linux-kernel - Re: [PATCH 1/8] mm/vmscan: remove redundant folio_test

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Date:   Wed, 30 Mar 2022 09:46:59 +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 1/8] mm/vmscan: remove redundant folio_test_swapbacked
 check when folio is file lru

On 2022/3/29 21:46, Matthew Wilcox wrote:
> On Tue, Mar 29, 2022 at 09:26:12PM +0800, Miaohe Lin wrote:
>> When folio is file lru, folio_test_swapbacked is guaranteed to be true. So
>> it's unnecessary to check it here again. No functional change intended.
> 
> ummm ... is your logic right here?  The condition is:
> 
> 	if (!a || (b && !c))
> 

Because a is !c, so c is !a. Then we have:

!a || (b && !c) ==> !a || (b && !!a) ==> !a || (b && a) ==> !a || b.

Or am I miss something?

Thanks.

> I don't see how it follows that a => c means we can do any
> simplification at all.
> 
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/vmscan.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 1678802e03e7..7c1a9713bfc9 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1434,8 +1434,7 @@ static void folio_check_dirty_writeback(struct folio *folio,
>>  	 * Anonymous pages are not handled by flushers and must be written
>>  	 * from reclaim context. Do not stall reclaim based on them
>>  	 */
>> -	if (!folio_is_file_lru(folio) ||
>> -	    (folio_test_anon(folio) && !folio_test_swapbacked(folio))) {
>> +	if (!folio_is_file_lru(folio) || folio_test_anon(folio)) {
>>  		*dirty = false;
>>  		*writeback = false;
>>  		return;
>> -- 
>> 2.23.0
>>
>>
> 
> .
>