linux-kernel - Re: [PATCH] mm/hotplug: prevent memory leak when reuse pgdat

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Message-ID: <20190813075707.GA17933@dhcp22.suse.cz>
Date:   Tue, 13 Aug 2019 09:57:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     akpm@...ux-foundation.org, osalvador@...e.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hotplug: prevent memory leak when reuse pgdat

On Tue 13-08-19 10:06:08, Wei Yang wrote:
> When offline a node in try_offline_node, pgdat is not released. So that
> pgdat could be reused in hotadd_new_pgdat. While we re-allocate
> pgdat->per_cpu_nodestats if this pgdat is reused.
> 
> This patch prevents the memory leak by just allocate per_cpu_nodestats
> when it is a new pgdat.

Yes this makes sense! I was slightly confused why we haven't initialized
the allocated pcp area because __alloc_percpu does GFP_KERNEL without
__GFP_ZERO but then I've just found out that the zeroying is done
regardless. A bit unexpected...

> NOTE: This is not tested since I didn't manage to create a case to
> offline a whole node. If my analysis is not correct, please let me know.
> 
> Signed-off-by: Wei Yang <richardw.yang@...ux.intel.com>

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  mm/memory_hotplug.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c73f09913165..efaf9e6f580a 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -933,8 +933,11 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>  		if (!pgdat)
>  			return NULL;
>  
> +		pgdat->per_cpu_nodestats =
> +			alloc_percpu(struct per_cpu_nodestat);
>  		arch_refresh_nodedata(nid, pgdat);
>  	} else {
> +		int cpu;
>  		/*
>  		 * Reset the nr_zones, order and classzone_idx before reuse.
>  		 * Note that kswapd will init kswapd_classzone_idx properly
> @@ -943,6 +946,12 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>  		pgdat->nr_zones = 0;
>  		pgdat->kswapd_order = 0;
>  		pgdat->kswapd_classzone_idx = 0;
> +		for_each_online_cpu(cpu) {
> +			struct per_cpu_nodestat *p;
> +
> +			p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
> +			memset(p, 0, sizeof(*p));
> +		}
>  	}
>  
>  	/* we can use NODE_DATA(nid) from here */
> @@ -952,7 +961,6 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>  
>  	/* init node's zones as empty zones, we don't have any present pages.*/
>  	free_area_init_core_hotplug(nid);
> -	pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
>  
>  	/*
>  	 * The node we allocated has no zone fallback lists. For avoiding
> -- 
> 2.17.1
> 

-- 
Michal Hocko
SUSE Labs