Re: [PATCH v2] mm: move pgscan and pgsteal to node stats

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 2/18/26 04:29, JP Kobryn (Meta) wrote:
> From: JP Kobryn <jp.kobryn@linux.dev>
> 
> There are situations where reclaim kicks in on a system with free memory.
> One possible cause is a NUMA imbalance scenario where one or more nodes are
> under pressure. It would help if we could easily identify such nodes.
> 
> Move the pgscan and pgsteal counters from vm_event_item to node_stat_item
> to provide per-node reclaim visibility. With these counters as node stats,
> the values are now displayed in the per-node section of /proc/zoneinfo,
> which allows for quick identification of the affected nodes.
> 
> /proc/vmstat continues to report the same counters, aggregated across all
> nodes. But the ordering of these items within the readout changes as they
> move from the vm events section to the node stats section.
> 
> Memcg accounting of these counters is preserved. The relocated counters
> remain visible in memory.stat alongside the existing aggregate pgscan and
> pgsteal counters.
> 
> However, this change affects how the global counters are accumulated.
> Previously, the global event count update was gated on !cgroup_reclaim(),
> excluding memcg-based reclaim from /proc/vmstat. Now that
> mod_lruvec_state() is being used to update the counters, the global
> counters will include all reclaim. This is consistent with how pgdemote
> counters are already tracked.

Hm so that leaves PGREFILL (scanned in the active list) the odd one out,
right? Not being per-node and gated on !cgroup_reclaim() for global stats.
Should we change it too for full consistency?

> Finally, the virtio_balloon driver is updated to use
> global_node_page_state() to fetch the counters, as they are no longer
> accessible through the vm_events array.
> 
> Signed-off-by: JP Kobryn <jp.kobryn@linux.dev>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  drivers/virtio/virtio_balloon.c |  8 ++---
>  include/linux/mmzone.h          | 12 ++++++++
>  include/linux/vm_event_item.h   | 12 --------
>  mm/memcontrol.c                 | 52 +++++++++++++++++++++++----------
>  mm/vmscan.c                     | 32 ++++++++------------
>  mm/vmstat.c                     | 24 +++++++--------
>  6 files changed, 76 insertions(+), 64 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 4e549abe59ff..ab945532ceef 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -369,13 +369,13 @@ static inline unsigned int update_balloon_vm_stats(struct virtio_balloon *vb)
>  	update_stat(vb, idx++, VIRTIO_BALLOON_S_ALLOC_STALL, stall);
>  
>  	update_stat(vb, idx++, VIRTIO_BALLOON_S_ASYNC_SCAN,
> -		    pages_to_bytes(events[PGSCAN_KSWAPD]));
> +		    pages_to_bytes(global_node_page_state(PGSCAN_KSWAPD)));
>  	update_stat(vb, idx++, VIRTIO_BALLOON_S_DIRECT_SCAN,
> -		    pages_to_bytes(events[PGSCAN_DIRECT]));
> +		    pages_to_bytes(global_node_page_state(PGSCAN_DIRECT)));
>  	update_stat(vb, idx++, VIRTIO_BALLOON_S_ASYNC_RECLAIM,
> -		    pages_to_bytes(events[PGSTEAL_KSWAPD]));
> +		    pages_to_bytes(global_node_page_state(PGSTEAL_KSWAPD)));
>  	update_stat(vb, idx++, VIRTIO_BALLOON_S_DIRECT_RECLAIM,
