Re: [PATCH] mm/vmscan: don't try to reclaim freed folios

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/5/27 23:02, Matthew Wilcox wrote:
> On Fri, May 27, 2022 at 04:04:51PM +0800, Miaohe Lin wrote:
>> If folios were freed from under us, there's no need to reclaim them. Skip
>> these folios to save lots of cpu cycles and avoid possible unnecessary
>> disk IO.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/vmscan.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index f7d9a683e3a7..646dd1efad32 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1556,12 +1556,18 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>>  		folio = lru_to_folio(page_list);
>>  		list_del(&folio->lru);
>>  
>> +		nr_pages = folio_nr_pages(folio);
>> +		if (folio_ref_count(folio) == 1) {
>> +			/* folio was freed from under us. So we are done. */
>> +			WARN_ON(!folio_put_testzero(folio));
> 
> What?  No.  This can absolutely happen.  We have a refcount on the folio,
> which means that any other thread can temporarily raise the refcount,

IIUC, the folio is only in the isolated page_list now and it's not in the page cache, swap cache, pagetable or
under any use. So there should be no way that any other thread can temporarily raise the refcount when
folio_ref_count == 1. Or am I miss something?

> so this WARN_ON can trigger.  Also, we don't hold the folio locked,
> or an extra reference, so nr_pages is unstable because it can be split.

Yes, you're right. When folio_ref_count != 1, nr_pages is unstable. Will fix it if v2 is possible. :)

Thanks a lot for review and comment!

> 
>> +			goto free_it;
>> +		}
>> +
>>  		if (!folio_trylock(folio))
>>  			goto keep;
>>  
>>  		VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
>>  
>> -		nr_pages = folio_nr_pages(folio);
>>  
>>  		/* Account the number of base pages */
>>  		sc->nr_scanned += nr_pages;
>> -- 
>> 2.23.0
>>
>>
> 
> .
>