linux-kernel - Re: [v2 2/2] mm: khugepaged: avoid overriding min_free

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Date:   Wed, 16 Sep 2020 09:59:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Vijay Balakrishna <vijayb@...ux.microsoft.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Song Liu <songliubraving@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Allen Pais <apais@...rosoft.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [v2 2/2] mm: khugepaged: avoid overriding min_free_kbytes set by
 user

On Tue 15-09-20 14:10:58, Vijay Balakrishna wrote:
> set_recommended_min_free_kbytes need to honor min_free_kbytes set by the
> user.  Post start-of-day THP enable or memory hotplug operations can
> lose user specified min_free_kbytes, in particular when it is higher than
> calculated recommended value.  Also modifying "recommended_min" variable
> type to "int" from "unsigned long" to avoid undesired result noticed
> during testing.  It is due to comparing "unsigned long" with "int" type.
> 
> Signed-off-by: Vijay Balakrishna <vijayb@...ux.microsoft.com>
> Cc: stable@...r.kernel.org
> Reviewed-by: Pavel Tatashin <pasha.tatashin@...een.com>
> ---
>  mm/khugepaged.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4f7107476a6f..b4b753ba411a 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2253,7 +2253,7 @@ static void set_recommended_min_free_kbytes(void)
>  {
>  	struct zone *zone;
>  	int nr_zones = 0;
> -	unsigned long recommended_min;
> +	int recommended_min;
>  
>  	for_each_populated_zone(zone) {
>  		/*
> @@ -2280,12 +2280,12 @@ static void set_recommended_min_free_kbytes(void)
>  
>  	/* don't ever allow to reserve more than 5% of the lowmem */
>  	recommended_min = min(recommended_min,
> -			      (unsigned long) nr_free_buffer_pages() / 20);
> +			      (int) nr_free_buffer_pages() / 20);

nr_free_buffer_pages can oveflow in int on very large machines.

>  	recommended_min <<= (PAGE_SHIFT-10);
>  
> -	if (recommended_min > min_free_kbytes) {
> +	if (recommended_min > user_min_free_kbytes) {

This can decrease the size theoretically. Because user_min_free_kbytes
is -1 by default and recommended_min might be <= min_free_kbytes.

You need to check both. Also can we make user_min_free_kbytes 0 by
default? From a quick look, nobody should really care.

>  		if (user_min_free_kbytes >= 0)
> -			pr_info("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n",
> +			pr_info("raising min_free_kbytes from %d to %d to help transparent hugepage allocations\n",
>  				min_free_kbytes, recommended_min);
>  
>  		min_free_kbytes = recommended_min;
> -- 
> 2.28.0
> 

-- 
Michal Hocko
SUSE Labs