Re: [RFC PATCH v2 3/3] mm/vmscan: avoid pointless large folio splits without swap

[Xueyuan Chen <xueyuan.chen21@gmail.com>]

On Fri, Jul 10, 2026 at 03:21:44PM +0800, Baolin Wang wrote:
>
>
>On 7/9/26 10:51 PM, Xueyuan Chen wrote:
>> From: "Barry Song (Xiaomi)" <baohua@kernel.org>
>> 
>> When swap is disabled, exhausted, or unavailable due to memcg swap
>> limits, splitting a large anonymous folio cannot make swapout progress.
>> The fallback only destroys the large folio and inflates split statistics.
>> 
>> Use -E2BIG from folio_alloc_swap() as the explicit signal that splitting
>> the folio might allow swapout of smaller pieces. For other allocation
>> failures, keep the existing activation path and avoid the split.
>> 
>> This preserves the split fallback for fragmented or partially available
>> swap, while avoiding it when there is no backing space for any part of the
>> folio.
>> 
>> Reported-by: Nanzhe Zhao <zhaonanzhe@xiaomi.com>
>> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
>> ---
>>   mm/vmscan.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index bd1b1aa12581..40340a88f78e 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1260,6 +1260,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>   		 */
>>   		if (folio_test_anon(folio) && folio_test_swapbacked(folio) &&
>>   				!folio_test_swapcache(folio)) {
>> +			int ret;
>> +
>>   			if (!(sc->gfp_mask & __GFP_IO))
>>   				goto keep_locked;
>>   			if (folio_maybe_dma_pinned(folio))
>> @@ -1278,10 +1280,11 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>   				    split_folio_to_list(folio, folio_list))
>>   					goto activate_locked;
>>   			}
>> -			if (folio_alloc_swap(folio)) {
>> +			ret = folio_alloc_swap(folio);
>> +			if (ret) {
>>   				int __maybe_unused order = folio_order(folio);
>>   
>> -				if (!folio_test_large(folio))
>> +				if (!folio_test_large(folio) || ret != -E2BIG)
>>   					goto activate_locked_split;
>
>Like I said in v1 [1], please apply the same change to shmem swap as well.
>
>[1] 
>https://lore.kernel.org/all/6e89f868-ca7a-484f-aeea-5d8d029714f2@linux.alibaba.com/
>

Hi Baolin,

Thanks for pointing this out, and sorry for missing your v1 comment.

I'll apply the same folio_alloc_swap() error handling to the shmem swap
path in the next version.

Thanks,
Xueyuan