Re: [PATCH] memcg: use round-robin victim selection in refill_stock

[Michal Hocko]

On Thu 21-05-26 15:37:51, Shakeel Butt wrote:
> Harry Yoo reported that get_random_u32_below() is not safe to call in
> the nmi context and memcg charge draining can happen in nmi context.
> 
> More specifically get_random_u32_below() is neither reentrant- nor
> NMI-safe: it acquires a per-cpu local_lock via local_lock_irqsave() on
> the batched_entropy_u32 state. An NMI that lands on a CPU mid-update of
> the ChaCha batch state and recurses into the random subsystem would
> corrupt that state. The memcg_stock local_trylock prevents re-entry
> on the percpu stock itself, but cannot protect an unrelated
> subsystem's per-cpu lock.
> 
> Replace the random pick with a per-cpu round-robin counter stored in
> memcg_stock_pcp and serialized by the same local_trylock that already
> guards cached[] and nr_pages[]. No atomics, no random calls, no extra
> locks needed.
> 
> Fixes: f735eebe55f8f ("memcg: multi-memcg percpu charge cache")
> Reported-by: Harry Yoo <harry@xxxxxxxxxx>
> Closes: https://lore.kernel.org/4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@xxxxxxxxxx/
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  mm/memcontrol.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0eb50e639f0a..6392a2704441 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2031,6 +2031,7 @@ struct memcg_stock_pcp {
>  
>  	struct work_struct work;
>  	unsigned long flags;
> +	uint8_t drain_idx;
>  };
>  
>  static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = {
> @@ -2214,7 +2215,9 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  	if (!success) {
>  		i = empty_slot;
>  		if (i == -1) {
> -			i = get_random_u32_below(NR_MEMCG_STOCK);
> +			i = stock->drain_idx++;
> +			if (stock->drain_idx == NR_MEMCG_STOCK)
> +				stock->drain_idx = 0;
>  			drain_stock(stock, i);
>  		}
>  		css_get(&memcg->css);
> -- 
> 2.53.0-Meta

-- 
Michal Hocko
SUSE Labs