linux-kernel - Re: [PATCH 4/7] mm/khugepaged: minor cleanup for collapse

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Message-ID: <YqoAydWk/IcMp5d2@google.com>
Date:   Wed, 15 Jun 2022 08:54:49 -0700
From:   Zach O'Keefe <zokeefe@...gle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, aarcange@...hat.com,
        willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, apopple@...dia.com, david@...hat.com,
        surenb@...gle.com, peterx@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] mm/khugepaged: minor cleanup for collapse_file

On 11 Jun 16:47, Miaohe Lin wrote:
> nr_none is always 0 for non-shmem case because the page can be read from
> the backend store. So when nr_none ! = 0, it must be in is_shmem case.
> Also only adjust the nrpages and uncharge shmem when nr_none != 0 to save
> cpu cycles.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/khugepaged.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1b5dd3820eac..8e6fad7c7bd9 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1885,8 +1885,7 @@ static void collapse_file(struct mm_struct *mm,
>  
>  	if (nr_none) {
>  		__mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
> -		if (is_shmem)
> -			__mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
> +		__mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
>  	}


Might be worth a small comment here - even though folks can see in above code
that this is only incremented in shmem path, might be nice to say why it's
always 0 for non-shmem (or conversely, why it's only possible to be non 0 on
shmem).

>  
>  	/* Join all the small entries into a single multi-index entry */
> @@ -1950,10 +1949,10 @@ static void collapse_file(struct mm_struct *mm,
>  
>  		/* Something went wrong: roll back page cache changes */
>  		xas_lock_irq(&xas);
> -		mapping->nrpages -= nr_none;
> -
> -		if (is_shmem)
> +		if (nr_none) {
> +			mapping->nrpages -= nr_none;
>  			shmem_uncharge(mapping->host, nr_none);
> +		}
>  
>  		xas_set(&xas, start);
>  		xas_for_each(&xas, page, end - 1) {
> -- 
> 2.23.0
> 
> 

Otherwise,

Reviewed-by: Zach O'Keefe <zokeefe@...gle.com>