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

[Zi Yan]

On 27 Aug 2026, at 11:23, David Hildenbrand (Arm) wrote:

> On 8/26/26 18:20, Kiryl Shutsemau 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:
>
> Yes.
>
>>
>>     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/
>
> Fixes?
>
>> 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;
>>
>> -	/*
>> -	 * 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;
>>  }
>>
>
> Who will clean up the stats that we used to clean up? That should be mentioned
> in the commit log, otherwise it looks like some piece of the puzzle is missing.

It is kinda implied in the last paragraph in the commit log (copied below),
but I agree that stating it explicitly is better.

|> 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().



Best Regards,
Yan, Zi