Re: [PATCH RFC 04/15] mm/mglru: make generation page counters atomic

[Baoquan He <baoquan.he@linux.dev>]

On 08/04/26 at 03:47am, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> No feature change, convert them to atomic so we can update them without
> holding the LRU lock. There is no risk of overflow. The reader always
> compares and uses zero instead if the counter values are negative. It
> follows final consistency.

It's better to tell this is a preparation patch, and who will use them
in the coming patch or function. Otherwise we can't see why we need it
if it's no feature change.

The code change looks good to me.

> 
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  include/linux/mm_inline.h |  6 ++----
>  include/linux/mmzone.h    |  2 +-
>  mm/vmscan.c               | 20 ++++++++++----------
>  3 files changed, 13 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 4076e3f7dcc8..018a2f54a5c9 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -245,11 +245,9 @@ static inline void lru_gen_update_size(struct lruvec *lruvec, struct folio *foli
>  	VM_WARN_ON_ONCE(old_gen == -1 && new_gen == -1);
>  
>  	if (old_gen >= 0)
> -		WRITE_ONCE(lrugen->nr_pages[old_gen][type][zone],
> -			   lrugen->nr_pages[old_gen][type][zone] - delta);
> +		atomic_long_sub(delta, &lrugen->nr_pages[old_gen][type][zone]);
>  	if (new_gen >= 0)
> -		WRITE_ONCE(lrugen->nr_pages[new_gen][type][zone],
> -			   lrugen->nr_pages[new_gen][type][zone] + delta);
> +		atomic_long_add(delta, &lrugen->nr_pages[new_gen][type][zone]);
>  
>  	/* addition */
>  	if (old_gen < 0) {
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 8048c6b0544d..4225dab760ba 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -572,7 +572,7 @@ struct lru_gen_folio {
>  	/* the multi-gen LRU lists, lazily sorted on eviction */
>  	struct list_head folios[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
>  	/* the multi-gen LRU sizes, eventually consistent */
> -	long nr_pages[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
> +	atomic_long_t nr_pages[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
>  	/* the exponential moving average of refaulted */
>  	unsigned long avg_refaulted[ANON_AND_FILE][MAX_NR_TIERS];
>  	/* the exponential moving average of evicted+protected */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f5b0a7c63a3a..b02d2ec8ff4b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3354,8 +3354,7 @@ static void reset_batch_size(struct lru_gen_mm_walk *walk)
>  			continue;
>  
>  		walk->nr_pages[gen][type][zone] = 0;
> -		WRITE_ONCE(lrugen->nr_pages[gen][type][zone],
> -			   lrugen->nr_pages[gen][type][zone] + delta);
> +		atomic_long_add(delta, &lrugen->nr_pages[gen][type][zone]);
>  
>  		if (lru_gen_is_active(lruvec, gen))
>  			lru += LRU_ACTIVE;
> @@ -4044,8 +4043,8 @@ static bool inc_max_seq(struct lruvec *lruvec, unsigned long seq, int swappiness
>  	for (type = 0; type < ANON_AND_FILE; type++) {
>  		for (zone = 0; zone < MAX_NR_ZONES; zone++) {
>  			enum lru_list lru = type * LRU_INACTIVE_FILE;
> -			long delta = lrugen->nr_pages[prev][type][zone] -
> -				     lrugen->nr_pages[next][type][zone];
> +			long delta = atomic_long_read(&lrugen->nr_pages[prev][type][zone]) -
> +				     atomic_long_read(&lrugen->nr_pages[next][type][zone]);
>  
>  			if (!delta)
>  				continue;
> @@ -4163,7 +4162,8 @@ static unsigned long lruvec_evictable_size(struct lruvec *lruvec, int swappiness
>  		for (seq = min_seq[type]; seq <= max_seq; seq++) {
>  			gen = lru_gen_from_seq(seq);
>  			for (zone = 0; zone < MAX_NR_ZONES; zone++)
> -				total += max(READ_ONCE(lrugen->nr_pages[gen][type][zone]), 0L);
> +				total += max(atomic_long_read(&lrugen->nr_pages[gen][type][zone]),
> +					     0L);
>  		}
>  	}
>  
> @@ -4598,7 +4598,7 @@ static void __lru_gen_reparent_memcg(struct lruvec *child_lruvec, struct lruvec
>  
>  	for (i = 0; i < get_nr_gens(child_lruvec, type); i++) {
>  		int gen = lru_gen_from_seq(child_lrugen->max_seq - i);
> -		long nr_pages = child_lrugen->nr_pages[gen][type][zone];
> +		long nr_pages = atomic_long_read(&child_lrugen->nr_pages[gen][type][zone]);
>  		int child_lru_active = lru_gen_is_active(child_lruvec, gen) ? LRU_ACTIVE : 0;
>  		int parent_lru_active = lru_gen_is_active(parent_lruvec, gen) ? LRU_ACTIVE : 0;
>  
> @@ -4606,9 +4606,8 @@ static void __lru_gen_reparent_memcg(struct lruvec *child_lruvec, struct lruvec
>  		list_splice_tail_init(&child_lrugen->folios[gen][type][zone],
>  				      &parent_lrugen->folios[gen][type][zone]);
>  
> -		WRITE_ONCE(child_lrugen->nr_pages[gen][type][zone], 0);
> -		WRITE_ONCE(parent_lrugen->nr_pages[gen][type][zone],
> -			   parent_lrugen->nr_pages[gen][type][zone] + nr_pages);
> +		atomic_long_set(&child_lrugen->nr_pages[gen][type][zone], 0);
> +		atomic_long_add(nr_pages, &parent_lrugen->nr_pages[gen][type][zone]);
>  
>  		if (lru_gen_is_active(child_lruvec, gen) != lru_gen_is_active(parent_lruvec, gen)) {
>  			__update_lru_size(child_lruvec, lru + child_lru_active, zone, -nr_pages);
> @@ -5650,7 +5649,8 @@ static int lru_gen_seq_show(struct seq_file *m, void *v)
>  			char mark = full && seq < min_seq[type] ? 'x' : ' ';
>  
>  			for (zone = 0; zone < MAX_NR_ZONES; zone++)
> -				size += max(READ_ONCE(lrugen->nr_pages[gen][type][zone]), 0L);
> +				size += max(atomic_long_read(&lrugen->nr_pages[gen][type][zone]),
> +					    0L);
>  
>  			seq_printf(m, " %10lu%c", size, mark);
>  		}
> 
> -- 
> 2.55.0
> 
>