Re: [PATCH v5 2/2] mm: add swapiness= arg to memory.reclaim

[Dan Schatzberg <schatzberg.dan@gmail.com>]

On Thu, Dec 21, 2023 at 10:29:59AM +0100, Michal Hocko wrote:
> On Wed 20-12-23 07:26:51, Dan Schatzberg wrote:
> >  ...
> 
> LGTM
> Acked-by: Michal Hocko <mhocko@suse.com.
> 
> Just one minor thing. It would be really great to prevent from potential
> incorrect use of mem_cgroup_swappiness. This should be internal function
> to memcg. Now, having scan_control internal to vmscan.c makes that
> harder and moving it out to swap.h or internal.h sounds overreaching.
> 
> We could do this at least to reduce those mistakes. I can make it a
> proper patch if this seems reasonable or you can fold it into your patch
> directly.
> --- 
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f98dff23b758..5f3a182e9515 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -92,8 +92,10 @@ struct scan_control {
>  	unsigned long	anon_cost;
>  	unsigned long	file_cost;
>  
> -	/* Swappiness value for reclaim. NULL will fall back to per-memcg/global value */
> +#ifdef CONFIG_MEMCG
> +	/* Swappiness value for reclaim. Always use sc_swappiness()! */
>  	int *swappiness;
> +#endif
>  
>  	/* Can active folios be deactivated as part of reclaim? */
>  #define DEACTIVATE_ANON 1
> @@ -230,6 +232,13 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  #endif
>  	return false;
>  }
> +
> +static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
> +{
> +	if (sc->swappiness)
> +		return *sc->swappiness;
> +	return mem_cgroup_swappiness(memcg);
> +}
>  #else
>  static bool cgroup_reclaim(struct scan_control *sc)
>  {
> @@ -245,6 +254,10 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  {
>  	return true;
>  }
> +static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
> +{
> +	return READ_ONCE(vm_swappiness);
> +}
>  #endif
>  
>  static void set_task_reclaim_state(struct task_struct *task,
> @@ -2330,8 +2343,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>  	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>  	unsigned long anon_cost, file_cost, total_cost;
> -	int swappiness = sc->swappiness ?
> -		*sc->swappiness : mem_cgroup_swappiness(memcg);
> +	int swappiness = sc_swappiness(sc, memcg);
>  	u64 fraction[ANON_AND_FILE];
>  	u64 denominator = 0;	/* gcc */
>  	enum scan_balance scan_balance;
> @@ -2612,10 +2624,7 @@ static int get_swappiness(struct lruvec *lruvec, struct scan_control *sc)
>  	    mem_cgroup_get_nr_swap_pages(memcg) < MIN_LRU_BATCH)
>  		return 0;
>  
> -	if (sc->swappiness)
> -		return *sc->swappiness;
> -
> -	return mem_cgroup_swappiness(memcg);
> +	return sc_swappiness(sc, memcg);
>  }
>  
>  static int get_nr_gens(struct lruvec *lruvec, int type)
> -- 
> Michal Hocko
> SUSE Labs

Thanks for the review Michal and sorry for the delayed response. Your
patch looks reasonable to me but I'm a bit unclear about the need for
#ifdef - mem_cgroup_swappiness already works correctly regardless of
CONFIG_MEMCG or not - why not make sc->swappiness and sc_swappiness()
unconditional?

Happy to roll that into the next version of my patch.