> -		    pages_to_bytes(events[PGSTEAL_DIRECT]));
> +		    pages_to_bytes(global_node_page_state(PGSTEAL_DIRECT)));
>  
>  #ifdef CONFIG_HUGETLB_PAGE
>  	update_stat(vb, idx++, VIRTIO_BALLOON_S_HTLB_PGALLOC,
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 3e51190a55e4..1aa9c7aec889 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -255,6 +255,18 @@ enum node_stat_item {
>  	PGDEMOTE_DIRECT,
>  	PGDEMOTE_KHUGEPAGED,
>  	PGDEMOTE_PROACTIVE,
> +	PGSTEAL_KSWAPD,
> +	PGSTEAL_DIRECT,
> +	PGSTEAL_KHUGEPAGED,
> +	PGSTEAL_PROACTIVE,
> +	PGSTEAL_ANON,
> +	PGSTEAL_FILE,
> +	PGSCAN_KSWAPD,
> +	PGSCAN_DIRECT,
> +	PGSCAN_KHUGEPAGED,
> +	PGSCAN_PROACTIVE,
> +	PGSCAN_ANON,
> +	PGSCAN_FILE,
>  #ifdef CONFIG_HUGETLB_PAGE
>  	NR_HUGETLB,
>  #endif
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 22a139f82d75..1fa3b3ad0ff9 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -40,19 +40,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		PGLAZYFREED,
>  		PGREFILL,
>  		PGREUSE,
> -		PGSTEAL_KSWAPD,
> -		PGSTEAL_DIRECT,
> -		PGSTEAL_KHUGEPAGED,
> -		PGSTEAL_PROACTIVE,
> -		PGSCAN_KSWAPD,
> -		PGSCAN_DIRECT,
> -		PGSCAN_KHUGEPAGED,
> -		PGSCAN_PROACTIVE,
>  		PGSCAN_DIRECT_THROTTLE,
> -		PGSCAN_ANON,
> -		PGSCAN_FILE,
> -		PGSTEAL_ANON,
> -		PGSTEAL_FILE,
>  #ifdef CONFIG_NUMA
>  		PGSCAN_ZONE_RECLAIM_SUCCESS,
>  		PGSCAN_ZONE_RECLAIM_FAILED,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 007413a53b45..e89e77457701 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -328,6 +328,18 @@ static const unsigned int memcg_node_stat_items[] = {
>  	PGDEMOTE_DIRECT,
>  	PGDEMOTE_KHUGEPAGED,
>  	PGDEMOTE_PROACTIVE,
> +	PGSTEAL_KSWAPD,
> +	PGSTEAL_DIRECT,
> +	PGSTEAL_KHUGEPAGED,
> +	PGSTEAL_PROACTIVE,
> +	PGSTEAL_ANON,
> +	PGSTEAL_FILE,
> +	PGSCAN_KSWAPD,
> +	PGSCAN_DIRECT,
> +	PGSCAN_KHUGEPAGED,
> +	PGSCAN_PROACTIVE,
> +	PGSCAN_ANON,
> +	PGSCAN_FILE,
>  #ifdef CONFIG_HUGETLB_PAGE
>  	NR_HUGETLB,
>  #endif
> @@ -441,14 +453,6 @@ static const unsigned int memcg_vm_event_stat[] = {
>  #endif
>  	PSWPIN,
>  	PSWPOUT,
> -	PGSCAN_KSWAPD,
> -	PGSCAN_DIRECT,
> -	PGSCAN_KHUGEPAGED,
> -	PGSCAN_PROACTIVE,
> -	PGSTEAL_KSWAPD,
> -	PGSTEAL_DIRECT,
> -	PGSTEAL_KHUGEPAGED,
> -	PGSTEAL_PROACTIVE,
>  	PGFAULT,
>  	PGMAJFAULT,
>  	PGREFILL,
> @@ -1382,6 +1386,14 @@ static const struct memory_stat memory_stats[] = {
>  	{ "pgdemote_direct",		PGDEMOTE_DIRECT		},
>  	{ "pgdemote_khugepaged",	PGDEMOTE_KHUGEPAGED	},
>  	{ "pgdemote_proactive",		PGDEMOTE_PROACTIVE	},
> +	{ "pgsteal_kswapd",		PGSTEAL_KSWAPD		},
> +	{ "pgsteal_direct",		PGSTEAL_DIRECT		},
> +	{ "pgsteal_khugepaged",		PGSTEAL_KHUGEPAGED	},
> +	{ "pgsteal_proactive",		PGSTEAL_PROACTIVE	},
> +	{ "pgscan_kswapd",		PGSCAN_KSWAPD		},
> +	{ "pgscan_direct",		PGSCAN_DIRECT		},
> +	{ "pgscan_khugepaged",		PGSCAN_KHUGEPAGED	},
> +	{ "pgscan_proactive",		PGSCAN_PROACTIVE	},
>  #ifdef CONFIG_NUMA_BALANCING
>  	{ "pgpromote_success",		PGPROMOTE_SUCCESS	},
>  #endif
> @@ -1425,6 +1437,14 @@ static int memcg_page_state_output_unit(int item)
>  	case PGDEMOTE_DIRECT:
>  	case PGDEMOTE_KHUGEPAGED:
>  	case PGDEMOTE_PROACTIVE:
> +	case PGSTEAL_KSWAPD:
> +	case PGSTEAL_DIRECT:
> +	case PGSTEAL_KHUGEPAGED:
> +	case PGSTEAL_PROACTIVE:
