linux-kernel - Re: [PATCH] mm/swap: take folio refcount after testing the LRU flag

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Message-ID: <792b62dd-ef63-4609-a1eb-e2ccfd90b5ee@126.com>
Date: Mon, 12 Aug 2024 19:18:40 +0800
From: Ge Yang <yangge1116@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 21cnbao@...il.com,
 baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH] mm/swap: take folio refcount after testing the LRU flag



在 2024/8/12 17:10, David Hildenbrand 写道:
> On 10.08.24 08:15, yangge1116@....com wrote:
>> From: yangge <yangge1116@....com>
>>
>> Whoever passes a folio to __folio_batch_add_and_move() must hold
>> a reference, otherwise something else would already be messed up.
>> If the folio is referenced, it will not be freed elsewhere, so we
>> can safely clear the folio's lru flag. As discussed with David
>> in [1], we should take the reference after testing the LRU flag,
>> not before.
>>
>> Link: 
>> https://lore.kernel.org/lkml/d41865b4-d6fa-49ba-890a-921eefad27dd@redhat.com/ [1]
>> Signed-off-by: yangge <yangge1116@....com>
>> ---
>>   mm/swap.c | 8 +++-----
>>   1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 67a2467..6b83898 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -226,12 +226,10 @@ static void __folio_batch_add_and_move(struct 
>> folio_batch __percpu *fbatch,
>>   {
>>       unsigned long flags;
>> -    folio_get(folio);
>> -
>> -    if (on_lru && !folio_test_clear_lru(folio)) {
>> -        folio_put(folio);
>> +    if (on_lru && !folio_test_clear_lru(folio))
>>           return;
>> -    }
>> +
>> +    folio_get(folio);
>>       if (disable_irq)
>>           local_lock_irqsave(&cpu_fbatches.lock_irq, flags);
> 
> Ah, "mm/swap: remove boilerplate" reduced them to a single instance, good.
> 
> You can throw in a
> 
> VM_WARN_ON_ONCE(!folio_ref_count(folio));
> 
> before the folio_get(folio)
> 
> so we would realize when something very unexpected happens.

Ok, thanks.

> 
> Acked-by: David Hildenbrand <david@...hat.com>
>