linux-kernel - Re: [PATCH V2] mm/swap: take folio refcount after testing the LRU flag

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Message-ID: <2da47dad-46ff-4e04-b697-c85f30a4341a@redhat.com>
Date: Mon, 12 Aug 2024 22:12:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Yu Zhao <yuzhao@...gle.com>, yangge1116@....com
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, 21cnbao@...il.com,
 baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH V2] mm/swap: take folio refcount after testing the LRU
 flag

On 12.08.24 21:06, Yu Zhao wrote:
> On Mon, Aug 12, 2024 at 5:22 AM <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>
>> Acked-by: David Hildenbrand <david@...hat.com>
>> ---
>>   mm/swap.c | 9 ++++-----
>>   1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> V2:
>>     Add sanity check suggested by David
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 67a2467..c048659 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -226,12 +226,11 @@ 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;
>> -       }
>> +
>> +       VM_WARN_ON_ONCE(!folio_ref_count(folio));
>> +       folio_get(folio);
> 
> No need to check folio_ref_count() here, because folio_get() already
> does it with a better check folio_ref_zero_or_close_to_overflow().
> 

Ah, good point, thanks!

-- 
Cheers,

David / dhildenb