> +	case PGSCAN_KSWAPD:
> +	case PGSCAN_DIRECT:
> +	case PGSCAN_KHUGEPAGED:
> +	case PGSCAN_PROACTIVE:
>  #ifdef CONFIG_NUMA_BALANCING
>  	case PGPROMOTE_SUCCESS:
>  #endif
> @@ -1496,15 +1516,15 @@ static void memcg_stat_format(struct mem_cgroup *memcg, struct seq_buf *s)
>  
>  	/* Accumulated memory events */
>  	seq_buf_printf(s, "pgscan %lu\n",
> -		       memcg_events(memcg, PGSCAN_KSWAPD) +
> -		       memcg_events(memcg, PGSCAN_DIRECT) +
> -		       memcg_events(memcg, PGSCAN_PROACTIVE) +
> -		       memcg_events(memcg, PGSCAN_KHUGEPAGED));
> +		       memcg_page_state(memcg, PGSCAN_KSWAPD) +
> +		       memcg_page_state(memcg, PGSCAN_DIRECT) +
> +		       memcg_page_state(memcg, PGSCAN_PROACTIVE) +
> +		       memcg_page_state(memcg, PGSCAN_KHUGEPAGED));
>  	seq_buf_printf(s, "pgsteal %lu\n",
> -		       memcg_events(memcg, PGSTEAL_KSWAPD) +
> -		       memcg_events(memcg, PGSTEAL_DIRECT) +
> -		       memcg_events(memcg, PGSTEAL_PROACTIVE) +
> -		       memcg_events(memcg, PGSTEAL_KHUGEPAGED));
> +		       memcg_page_state(memcg, PGSTEAL_KSWAPD) +
> +		       memcg_page_state(memcg, PGSTEAL_DIRECT) +
> +		       memcg_page_state(memcg, PGSTEAL_PROACTIVE) +
> +		       memcg_page_state(memcg, PGSTEAL_KHUGEPAGED));
>  
>  	for (i = 0; i < ARRAY_SIZE(memcg_vm_event_stat); i++) {
>  #ifdef CONFIG_MEMCG_V1
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 44e4fcd6463c..dd6d87340941 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1984,7 +1984,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  	unsigned long nr_taken;
>  	struct reclaim_stat stat;
>  	bool file = is_file_lru(lru);
> -	enum vm_event_item item;
> +	enum node_stat_item item;
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>  	bool stalled = false;
>  
> @@ -2010,10 +2010,8 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  
>  	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
>  	item = PGSCAN_KSWAPD + reclaimer_offset(sc);
> -	if (!cgroup_reclaim(sc))
> -		__count_vm_events(item, nr_scanned);
> -	count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);
> -	__count_vm_events(PGSCAN_ANON + file, nr_scanned);
> +	mod_lruvec_state(lruvec, item, nr_scanned);
> +	mod_lruvec_state(lruvec, PGSCAN_ANON + file, nr_scanned);
>  
>  	spin_unlock_irq(&lruvec->lru_lock);
>  
> @@ -2030,10 +2028,8 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  					stat.nr_demoted);
>  	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
>  	item = PGSTEAL_KSWAPD + reclaimer_offset(sc);
> -	if (!cgroup_reclaim(sc))
> -		__count_vm_events(item, nr_reclaimed);
> -	count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
> -	__count_vm_events(PGSTEAL_ANON + file, nr_reclaimed);
> +	mod_lruvec_state(lruvec, item, nr_reclaimed);
> +	mod_lruvec_state(lruvec, PGSTEAL_ANON + file, nr_reclaimed);
>  
>  	lru_note_cost_unlock_irq(lruvec, file, stat.nr_pageout,
>  					nr_scanned - nr_reclaimed);
> @@ -4542,7 +4538,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  {
>  	int i;
>  	int gen;
> -	enum vm_event_item item;
> +	enum node_stat_item item;
>  	int sorted = 0;
>  	int scanned = 0;
>  	int isolated = 0;
> @@ -4601,13 +4597,11 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	}
