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

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Message-ID: <ce346099-e7a8-b9c1-5556-f0b8b7fa4d70@huawei.com>
Date:   Thu, 31 Mar 2022 16:07:22 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH 1/8] mm/vmscan: remove redundant folio_test_swapbacked
 check when folio is file lru

On 2022/3/31 16:02, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
> 
>> On 2022/3/31 14:37, Huang, Ying wrote:
>>> Muchun Song <songmuchun@...edance.com> writes:
>>>
>>>> On Tue, Mar 29, 2022 at 9:26 PM Miaohe Lin <linmiaohe@...wei.com> 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.
>>>>>
>>>>> 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)) {
>>>>
>>>> At least your login is no problem since folio_is_file_lru() is equal to
>>>> !folio_test_swapbacked().  But the new code is not clear to me.
>>>> The old code is easy to understand, e.g. folio_test_anon(folio) &&
>>>> !folio_test_swapbacked(folio) tells us that the anon pages which
>>>> do not need to be swapped should be skipped.
>>>
>>> That is for MADV_FREE pages.  The code is introduced in commit
>>> 802a3a92ad7a ("mm: reclaim MADV_FREE pages").
>>>
>>> So I think the original code is better.  It's an implementation detail
>>> that folio_is_file_lru() equals !folio_test_swapbacked().  It may be
>>> better to add some comments here for MADV_FREE pages.
>>>
>>
>> Do you tend to drop this patch or adding a comment with the change in this patch or something else?
> 
> I suggest to drop the code change and add a comment about MADV_FREE.

Will do. Thanks.

> 
> Best Regards,
> Huang, Ying
> 
>> Thanks.
>>
>>>> So I'm neutral on the patch.
>>>
>>> Best Regards,
>>> Huang, Ying
>>> .
>>>
> .
>