Re: [PATCH v5 9/9] mm: switch deferred split shrinker to list_lru

[Lance Yang <lance.yang@linux.dev>]

On 2026/6/9 15:35, Lorenzo Stoakes wrote:
> On Tue, Jun 09, 2026 at 11:20:58AM +0800, Lance Yang wrote:
>>
>> On Mon, Jun 01, 2026 at 02:17:28PM -0400, Johannes Weiner wrote:
>>> On Mon, Jun 01, 2026 at 09:21:35PM +0800, Lance Yang wrote:
>>>>
>>>> On Wed, May 27, 2026 at 04:45:16PM -0400, Johannes Weiner wrote:
>>>> [...]
>>>>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>>>>> index 04f5ce992401..9c3a5cf99778 100644
>>>>> --- a/mm/swap_state.c
>>>>> +++ b/mm/swap_state.c
>>>>> @@ -465,6 +465,16 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci,
>>>>> 		return ERR_PTR(-ENOMEM);
>>>>> 	}
>>>>>
>>>>
>>>> Shouldn't this be limited to anon swapin?
>>>>
>>>> e.g. vmf && vma_is_anonymous(vmf->vma)
>>>>
>>>>> +	if (order > 1 && folio_memcg_alloc_deferred(folio)) {
>>>>
>>>> __swap_cache_alloc() is also used by shmem direct swapin, so shmem can
>>>> get here too when handling a large swap entry:
>>>>
>>>> shmem_get_folio_gfp()
>>>>    shmem_swapin_folio()
>>>>      shmem_swap_alloc_folio()
>>>>        swapin_sync()
>>>>          swap_cache_alloc_folio()
>>>>            __swap_cache_alloc()
>>>>              folio_memcg_alloc_deferred()
>>>
>>> Good catch, I think you're right. I shouldn't have dismissed that
>>> branch due to "/* Direct swapin skipping swap cache & readahead */"
>>>
>>>> @Baolin please correct me if I got it wrong :)
>>>>
>>>> folio_memcg_alloc_deferred() itself doesn't filter shmem out either; it
>>>> only allocates the memcg list_lru metadata for deferred_split_lru:
>>>>
>>>> int folio_memcg_alloc_deferred(struct folio *folio)
>>>> {
>>>> 	if (mem_cgroup_disabled())
>>>> 		return 0;
>>>> 	return folio_memcg_list_lru_alloc(folio, &deferred_split_lru, GFP_KERNEL);
>>>> }
>>>>
>>>> Since deferred_split_lru only queues anon large folios, doing this for
>>>> shmem swapin doesn't buy us anything :)
>>>
>>> Yes, agreed. I don't think it's a big deal / show stopper in terms of
>>> user-visible effect, but of course still worth fixing.
>>>
>>> I'll send a follow-up patch.
>>
>> Thanks.
>>
>> Looks like this has already landed in mm-stable. If you're okay with it,
>> I can send the follow-up.
>>
>> From: Lance Yang <lance.yang@linux.dev>
>> Date: Tue, 9 Jun 2026 10:56:45 +0800
>> Subject: [PATCH] mm: prepare deferred split metadata only for anon swapin
>>
>> __swap_cache_alloc() prepares deferred split metadata for large swapcache
>> folios.
>>
>> That also covers shmem swapin, because shmem_swap_alloc_folio() can call
>> swapin_sync() with a large order[1]. But shmem folios are not queued on
>> the deferred split queue, so preparing the metadata doesn't buy us
>> anything there.
>>
>> So let's limit it to anon swapin.
>>
>> [1] https://lore.kernel.org/all/20260601132135.14272-1-lance.yang@linux.dev/
>>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/swap_state.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>> index 9c3a5cf99778..7adac957c2b8 100644
>> --- a/mm/swap_state.c
>> +++ b/mm/swap_state.c
>> @@ -465,7 +465,8 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci,
>>   		return ERR_PTR(-ENOMEM);
>>   	}
>>
>> -	if (order > 1 && folio_memcg_alloc_deferred(folio)) {
>> +	if (order > 1 && vma && vma_is_anonymous(vma) &&
> 
> A folio can be anon for a non-shmem file-backed VMA though?
> E.g. MAP_PRIVATE-mapped file-backed mappings?

Ah, good point! vma_is_anonymous() is too strong here ...

Maybe use !vma_is_shmem(vma) instead?

> Not sure if that's something that'd be a factor here/meaningful though.
> 
>> +	    folio_memcg_alloc_deferred(folio)) {
>>   		spin_lock(&ci->lock);
>>   		__swap_cache_do_del_folio(ci, folio, entry, shadow);
>>   		spin_unlock(&ci->lock);
>> --
>> 2.39.3 (Apple Git-146)
> 
> Cheers, Lorenzo