linux-kernel - Re: [PATCH 2/8] mm/vmscan: remove unneeded can_split_huge

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Date:   Thu, 31 Mar 2022 18:47:49 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/8] mm/vmscan: remove unneeded can_split_huge_page check

On 2022/3/31 16:38, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
> 
>> can_split_huge_page is introduced via commit b8f593cd0896 ("mm, THP, swap:
>> check whether THP can be split firstly") to avoid deleting the THP from
>> the swap cache and freeing the swap cluster when the THP cannot be split.
>> But since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
>> swapped out"), splitting THP is delayed until THP is swapped out. There's
>> no need to delete the THP from the swap cache and free the swap cluster
>> anymore. Thus we can remove this unneeded can_split_huge_page check now to
>> simplify the code logic.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/vmscan.c | 3 ---
>>  1 file changed, 3 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7c1a9713bfc9..09b452c4d256 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1691,9 +1691,6 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>>  				if (folio_maybe_dma_pinned(folio))
>>  					goto keep_locked;
>>  				if (PageTransHuge(page)) {
>> -					/* cannot split THP, skip it */
>> -					if (!can_split_folio(folio, NULL))
>> -						goto activate_locked;
>>  					/*
>>  					 * Split pages without a PMD map right
>>  					 * away. Chances are some or all of the
> 
> I'm OK with the change itself.  But THP still needs to be split after
> being swapped out.  The reason we don't need to check can_split_folio()

Could you please explain the relevant code path more detailed slightly?
IIUC, if THP is swapped out, it will be freed via destroy_compound_page
after __remove_mapping in shrink_page_list. So THP can be freed without
split. Or am I miss something ?

> is that folio_maybe_dma_pinned() is checked before.  Which will avoid
> the long term pinned pages to be swapped out.  And we can live with
> short term pinned pages.  Without can_split_folio() checking we can
> simplify the code as follows,
> 
> 	/*
> 	 * Split pages without a PMD map right
> 	 * away. Chances are some or all of the
> 	 * tail pages can be freed without IO.
> 	 */
> 	if (PageTransHuge(page) && !compound_mapcount(page) &&
>             split_huge_page_to_list(page, page_list))
> 		goto keep_locked;
>                                                                 
> activate_locked can be changed to keep_locked too, because it's just
> short term pinning.
> 

The change above looks good to me. Many thanks. Should I add a Suggested-by
tag for you?

> Best Regards,
> Huang, Ying

Thanks.

> 
> .
>