linux-kernel - Re: [PATCH v2] mm: use kvzalloc for swap_info

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Message-ID: <20181105141156.GB10132@dhcp22.suse.cz>
Date:   Mon, 5 Nov 2018 15:11:56 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        linux-kernel@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>
Subject: Re: [PATCH v2] mm: use kvzalloc for swap_info_struct allocation

On Mon 05-11-18 14:17:01, Vasily Averin wrote:
> commit a2468cc9bfdf ("swap: choose swap device according to numa node")
> changed 'avail_lists' field of 'struct swap_info_struct' to an array.
> In popular linux distros it increased size of swap_info_struct up to
> 40 Kbytes and now swap_info_struct allocation requires order-4 page.
> Switch to kvzmalloc allows to avoid unexpected allocation failures.

While this fixes the most visible issue is this a good long term
solution? Aren't we wasting memory without a good reason? IIRC our limit
for swap files/devices is much smaller than potential NUMA nodes numbers
so we can safely expect that would be only few numa affine nodes. I am
not really familiar with the rework which has added numa node awareness
but I wouls assueme that we should either go with one global table with
a linked list of possible swap_info structure per numa node or use a
sparse array.

That being said I am not really objecting to this patch as it is simple
and backportable to older (stable kernels).
 
I would even dare to add
Fixes: a2468cc9bfdf ("swap: choose swap device according to numa node")

because not being able to add a swap space on a fragmented system looks
like a regression to me.

> Acked-by: Aaron Lu <aaron.lu@...el.com>
> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>

Acked-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/swapfile.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 644f746e167a..8688ae65ef58 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2813,7 +2813,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>  	unsigned int type;
>  	int i;
>  
> -	p = kzalloc(sizeof(*p), GFP_KERNEL);
> +	p = kvzalloc(sizeof(*p), GFP_KERNEL);
>  	if (!p)
>  		return ERR_PTR(-ENOMEM);
>  
> @@ -2824,7 +2824,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>  	}
>  	if (type >= MAX_SWAPFILES) {
>  		spin_unlock(&swap_lock);
> -		kfree(p);
> +		kvfree(p);
>  		return ERR_PTR(-EPERM);
>  	}
>  	if (type >= nr_swapfiles) {
> @@ -2838,7 +2838,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>  		smp_wmb();
>  		nr_swapfiles++;
>  	} else {
> -		kfree(p);
> +		kvfree(p);
>  		p = swap_info[type];
>  		/*
>  		 * Do not memset this entry: a racing procfs swap_next()
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs