19 ++++---------------
>>  1 file changed, 4 insertions(+), 15 deletions(-)
>> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ced400f72d43..6281ed993243 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4590,22 +4590,11 @@ static enum lru_status deferred_split_isolate(struct list_head *item,
>>  	struct folio *folio = container_of(item, struct folio, _deferred_list);
>>  	struct list_head *freeable = cb_arg;
>>  
>> -	if (folio_try_get(folio)) {
>> -		list_lru_isolate_move(lru, item, freeable);
>> -		return LRU_REMOVED;
>> -	}
>> +	/* Lost race to folio_put() or the folio is under folio_ref_freeze() */
>> +	if (!folio_try_get(folio))
>> +		return LRU_SKIP;
>
> I think we might have a problem here for ZONE_DEVICE folios?

For coherent ZONE_DEVICE folios, yes. IIRC, private ZONE_DEVICE folios
are not added to deferred split queue.
>
> This assumes the final put always dequeues the folio, but ZONE_DEVICE folios
> bypass the generic folio_unqueue_deferred_split() path.
> With memcg disabled, this can leave a recycled folio linked on the
> deferred-split list?
>
> Should we dequeue folios in free_zone_device_folio()?

I think so, before mem_cgroup_uncharge().

But it is a pre-existing issue. We need a separate patch unqueuing
folios in free_zone_device_folio() to fix commit a30b48bf1b24
("mm/migrate_device: implement THP migration of zone device pages").

>>  
>> -	/*
>> -	 * We lost race with folio_put(). Read folio state before the
>> -	 * isolate: folio_unqueue_deferred_split() checks list_empty()
>> -	 * locklessly, so once removed the folio can be freed any time.
>> -	 */
>> -	if (folio_test_partially_mapped(folio)) {
>> -		folio_clear_partially_mapped(folio);
>> -		mod_mthp_stat(folio_order(folio),
>> -			      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
>> -	}
>> -	list_lru_isolate(lru, item);
>> +	list_lru_isolate_move(lru, item, freeable);
>>  	return LRU_REMOVED;
>>  }
>>  
>> -- 
>> 2.54.0
>> 
>> 



-- 
Best Regards,
Yan, Zi