>  
>  	item = PGSCAN_KSWAPD + reclaimer_offset(sc);
> -	if (!cgroup_reclaim(sc)) {
> -		__count_vm_events(item, isolated);
> +	if (!cgroup_reclaim(sc))
>  		__count_vm_events(PGREFILL, sorted);
> -	}
> -	count_memcg_events(memcg, item, isolated);
> +	mod_lruvec_state(lruvec, item, isolated);
>  	count_memcg_events(memcg, PGREFILL, sorted);
> -	__count_vm_events(PGSCAN_ANON + type, isolated);
> +	mod_lruvec_state(lruvec, PGSCAN_ANON + type, isolated);
>  	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, scan_batch,
>  				scanned, skipped, isolated,
>  				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
> @@ -4692,7 +4686,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	LIST_HEAD(clean);
>  	struct folio *folio;
>  	struct folio *next;
> -	enum vm_event_item item;
> +	enum node_stat_item item;
>  	struct reclaim_stat stat;
>  	struct lru_gen_mm_walk *walk;
>  	bool skip_retry = false;
> @@ -4756,10 +4750,8 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  					stat.nr_demoted);
>  
>  	item = PGSTEAL_KSWAPD + reclaimer_offset(sc);
> -	if (!cgroup_reclaim(sc))
> -		__count_vm_events(item, reclaimed);
> -	count_memcg_events(memcg, item, reclaimed);
> -	__count_vm_events(PGSTEAL_ANON + type, reclaimed);
> +	mod_lruvec_state(lruvec, item, reclaimed);
> +	mod_lruvec_state(lruvec, PGSTEAL_ANON + type, reclaimed);
>  
>  	spin_unlock_irq(&lruvec->lru_lock);
>  
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 99270713e0c1..d952c1e763e6 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1276,6 +1276,18 @@ const char * const vmstat_text[] = {
>  	[I(PGDEMOTE_DIRECT)]			= "pgdemote_direct",
>  	[I(PGDEMOTE_KHUGEPAGED)]		= "pgdemote_khugepaged",
>  	[I(PGDEMOTE_PROACTIVE)]			= "pgdemote_proactive",
> +	[I(PGSTEAL_KSWAPD)]			= "pgsteal_kswapd",
> +	[I(PGSTEAL_DIRECT)]			= "pgsteal_direct",
> +	[I(PGSTEAL_KHUGEPAGED)]			= "pgsteal_khugepaged",
> +	[I(PGSTEAL_PROACTIVE)]			= "pgsteal_proactive",
> +	[I(PGSTEAL_ANON)]			= "pgsteal_anon",
> +	[I(PGSTEAL_FILE)]			= "pgsteal_file",
> +	[I(PGSCAN_KSWAPD)]			= "pgscan_kswapd",
> +	[I(PGSCAN_DIRECT)]			= "pgscan_direct",
> +	[I(PGSCAN_KHUGEPAGED)]			= "pgscan_khugepaged",
> +	[I(PGSCAN_PROACTIVE)]			= "pgscan_proactive",
> +	[I(PGSCAN_ANON)]			= "pgscan_anon",
> +	[I(PGSCAN_FILE)]			= "pgscan_file",
>  #ifdef CONFIG_HUGETLB_PAGE
>  	[I(NR_HUGETLB)]				= "nr_hugetlb",
>  #endif
> @@ -1320,19 +1332,7 @@ const char * const vmstat_text[] = {
>  
>  	[I(PGREFILL)]				= "pgrefill",
>  	[I(PGREUSE)]				= "pgreuse",
> -	[I(PGSTEAL_KSWAPD)]			= "pgsteal_kswapd",
> -	[I(PGSTEAL_DIRECT)]			= "pgsteal_direct",
> -	[I(PGSTEAL_KHUGEPAGED)]			= "pgsteal_khugepaged",
> -	[I(PGSTEAL_PROACTIVE)]			= "pgsteal_proactive",
> -	[I(PGSCAN_KSWAPD)]			= "pgscan_kswapd",
> -	[I(PGSCAN_DIRECT)]			= "pgscan_direct",
> -	[I(PGSCAN_KHUGEPAGED)]			= "pgscan_khugepaged",
> -	[I(PGSCAN_PROACTIVE)]			= "pgscan_proactive",
>  	[I(PGSCAN_DIRECT_THROTTLE)]		= "pgscan_direct_throttle",
> -	[I(PGSCAN_ANON)]			= "pgscan_anon",
> -	[I(PGSCAN_FILE)]			= "pgscan_file",
> -	[I(PGSTEAL_ANON)]			= "pgsteal_anon",
> -	[I(PGSTEAL_FILE)]			= "pgsteal_file",
>  
>  #ifdef CONFIG_NUMA
>  	[I(PGSCAN_ZONE_RECLAIM_SUCCESS)]	= "zone_reclaim_success",