Re: [PATCH 5/9] mm, page_alloc: make per_cpu_pageset accessible only after init

David Hildenbrand
2020-09-25T10:25:50Z

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@suse.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@redhat.com>

--
Thanks,

David / dhildenb