Re: [PATCH RFC 02/28] mm: memcontrol: use folio_memcg_charged() to avoid potential rcu lock holding

[Johannes Weiner]

On Tue, Apr 15, 2025 at 10:45:06AM +0800, Muchun Song wrote:
> If a folio isn't charged to the memory cgroup, holding an rcu read lock
> is needless. Users only want to know its charge status, so use
> folio_memcg_charged() here.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  mm/memcontrol.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 61488e45cab2..0fc76d50bc23 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -797,20 +797,17 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  void __lruvec_stat_mod_folio(struct folio *folio, enum node_stat_item idx,
>  			     int val)
>  {
> -	struct mem_cgroup *memcg;
>  	pg_data_t *pgdat = folio_pgdat(folio);
>  	struct lruvec *lruvec;
>  
> -	rcu_read_lock();
> -	memcg = folio_memcg(folio);
> -	/* Untracked pages have no memcg, no lruvec. Update only the node */
> -	if (!memcg) {
> -		rcu_read_unlock();
> +	if (!folio_memcg_charged(folio)) {
> +		/* Untracked pages have no memcg, no lruvec. Update only the node */
>  		__mod_node_page_state(pgdat, idx, val);
>  		return;
>  	}
>  
> -	lruvec = mem_cgroup_lruvec(memcg, pgdat);
> +	rcu_read_lock();
> +	lruvec = mem_cgroup_lruvec(folio_memcg(folio), pgdat);
>  	__mod_lruvec_state(lruvec, idx, val);
>  	rcu_read_unlock();

Hm, but untracked pages are the rare exception. It would seem better
for that case to take the rcu_read_lock() unnecessarily, than it is to
look up folio->memcg_data twice in the fast path?