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

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Message-Id: <CD857422-95FD-477D-809A-C7ED0780E188@linux.dev>
Date: Fri, 18 Apr 2025 10:38:10 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Muchun Song <songmuchun@...edance.com>,
 mhocko@...nel.org,
 roman.gushchin@...ux.dev,
 shakeel.butt@...ux.dev,
 akpm@...ux-foundation.org,
 david@...morbit.com,
 zhengqi.arch@...edance.com,
 yosry.ahmed@...ux.dev,
 nphamcs@...il.com,
 chengming.zhou@...ux.dev,
 linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org,
 linux-mm@...ck.org,
 hamzamahfooz@...ux.microsoft.com,
 apais@...ux.microsoft.com
Subject: Re: [PATCH RFC 02/28] mm: memcontrol: use folio_memcg_charged() to
 avoid potential rcu lock holding



> On Apr 17, 2025, at 22:48, Johannes Weiner <hannes@...xchg.org> wrote:
> 
> 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@...edance.com>
>> ---
>> 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?

Yep, you are right. I'll drop this next version. Thanks.

Muchun,
Thanks.