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

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Date:   Wed, 8 Jun 2022 22:09:52 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmscan: don't try to reclaim freed folios

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@...wei.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,
> 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.

When I reread the code, I found caller holds an extra reference to the folio when
calling isolate_lru_pages(), so folio can't be split and thus nr_pages should be
stable indeed? Or am I miss something again?

Thanks!

> 
>> +			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
>>
>>
> 
> .
>