linux-kernel - Re: [PATCH 5/9] mm, page_alloc: make per_cpu

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Date:   Fri, 25 Sep 2020 12:25:38 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Oscar Salvador <osalvador@...e.de>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 5/9] mm, page_alloc: make per_cpu_pageset accessible only
 after init

On 22.09.20 16:37, Vlastimil Babka wrote:
> setup_zone_pageset() replaces the boot_pageset by allocating and initializing a
> proper percpu one. Currently it assigns zone->pageset with the newly allocated
> one before initializing it. That's currently not an issue, because the zone
> should not be in any zonelist, thus not visible to allocators at this point.
> 
> Memory ordering between the pcplist contents and its visibility is also not
> guaranteed here, but that also shouldn't be an issue because online_pages()
> does a spin_unlock(pgdat->node_size_lock) before building the zonelists.
> 
> However it's best that we don't silently rely on operations that can be changed
> in the future. Make sure only properly initialized pcplists are visible, using
> smp_store_release(). The read side has a data dependency via the zone->pageset
> pointer instead of an explicit read barrier.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/page_alloc.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 99b74c1c2b0a..de3b48bda45c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6246,15 +6246,17 @@ static void zone_set_pageset_high_and_batch(struct zone *zone)
>  
>  void __meminit setup_zone_pageset(struct zone *zone)
>  {
> +	struct per_cpu_pageset __percpu * new_pageset;
>  	struct per_cpu_pageset *p;
>  	int cpu;
>  
> -	zone->pageset = alloc_percpu(struct per_cpu_pageset);
> +	new_pageset = alloc_percpu(struct per_cpu_pageset);
>  	for_each_possible_cpu(cpu) {
> -		p = per_cpu_ptr(zone->pageset, cpu);
> +		p = per_cpu_ptr(new_pageset, cpu);
>  		pageset_init(p);
>  	}
>  
> +	smp_store_release(&zone->pageset, new_pageset);
>  	zone_set_pageset_high_and_batch(zone);
>  }
>  
> 

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb